/**
 * Static utilities.
 */
class MDUtils {
	// Modified from https://urlregex.com/ to remove capture groups. Matches fully qualified URLs only.
	static baseURLRegex = /(?:(?:(?:[a-z]{3,9}:(?:\/\/)?)(?:[\-;:&=\+\$,\w]+@)?[a-z0-9\.\-]+|(?:www\.|[\-;:&=\+\$,\w]+@)[a-z0-9\.\-]+)(?:(?:\/[\+~%\/\.\w\-_]*)?\??(?:[\-\+=&;%@\.\w_]*)#?(?:[\.\!\/\\\w]*))?)/i;
	// Modified from https://emailregex.com/ to remove capture groups.
	static baseEmailRegex = /(?:(?:[^<>()\[\]\\.,;:\s@"]+(?:\.[^<>()\[\]\\.,;:\s@"]+)*)|(?:".+"))@(?:(?:\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(?:(?:[a-z\-0-9]+\.)+[a-z]{2,}))/i;

	/**
	 * Escapes special HTML characters.
	 *
	 * @param {string} str - string to escape
	 * @param {boolean} encodeNewlinesAsBreaks - whether to convert newline characters to `<br>` tags
	 * @returns {string} escaped HTML
	 */
	static escapeHTML(str, encodeNewlinesAsBreaks=false) {
		if (typeof str !== 'string') return '';
		var html = str.replace(/&/g, '&amp;').replace(/</g, '&lt;')
			.replace(/>/g, '&gt;').replace(/"/g, '&quot;');
		if (encodeNewlinesAsBreaks) {
			html = html.replace(/\n/g, "<br>\n");
		}
		return html;
	}

	/**
	 * Converts HTML entities to characters. HTML tags are stripped.
	 *
	 * @param {string} html
	 * @returns {string} plain text
	 */
	static unescapeHTML(html, decodeBRsAsNewlines=false) {
		if (decodeBRsAsNewlines) {
			html = html.replace(/<br[\/]?>\n?/g, "\n");
		}
		const doc = (new DOMParser()).parseFromString(html, "text/html");
		return doc.documentElement.textContent;
	}

	/**
	 * Encodes characters as HTML numeric entities to make it marginally more
	 * difficult for web scrapers to grab sensitive info. If `text` starts with
	 * `mailto:` only the email address following it will be obfuscated.
	 *
	 * @param {string} text - text to escape
	 * @returns {string} escaped HTML
	 */
	static escapeObfuscated(text) {
		if (text.startsWith('mailto:')) {
			return 'mailto:' + this.escapeObfuscated(text.substring(7));
		}
		var html = '';
		for (var p = 0; p < text.length; p++) {
			const cp = text.codePointAt(p);
			html += `&#${cp};`;
		}
		return html;
	}

	/**
	 * Removes illegal characters from an HTML attribute name.
	 *
	 * @param {string} name
	 * @returns {string}
	 */
	static scrubAttributeName(name) {
		return name.replace(/[\t\n\f \/>"'=]+/, '');
	}

	/**
	 * Strips one or more leading indents from a line or lines of markdown. An
	 * indent is defined as 4 spaces or one tab. Incomplete indents (i.e. 1-3
	 * spaces) are treated like one indent level.
	 *
	 * @param {string|string[]} line - string or strings to strip
	 * @param {number} levels - how many indent levels to strip
	 * @returns {string|string[]} stripped lines
	 */
	static stripIndent(line, levels=1) {
		const regex = new RegExp(`^(?: {1,4}|\t){${levels}}`);
		return (line instanceof Array) ? line.map((l) => l.replace(regex, '')) : line.replace(regex, '');
	}

	/**
	 * Counts the number of indent levels in a line of text. Partial indents
	 * (1 to 3 spaces) are counted as one indent level unless `fullIndentsOnly`
	 * is `true`.
	 *
	 * @param {string} line - line of markdown
	 * @param {boolean} fullIndentsOnly - whether to only count full indent levels (4 spaces or a tab)
	 * @returns {number} number of indent levels found
	 */
	static countIndents(line, fullIndentsOnly=false) {
		// normalize indents to tabs
		return line.replace(fullIndentsOnly
			? /(?: {4}|\t)/g
			: /(?: {1,4}|\t)/g,
		"\t")
		// remove content after indent
		.replace(/^(\t*)(.*?)$/, '$1')
		// count tabs
		.length;
	}

	/**
	 * Returns a copy of an array without any whitespace-only lines at the end.
	 *
	 * @param {String[]} lines - text lines
	 * @returns {String[]} - text lines without trailing blank lines
	 */
	static withoutTrailingBlankLines(lines) {
		var stripped = lines.slice();
		while (stripped.length > 0 && stripped[stripped.length - 1].trim().length == 0) {
			stripped.pop();
		}
		return stripped;
	}

	/**
	 * Tests if an array of lines contains at least one blank. A blank line
	 * can contain whitespace.
	 *
	 * @param {String[]} lines
	 * @returns {boolean} whether `lines` contains any whitespace-only lines
	 */
	static containsBlankLine(lines) {
		for (const line of lines) {
			if (line.trim().length == 0) return true;
		}
		return false;
	}

	/**
	 * Describes the type of a variable for debugging.
	 *
	 * @param {any} value - value
	 * @returns {String} description of type
	 */
	static typename(value) {
		if (value === null) return 'null';
		if (value instanceof Object) {
			return value.constructor.name;
		}
		return typeof value;
	}

	static #equalArrays(a, b) {
		if (a === b) return true;
		if (!(a instanceof Array) || !(b instanceof Array)) return false;
		if (a == null || b == null) return false;
		if (a.length != b.length) return false;
		for (var i = 0; i < a.length; i++) {
			if (!this.equal(a[i], b[i])) return false;
		}
		return true;
	}

	static #equalObjects(a, b) {
		if (a === b) return true;
		if (!(a instanceof Object) || !(b instanceof Object)) return false;
		if (a == null || b == null) return false;
		if (a.equals !== undefined) {
			return a.equals(b);
		}
		for (const key of Object.keys(a)) {
			if (!this.equal(a[key], b[key])) return false;
		}
		for (const key of Object.keys(b)) {
			if (!this.equal(a[key], b[key])) return false;
		}
		return true;
	}

	/**
	 * Tests for equality on lots of different kinds of values including objects
	 * and arrays. Will use `.equals` on objects that implement it.
	 *
	 * @param {any} a
	 * @param {any} b
	 * @returns {boolean}
	 */
	static equal(a, b, floatDifferencePercent=0.0) {
		if (a instanceof Array && b instanceof Array) {
			return this.#equalArrays(a, b);
		}
		if (a instanceof Object && b instanceof Object) {
			return this.#equalObjects(a, b);
		}
		if (typeof a == 'number' && typeof b == 'number') {
			if (a === b) return true;
			const delta = b - a;
			const ratio = delta / a;
			return Math.abs(ratio) <= floatDifferencePercent;
		}
		return a == b;
	}

	/**
	 * Escapes special characters in a string for inclusion as a literal in a
	 * regular expression.
	 *
	 * @param {string} text
	 */
	static escapeRegex(text) {
		// Partially following escaping scheme from not-yet-widely-supported RegExp.escape().
		// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/escape
		const escapeHex = function(ch) {
			const codepoint = ch.codePointAt(0);
			const s = '00' + codepoint.toString(16);
			return `\\x${s.substring(s.length - 2)}`;
		}
		var escaped = '';
		const l = text.length;
		for (var i = 0; i < l; i++) {
			const ch = text.substring(i, i + 1);
			if (i == 0 && /[a-zA-Z0-9]/.exec(ch)) {
				escaped += escapeHex(ch);
			} else if ("^$\\.*+?()[]{}|/".indexOf(ch) >= 0) {
				escaped += `\\${ch}`;
			} else if (",-=<>#&!%:;@~'`\"".indexOf(ch) >= 0) {
				escaped += escapeHex(ch);
			} else if (ch == '\f') {
				escaped += "\\f";
			} else if (ch == '\n') {
				escaped += "\\n";
			} else if (ch == '\r') {
				escaped += "\\r";
			} else if (ch == '\t') {
				escaped += "\\t";
			} else if (ch == '\v') {
				escaped += "\\v";
			} else {
				escaped += ch;
			}
		}
		return escaped;
	}
}

/**
 * Token type enum for `MDToken`.
 */
class MDTokenType {
	static Text = new MDTokenType('Text');
	/**
	 * Only used for the leading and trailing whitespace around a run of text,
	 * not every single whitespace character.
	 */
	static Whitespace = new MDTokenType('Whitespace');

	static Underscore = new MDTokenType('Underscore');
	static Asterisk = new MDTokenType('Asterisk');
	static Slash = new MDTokenType('Slash');
	static Tilde = new MDTokenType('Tilde');
	static Bang = new MDTokenType('Bang');
	static Backtick = new MDTokenType('Backtick');
	static Equal = new MDTokenType('Equal');
	static Caret = new MDTokenType('Caret');

	static Label = new MDTokenType('Label'); // content=label
	static URL = new MDTokenType('URL'); // content=URL, extra=title
	static Email = new MDTokenType('Email'); // content=email address, extra=title
	static SimpleLink = new MDTokenType('SimpleLink'); // content=URL
	static SimpleEmail = new MDTokenType('SimpleEmail'); // content=email address
	static Footnote = new MDTokenType('Footnote'); // content=symbol
	static Modifier = new MDTokenType('Modifier'); // modifier=MDTagModifier

	static HTMLTag = new MDTokenType('HTMLTag'); // tag=MDHTMLTag

	/** Wildcard for `MDToken.findFirstTokens` */
	static META_AnyNonWhitespace = new MDTokenType('META_AnyNonWhitespace');
	/** Wildcard for `MDToken.findFirstTokens` */
	static META_OptionalWhitespace = new MDTokenType('META_OptionalWhitespace');

	/** @type {string} */
	name;

	/**
	 * @param {string} name
	 */
	constructor(name) {
		this.name = name;
	}

	/** @returns {string} */
	toString() {
		return `${this.constructor.name}.${this.name}`;
	}
}

/**
 * Search results from `MDToken.findFirstTokens`.
 */
class MDTokenMatch {
	/** @type {MDToken{}} */
	tokens;
	/** @type {number} */
	index;

	constructor(tokens, index) {
		this.tokens = tokens;
		this.index = index;
	}
}

/**
 * Search results from `MDToken.findPairedTokens`.
 */
class MDPairedTokenMatch {
	/** @type {MDToken[]} */
	startTokens;
	/** @type {MDToken[]} */
	contentTokens;
	/** @type {MDToken[]} */
	endTokens;
	/** @type {number} */
	startIndex;
	/** @type {number} */
	contentIndex;
	/** @type {number} */
	endIndex;
	/** @type {number} */
	totalLength;

	constructor(startTokens, contentTokens, endTokens, startIndex, contentIndex, endIndex, totalLength) {
		this.startTokens = startTokens;
		this.contentTokens = contentTokens;
		this.endTokens = endTokens;
		this.startIndex = startIndex;
		this.contentIndex = contentIndex;
		this.endIndex = endIndex;
		this.totalLength = totalLength;
	}
}

/**
 * One lexical unit in inline markdown syntax parsing.
 */
class MDToken {
	/**
	 * The original verbatim token string. Required as a plaintext fallback if
	 * the token remains unresolved.
	 * @type {string}
	 */
	original;
	/** @type {MDTokenType} */
	type;
	/** @type {string|null} */
	content = null;
	/** @type {string|null} */
	extra = null;
	/** @type {MDHTMLTag|null} */
	tag = null;
	/** @type {MDTagModifier|null} */
	modifier = null;

	/**
	 * Creates a token.
	 *
	 * @param {string} original - verbatim token string
	 * @param {MDTokenType} type - token type
	 * @param {string|MDTagModifier|MDHTMLTag|null} content - primary content of the token
	 * @param {string|null} extra - additional content
	 */
	constructor(original, type, content=null, extra=null) {
		this.original = original;
		this.type = type;
		if (content instanceof MDTagModifier) {
			this.modifier = content;
		} else if (content instanceof MDHTMLTag) {
			this.tag = content;
		} else {
			this.content = content;
		}
		this.extra = extra;
	}

	toString() {
		return `(${this.constructor.name} type=${this.type.toString()} content=${this.content})`;
	}

	/**
	 * Attempts to parse a label token from the beginning of `line`. A label is
	 * of the form `[content]`. If found, returns an array:
	 * - `0`: the entire label including brackets
	 * - `1`: the content of the label
	 *
	 * @param {string} line
	 * @returns {string[]|null} match groups or null if not found
	 */
	static tokenizeLabel(line) {
		if (!line.startsWith('[')) return null;
		var parenCount = 0;
		var bracketCount = 0;
		for (var p = 1; p < line.length; p++) {
			let ch = line.substring(p, p + 1);
			if (ch == '\\') {
				p++;
			} else if (ch == '(') {
				parenCount++;
			} else if (ch == ')') {
				parenCount--;
				if (parenCount < 0) return null;
			} else if (ch == '[') {
				bracketCount++;
			} else if (ch == ']') {
				if (bracketCount > 0) {
					bracketCount--;
				} else {
					return [ line.substring(0, p + 1), line.substring(1, p) ];
				}
			}
		}
		return null;
	}

	static #urlWithTitleRegex = /^\((\S+?)\s+"(.*?)"\)/i;  // 1=URL, 2=title
	static #urlRegex = /^\((\S+?)\)/i;  // 1=URL

	/**
	 * Attempts to parse a URL token from the beginning of `line`. A URL token
	 * is of the form `(url)` or `(url "title")`. If found, returns an array:
	 * - `0`: the entire URL token including parentheses
	 * - `1`: the URL
	 * - `2`: the optional title, or `null`
	 *
	 * @param {string} line
	 * @returns {string[]} token tuple
	 */
	static tokenizeURL(line) {
		var groups;
		if (groups = this.#urlWithTitleRegex.exec(line)) {
			if (this.tokenizeEmail(line)) return null; // make sure it's not better described as an email address
			return groups;
		}
		if (groups = this.#urlRegex.exec(line)) {
			if (this.tokenizeEmail(line)) return null;
			return [...groups, null];
		}
		return null;
	}

	static #emailWithTitleRegex = new RegExp("^\\(\\s*(" + MDUtils.baseEmailRegex.source + ")\\s+\"(.*?)\"\\s*\\)", "i");  // 1=email, 2=title
	static #emailRegex = new RegExp("^\\(\\s*(" + MDUtils.baseEmailRegex.source + ")\\s*\\)", "i");  // 1=email

	/**
	 * Attempts to parse an email address from the beginning of `line`. An
	 * email address is of the form `(user@example.com)` or
	 * `(user@example.com "link title")`. If found, returns an array:
	 * - `0`: the entire token including parentheses
	 * - `1`: the email address
	 * - `2`: the optional link title, or `null`
	 *
	 * @param {string} line
	 * @returns {string[]} token tuple
	 */
	static tokenizeEmail(line) {
		var groups;
		if (groups = this.#emailWithTitleRegex.exec(line)) {
			return groups;
		}
		if (groups = this.#emailRegex.exec(line)) {
			return [...groups, null];
		}
		return null;
	}

	/**
	 * Searches an array of `MDToken` for the given pattern of `MDTokenType`s.
	 * If found, returns a `MDTokenMatch`, otherwise `null`.
	 *
	 * Special token types `META_AnyNonWhitespace` and `META_OptionalWhitespace`
	 * are special supported token types. Note that `META_OptionalWhitespace`
	 * may give a result with a variable number of tokens.
	 *
	 * @param {MDToken[]|MDNode[]} tokensToSearch - mixed array of `MDToken` and
	 *   `MDNode` elements
	 * @param {MDTokenType[]} pattern - contiguous run of token types to find
	 * @param {number} startIndex - token index to begin searching (defaults to 0)
	 * @returns {MDTokenMatch|null} match object, or `null` if not found
	 */
	static findFirstTokens(tokensToSearch, pattern, startIndex=0) {
		var matched = [];
		for (var t = startIndex; t < tokensToSearch.length; t++) {
			var matchedAll = true;
			matched = [];
			var patternOffset = 0;
			for (var p = 0; p < pattern.length; p++) {
				var t0 = t + p + patternOffset;
				if (t0 >= tokensToSearch.length) return null;
				let token = tokensToSearch[t0];
				let elem = pattern[p];
				if (elem == MDTokenType.META_OptionalWhitespace) {
					if (token instanceof MDToken && token.type == MDTokenType.Whitespace) {
						matched.push(token);
					} else {
						patternOffset--;
					}
				} else if (elem == MDTokenType.META_AnyNonWhitespace) {
					if (token instanceof MDToken && token.type == MDTokenType.Whitespace) {
						matchedAll = false;
						break;
					}
					matched.push(token);
				} else {
					if (!(token instanceof MDToken) || token.type != elem) {
						matchedAll = false;
						break;
					}
					matched.push(token);
				}
			}
			if (matchedAll) {
				return new MDTokenMatch(matched, t);
			}
		}
		return null;
	}

	/**
	 * Searches an array of MDToken for a given starting pattern and ending
	 * pattern and returns match info about both and the tokens in between.
	 *
	 * If `contentValidator` is specified, it will be called with the content
	 * tokens of a potential match. If the validator returns `true`, the result
	 * will be accepted and returned by this method. If the validator returns
	 * `false`, this method will keep looking for another matching pair. If no
	 * validator is given the first match will be returned regardless of content.
	 *
	 * If a match is found, a `MDPairedTokenMatch` is returned with details
	 * of the opening tokens, closing tokens, and content tokens between. Otherwise
	 * `null` is returned.
	 *
	 * @param {MDToken[]} tokensToSearch - array of `MDToken` to search in
	 * @param {MDTokenType[]} startPattern - array of `MDTokenType` to find first
	 * @param {MDTokenType[]} endPattern - array of `MDTokenType` to find positioned after `startPattern`
	 * @param {function|null} contentValidator - optional validator function. If provided, will be passed an array of inner `MDToken`, and the function can return `true` to accept the contents or `false` to keep searching
	 * @param {number} startIndex - token index where searching should begin
	 * @returns {MDPairedTokenMatch|null} match, or `null`
	 */
	static findPairedTokens(tokensToSearch, startPattern, endPattern, contentValidator=null, startIndex=0) {
		for (var s = startIndex; s < tokensToSearch.length; s++) {
			var startMatch = this.findFirstTokens(tokensToSearch, startPattern, s);
			if (startMatch === null) return null;
			var endStart = startMatch.index + startMatch.tokens.length;
			while (endStart < tokensToSearch.length) {
				var endMatch = this.findFirstTokens(tokensToSearch, endPattern, endStart);
				if (endMatch === null) break;
				var contents = tokensToSearch.slice(startMatch.index + startMatch.tokens.length, endMatch.index);
				if (contents.length > 0 && (contentValidator === null || contentValidator(contents))) {
					return new MDPairedTokenMatch(startMatch.tokens,
						contents,
						endMatch.tokens,
						startMatch.index,
						startMatch.index + startMatch.tokens.length,
						endMatch.index,
						endMatch.index + endMatch.tokens.length - startMatch.index);
				} else {
					// Contents rejected. Try next end match.
					endStart = endMatch.index + 1;
				}
			}
			// No end matches. Increment start match.
			s = startMatch.index;
		}
		return null;
	}

	equals(other) {
		if (!(other instanceof MDToken)) return false;
		if (other.original !== this.original) return false;
		if (!other.type.equals(this.type)) return false;
		if (other.content !== this.content) return false;
		if (other.extra !== this.extra) return false;
		if (!MDUtils.equal(other.tag, this.tag)) return false;
		if (!MDUtils.equals(other.modifier, this.modifier)) return false;
		return true
	}
}

/**
 * Parsing and rendering state. Passed around throughout the parsing process.
 *
 * States are hierarchical. A sub-state can be created by calling `.copy()` with
 * a new array of lines. The sub-state points back to its parent state. This
 * is done to parse inner content of a syntax as its own standalone document.
 *
 * If a custom `MDReader` implementation wants to store data in this object,
 * always do so on `state.root` to ensure it's stored on the original state,
 * not a child state. Otherwise data may be lost when the sub-state is discarded.
 */
class MDState {
	/**
	 * Ascends the parent chain to the root `MDState` instance. This should be
	 * used when referencing most stored fields except `lines` and `p`.
	 *
	 * @type {MDState}
	 */
	get root() { return this.#parent ? this.#parent.root : this; }

	/**
	 * Lines of the markdown document. The current line index is pointed to by `p`.
	 *
	 * @type {string[]}
	 */
	lines;

	/**
	 * The current line in `lines`.
	 *
	 * @returns {string|null} current line or `null` if out of content
	 */
	get currentLine() { return (this.p < this.lines.length) ? this.lines[this.p] : null; }

	/**
	 * Current line pointer into array `lines`.
	 *
	 * @type {number} line pointer
	 */
	p = 0;

	/** @type {MDState|null} */
	#parent = null;

	/**
	 * Array of `MDReader`s sorted by block reading priority.
	 * @type {MDReader[]}
	 */
	readersByBlockPriority = [];

	/**
	 * Array of `MDReader`s sorted by tokenization priority.
	 * @type {MDReader[]}
	 */
	readersByTokenPriority = [];

	/**
	 * Array of tuples of `pass:number` and `MDReader` sorted by substitution
	 * priority.
	 * @type {Array}
	 */
	readersBySubstitutePriority = [];

	/**
	 * Prefix to include in any generated `id` attributes on HTML elements.
	 * Useful for keeping elements unique in multiple parsed documents in the
	 * same HTML page.
	 *
	 * @type {string}
	 */
	elementIdPrefix = '';

	/**
	 * Filter for removing unapproved HTML tags, attributes, and values.
	 * @type {MDHTMLFilter}
	 */
	tagFilter;

	static #textWhitespaceRegex = /^(\s*)(?:(\S|\S.*\S)(\s*?))?$/; // 1=leading WS, 2=text, 3=trailing WS

	/**
	 * @param {string[]} lines - lines of markdown text
	 */
	constructor(lines) {
		this.lines = lines;
	}

	/**
	 * Creates a copy of this state with new lines. Useful for parsing nested
	 * content.
	 *
	 * @param {string[]} lines
	 * @returns {MDState} copied sub-state
	 */
	copy(lines) {
		let cp = new MDState(lines);
		cp.#parent = this;
		return cp;
	}

	/**
	 * Tests if there are at least `minCount` lines available to read. If `p`
	 * is not provided it will be relative to `this.p`.
	 *
	 * @param {number} minCount - minimum number of lines
	 * @param {number|null} p - line pointer, or `null` to use `this.p`
	 * @returns {boolean} whether at least the given number of lines is available
	 */
	hasLines(minCount, p=null) {
		let relativeTo = (p === null) ? this.p : p;
		return relativeTo + minCount <= this.lines.length;
	}

	/**
	 * Reads and returns an array of blocks from the current line pointer.
	 *
	 * @returns {MDBlockNode[]} parsed blocks
	 */
	readBlocks() {
		var blocks = [];
		while (this.hasLines(1)) {
			let block = this.#readNextBlock();
			if (block) {
				blocks.push(block);
			} else {
				break;
			}
		}
		return blocks;
	}

	/**
	 * Creates a simple `MDBlockNode` if no other registered blocks match.
	 *
	 * @returns {MDBlockNode|null} fallback block
	 */
	#readFallbackBlock() {
		if (this.p >= this.lines.length) return null;
		const lines = MDUtils.withoutTrailingBlankLines(this.lines.slice(this.p));
		if (lines.length == 0) return null;
		this.p = this.lines.length;
		return this.inlineMarkdownToNode(lines.join("\n"));
	}

	/**
	 * Attempts to read one block from the current line pointer. The pointer
	 * will be positioned just after the end of the block.
	 *
	 * @param {MDState} state
	 * @returns {MDBlockNode|null}
	 */
	#readNextBlock() {
		while (this.hasLines(1) && this.lines[this.p].trim().length == 0) {
			this.p++;
		}
		if (!this.hasLines(1)) return null;
		for (const reader of this.root.readersByBlockPriority) {
			const startP = this.p;
			const block = reader.readBlock(this);
			if (block) {
				if (this.p == startP) {
					throw new Error(`${reader.constructor.name} returned an ` +
						`${block.constructor.name} without incrementing MDState.p. ` +
						`This could lead to an infinite loop.`);
				}
				return block;
			}
		}
		const fallback = this.#readFallbackBlock();
		return fallback;
	}

	/**
	 * @param {string} line
	 * @returns {MDToken[]}
	 */
	#inlineMarkdownToTokens(line) {
		if (this.#parent) return this.#parent.#inlineMarkdownToTokens(line);

		var tokens = [];
		var text = '';
		var expectLiteral = false;

		/**
		 * Flushes accumulated content in `text` to `tokens`.
		 */
		const endText = function() {
			if (text.length == 0) return;
			const textGroups = MDState.#textWhitespaceRegex.exec(text);
			if (textGroups !== null) {
				if (textGroups[1].length > 0) {
					tokens.push(new MDToken(textGroups[1], MDTokenType.Whitespace, textGroups[1]));
				}
				if (textGroups[2] !== undefined && textGroups[2].length > 0) {
					tokens.push(new MDToken(textGroups[2], MDTokenType.Text, textGroups[2]));
				}
				if (textGroups[3] !== undefined && textGroups[3].length > 0) {
					tokens.push(new MDToken(textGroups[3], MDTokenType.Whitespace, textGroups[3]));
				}
			} else {
				tokens.push(new MDToken(text, MDTokenType.Text, text));
			}
			text = '';
		}

		for (var p = 0; p < line.length; p++) {
			const ch = line.substring(p, p + 1);
			const remainder = line.substring(p);
			if (expectLiteral) {
				text += ch;
				expectLiteral = false;
				continue;
			}
			if (ch == '\\') {
				expectLiteral = true;
				continue;
			}
			var found = false;
			for (const reader of this.root.readersByTokenPriority) {
				const token = reader.readToken(this, remainder);
				if (token === null) continue;
				if (token === undefined) {
					console.warn(`${reader.constructor.name}.readToken returned undefined instead of null`);
				}
				endText();
				tokens.push(token);
				if (token.original == null || token.original.length == 0) {
					throw new Error(`${reader.constructor.name} returned a token with an empty .original. This would cause an infinite loop.`);
				}
				p += token.original.length - 1;
				found = true;
				break;
			}
			if (!found) {
				text += ch;
			}
		}
		endText();
		return tokens;
	}

	/**
	 * Converts a line of markdown to an `MDInlineNode`.
	 *
	 * @param {string|string[]} line
	 * @returns {MDInlineNode}
	 */
	inlineMarkdownToNode(line) {
		let nodes = this.inlineMarkdownToNodes(line);
		return (nodes.length == 1) ? nodes[0] : new MDInlineNode(nodes);
	}

	/**
	 * Converts a line of markdown to an array of `MDInlineNode`s.
	 *
	 * @param {string|string[]} line
	 * @returns {MDInlineNode[]}
	 */
	inlineMarkdownToNodes(line) {
		var tokens = this.#inlineMarkdownToTokens((line instanceof Array) ? line.join('\n') : line);
		return this.tokensToNodes(tokens);
	}

	/**
	 * Converts a mixed array of `MDToken` and `MDInlineNode` elements into an array
	 * of only `MDInlineNode` via repeated `MDReader` substition.
	 *
	 * @param {MDToken[]|MDInlineNode[]} tokens
	 * @returns {MDInlineNode[]}
	 */
	tokensToNodes(tokens) {
		var nodes = tokens.slice();

		// Perform repeated substitutions, converting sequences of tokens into
		// nodes, until no more substitutions can be made.
		var anyChanges = false;
		do {
			anyChanges = false;
			for (const readerTuple of this.root.readersBySubstitutePriority) {
				/** @type {number} */
				const pass = readerTuple[0];
				/** @type {MDReader} */
				const reader = readerTuple[1];
				const changed = reader.substituteTokens(this, pass, nodes);
				if (!changed) continue;
				anyChanges = true;
				break;
			}
		} while (anyChanges);

		// Convert any remaining tokens to text nodes. Also apply any inline
		// CSS modifiers.
		var lastNode = null;
		const me = this;
		nodes = nodes.map(function(node) {
			if (node instanceof MDToken) {
				/** @type {MDToken} */
				const token = node;
				if (token.type == MDTokenType.Modifier && lastNode) {
					me.root.tagFilter.scrubModifier(token.modifier);
					token.modifier.applyTo(lastNode);
					lastNode = null;
					return new MDTextNode('');
				}
				lastNode = null;
				return new MDTextNode(token.original);
			} else if (node instanceof MDNode) {
				lastNode = (node instanceof MDTextNode) ? null : node;
				return node;
			} else {
				throw new Error(`Unexpected node type ${node.constructor.name}`);
			}
		});

		return nodes;
	}

	/**
	 * Mapping of reference symbols to URLs. Used by `MDReferencedLinkReader`
	 * and `MDReferencedImageReader`.
	 * @type {object} symbol -> URL
	 */
	#referenceToURL = {};

	/**
	 * Mapping of reference symbols to titles. Used by `MDReferencedLinkReader`
	 * and `MDReferencedImageReader`.
	 * @type {object} symbol -> title string
	 */
	#referenceToTitle = {};

	/**
	 * Defines a URL by reference symbol.
	 *
	 * @param {string} reference - case-insensitive reference symbol
	 * @param {string} url - URL to map the symbol to
	 * @param {string|null} title - optional link title
	 */
	defineURL(reference, url, title=null) {
		this.root.#referenceToURL[reference.toLowerCase()] = url;
		if (title !== null) this.root.#referenceToTitle[reference.toLowerCase()] = title;
	}

	/**
	 * Returns the URL associated with a reference symbol.
	 *
	 * @param {string} reference - case-insensitive reference symbol
	 * @returns {string|null} URL for the given reference, or `null` if not defined
	 */
	urlForReference(reference) {
		return this.root.#referenceToURL[reference.toLowerCase()] ?? null;
	}

	/**
	 * Returns the link title associated with a reference symbol.
	 *
	 * @param {string} reference - case-insensitive reference symbol
	 * @returns {string|null} link title for the given reference, or `null` if not defined
	 */
	urlTitleForReference(reference) {
		return this.root.#referenceToTitle[reference.toLowerCase()] ?? null;
	}
}

/**
 * Defines a set of allowable HTML tags, attributes, and CSS.
 */
class MDHTMLFilter {
	/**
	 * Mapping of permitted lowercase tag names to objects containing allowable
	 * attributes for those tags. Does not need to include those attributes
	 * defined in `allowableGlobalAttributes`.
	 *
	 * Values are objects with allowable lowercase attribute names mapped to
	 * allowable value patterns. A `*` means any value is acceptable. Multiple
	 * allowable values can be joined together with `|`. These special symbols
	 * represent certain kinds of values and can be used in combination or in
	 * place of literal values.
	 *
	 * - `{classlist}`: A list of legal CSS classnames, separated by spaces
	 * - `{int}`: An integer
	 * - `{none}`: No value (an attribute with no `=` or value, like `checked`)
	 * - `{style}`: One or more CSS declarations, separated by semicolons (simple
	 *   `key: value;` syntax only)
	 * - `{url}`: A URL
	 * @type {object}
	 */
	allowableTags = {
		'address': {
			'cite': '{url}',
		},
		'h1': {},
		'h2': {},
		'h3': {},
		'h4': {},
		'h5': {},
		'h6': {},
		'blockquote': {},
		'dl': {},
		'dt': {},
		'dd': {},
		'div': {},
		'hr': {},
		'ul': {},
		'ol': {
			'start': '{int}',
			'type': 'a|A|i|I|1',
		},
		'li': {
			'value': '{int}',
		},
		'p': {},
		'pre': {},
		'table': {},
		'thead': {},
		'tbody': {},
		'tfoot': {},
		'tr': {},
		'td': {},
		'th': {},
		'a': {
			'href': '{url}',
			'target': '*',
		},
		'abbr': {},
		'b': {},
		'br': {},
		'cite': {},
		'code': {},
		'data': {
			'value': '*',
		},
		'dfn': {},
		'em': {},
		'i': {},
		'kbd': {},
		'mark': {},
		'q': {
			'cite': '{url}',
		},
		's': {},
		'samp': {},
		'small': {},
		'span': {},
		'strong': {},
		'sub': {},
		'sup': {},
		'time': {
			'datetime': '*',
		},
		'u': {},
		'var': {},
		'wbr': {},
		'img': {
			'alt': '*',
			'href': '{url}',
		},
		'figure': {},
		'figcaption': {},
		'del': {},
		'ins': {},
		'details': {},
		'summary': {},
	};

	/**
	 * Mapping of allowable lowercase global attributes to their permitted
	 * values. Uses same value pattern syntax as described in `allowableTags`.
	 * @type {object}
	 */
	allowableGlobalAttributes = {
		'class': '{classlist}',
		'data-*': '*',
		'dir': 'ltr|rtl|auto',
		'id': '*',
		'lang': '*',
		'style': '{style}',
		'title': '*',
		'translate': 'yes|no|{none}',
	};

	/**
	 * Mapping of allowable CSS style names to their allowable value patterns.
	 * Multiple values can be delimited with `|` characters. Limited support
	 * so far.
	 *
	 * Recognized special values:
	 * - `{color}`: A hex or named color
	 *
	 * @type {object}
	 */
	allowableStyleKeys = {
		'background-color': '{color}',
		'color': '{color}',
	};

	/**
	 * Scrubs all forbidden attributes from an HTML tag. Assumes the tag name
	 * itself has already been whitelisted.
	 *
	 * @param {MDHTMLTag} tag - HTML tag
	 */
	scrubTag(tag) {
		for (const name of Object.keys(tag.attributes)) {
			if (!this.isValidAttributeName(tag.tagName, name)) {
				delete tag.attributes[name];
			}
			if (!this.isValidAttributeValue(tag.tagName, name, tag.attributes[name])) {
				delete tag.attributes[name];
			}
		}
	}

	/**
	 * Scrubs all forbidden attributes from an HTML modifier.
	 *
	 * @param {MDTagModifier} modifier
	 * @param {string|null} tagName - HTML tag name, if known, otherwise only
	 *   global attributes will be permitted
	 */
	scrubModifier(modifier, tagName) {
		if (modifier.cssClasses.length > 0) {
			const classList = modifier.cssClasses.join(' ');
			if (!this.isValidAttributeValue(tagName, 'class', classList)) {
				modifier.cssClasses = [];
			}
		}
		if (modifier.cssId !== null) {
			if (!this.isValidAttributeValue(tagName, 'id', modifier.cssId)) {
				modifier.cssId = null;
			}
		}
		if (!this.isValidAttributeName(tagName, 'style')) {
			modifier.cssStyles = {};
		} else {
			for (const key of Object.keys(modifier.cssStyles)) {
				const val = modifier.cssStyles[key];
				if (!this.isValidStyleValue(key, val)) {
					delete modifier.cssStyles[key];
				}
			}
		}
		for (const key of Object.keys(modifier.attributes)) {
			const val = modifier.attributes[key];
			if (!this.isValidAttributeValue(tagName, key, val)) {
				delete modifier.attributes[key];
			}
		}
	}

	/**
	 * Tests if an HTML tag name is permitted.
	 *
	 * @param {string} tagName
	 * @returns {boolean}
	 */
	isValidTagName(tagName) {
		return this.allowableTags[tagName.toLowerCase()] !== undefined;
	}

	/**
	 * Tests if an HTML attribute name is permitted.
	 *
	 * @param {string|null} tagName - HTML tag name or null to only check global
	 *   attributes
	 * @param {string} attributeName - attribute name
	 * @returns {boolean}
	 */
	isValidAttributeName(tagName, attributeName) {
		const lcAttributeName = attributeName.toLowerCase();
		if (this.allowableGlobalAttributes[lcAttributeName] !== undefined) {
			return true;
		}
		for (const pattern in this.allowableGlobalAttributes) {
			if (pattern.endsWith('*') && lcAttributeName.startsWith(pattern.substring(0, pattern.length - 1))) {
				return true;
			}
		}
		if (tagName === null) return false;
		const lcTagName = tagName.toLowerCase();
		const tagAttributes = this.allowableTags[lcTagName];
		if (tagAttributes) {
			return tagAttributes[lcAttributeName] !== undefined;
		}
		return false;
	}

	/**
	 * Tests if an attribute value is allowable.
	 *
	 * @param {string|null} tagName
	 * @param {string} attributeName
	 * @param {string} attributeValue
	 * @returns {boolean}
	 */
	isValidAttributeValue(tagName, attributeName, attributeValue) {
		const lcAttributeName = attributeName.toLowerCase();
		const globalPattern = this.allowableGlobalAttributes[lcAttributeName];
		if (globalPattern !== undefined) {
			return this.#attributeValueMatchesPattern(attributeValue, globalPattern);
		}
		for (const namePattern in this.allowableGlobalAttributes) {
			if (namePattern.endsWith('*') && lcAttributeName.startsWith(namePattern.substring(0, namePattern.length - 1))) {
				return this.#attributeValueMatchesPattern(attributeValue, this.allowableGlobalAttributes[namePattern]);
			}
		}
		if (tagName === null) return false;
		const lcTagName = tagName.toLowerCase();
		const tagAttributes = this.allowableTags[lcTagName];
		if (tagAttributes === undefined) return false;
		const valuePattern = tagAttributes[lcAttributeName];
		if (valuePattern === undefined) return false;
		return this.#attributeValueMatchesPattern(attributeValue, valuePattern);
	}

	static #permissiveURLRegex = /^\S+$/;
	static #integerRegex = /^[\-]?\d+$/;
	static #classListRegex = /^-?[_a-zA-Z]+[_a-zA-Z0-9-]*(?:\s+-?[_a-zA-Z]+[_a-zA-Z0-9-]*)*$/;

	/**
	 * @param {string} value
	 * @param {string} pattern
	 * @returns {boolean}
	 */
	#attributeValueMatchesPattern(value, pattern) {
		const options = pattern.split('|');
		for (const option of options) {
			switch (option) {
				case '*':
					return true;
				case '{classlist}':
					if (MDHTMLFilter.#classListRegex.exec(value)) return true;
					break;
				case '{int}':
					if (MDHTMLFilter.#integerRegex.exec(value)) return true;
					break;
				case '{none}':
					if (value === true) return true;
					break;
				case '{style}':
					if (this.isValidStyleDeclaration(value)) return true;
					break;
				case '{url}':
					if (MDHTMLFilter.#permissiveURLRegex.exec(value)) return true;
					break;
				default:
					if (value === option) return true;
					break;
			}
		}
		return false;
	}

	/**
	 * Tests if a string of one or more style `key: value;` declarations is
	 * fully allowable.
	 *
	 * @param {string} styles
	 * @returns {boolean}
	 */
	isValidStyleDeclaration(styles) {
		const settings = styles.split(';');
		for (const setting of settings) {
			if (setting.trim().length == 0) continue;
			const parts = setting.split(':');
			if (parts.length != 2) return false;
			const name = parts[0].trim();
			if (!this.isValidStyleKey(name)) return false;
			const value = parts[1].trim();
			if (!this.isValidStyleValue(name, value)) return false;
		}
		return true;
	}

	/**
	 * Tests if a CSS style key is allowable.
	 *
	 * @param {string} key - CSS key
	 * @returns {boolean}
	 */
	isValidStyleKey(key) {
		return this.allowableStyleKeys[key] !== undefined;
	}

	/**
	 * Tests if a CSS style value is allowable.
	 *
	 * @param {string} key
	 * @param {string} value
	 * @returns {boolean}
	 */
	isValidStyleValue(key, value) {
		const pattern = this.allowableStyleKeys[key];
		if (pattern === undefined) return false;
		const options = pattern.split('|');
		for (const option of options) {
			switch (option) {
				case '{color}':
					if (this.#isValidCSSColor(value)) return true;
				default:
					if (value === option) return true;
			}
		}
		return false;
	}

	static #styleColorRegex = /^#[0-9a-f]{3}(?:[0-9a-f]{3})?$|^[a-zA-Z]+$/i;

	#isValidCSSColor(value) {
		return MDHTMLFilter.#styleColorRegex.exec(value) !== null;
	}
}

/**
 * Represents a single HTML tag. Paired tags are represented separately.
 */
class MDHTMLTag {
	/**
	 * Verbatim string of the original parsed tag. Not modified. Should be
	 * considered unsafe for inclusion in the final document. Use `toString()`
	 * instead.
	 * @type {string}
	 */
	original;
	/** @type {string} */
	tagName;
	/** @type {boolean} */
	isCloser;
	/**
	 * Map of attribute names to value strings.
	 *
	 * @type {object}
	 */
	attributes;

	/**
	 * @param {string} original
	 * @param {string} tagName
	 * @param {boolean} isCloser
	 * @param {object} attributes
	 */
	constructor(original, tagName, isCloser, attributes) {
		this.original = original;
		this.tagName = tagName;
		this.isCloser = isCloser;
		this.attributes = attributes;
	}

	toString() {
		if (this.isCloser) {
			return `</${this.tagName}>`;
		}
		var html = '<';
		html += this.tagName;
		for (const key in this.attributes) {
			const safeName = MDUtils.scrubAttributeName(key);
			const value = this.attributes[key];
			if (value === true) {
				html += ` ${safeName}`;
			} else {
				const escapedValue = MDUtils.escapeHTML(`${value}`);
				html += ` ${safeName}="${escapedValue}"`;
			}
		}
		html += '>';
		return html;
	}

	equals(other) {
		if (!(other instanceof MDHTMLTag)) return false;
		if (other.tagName != this.tagName) return false;
		if (other.isCloser != this.isCloser) return false;
		return MDUtils.equal(other.attributes, this.attributes);
	}

	static #htmlTagNameFirstRegex = /[a-z]/i;
	static #htmlTagNameMedialRegex = /[a-z0-9]/i;
	static #htmlAttributeNameFirstRegex = /[a-z]/i;
	static #htmlAttributeNameMedialRegex = /[a-z0-9-]/i;
	static #whitespaceCharRegex = /\s/;

	/**
	 * Checks the start of the given string for presence of an HTML tag.
	 *
	 * @param {string} line
	 * @returns {MDHTMLTag|null} HTML tag if found, `null` otherwise
	 */
	static fromLineStart(line) {
		let expectOpenBracket = 0;
		let expectCloserOrName = 1;
		let expectName = 2;
		let expectAttributeNameOrEnd = 3;
		let expectEqualsOrAttributeOrEnd = 4;
		let expectAttributeValue = 5;
		let expectCloseBracket = 6;

		var isCloser = false;
		var tagName = '';
		var attributeName = '';
		var attributeValue = '';
		var attributeQuote = null;
		var attributes = {};
		var fullTag = null;
		let endAttribute = function(unescape=false) {
			if (attributeName.length > 0) {
				if (attributeValue.length > 0 || attributeQuote) {
					attributes[attributeName] = unescape ? MDUtils.unescapeHTML(attributeValue) : attributeValue;
				} else {
					attributes[attributeName] = true;
				}
			}
			attributeName = '';
			attributeValue = '';
			attributeQuote = null;
		};

		var expect = expectOpenBracket;
		for (var p = 0; p < line.length && fullTag === null; p++) {
			let ch = line.substring(p, p + 1);
			let isWhitespace = this.#whitespaceCharRegex.exec(ch) !== null;
			switch (expect) {
				case expectOpenBracket:
					if (ch != '<') return null;
					expect = expectCloserOrName;
					break;
				case expectCloserOrName:
					if (ch == '/') {
						isCloser = true;
					} else {
						p--;
					}
					expect = expectName;
					break;
				case expectName:
					if (tagName.length == 0) {
						if (this.#htmlTagNameFirstRegex.exec(ch) === null) return null;
						tagName += ch;
					} else {
						if (this.#htmlTagNameMedialRegex.exec(ch)) {
							tagName += ch;
						} else {
							p--;
							expect = (isCloser) ? expectCloseBracket : expectAttributeNameOrEnd;
						}
					}
					break;
				case expectAttributeNameOrEnd:
					if (attributeName.length == 0) {
						if (isWhitespace) {
							// skip whitespace
						} else if (ch == '/') {
							expect = expectCloseBracket;
						} else if (ch == '>') {
							fullTag = line.substring(0, p + 1);
							break;
						} else if (this.#htmlAttributeNameFirstRegex.exec(ch)) {
							attributeName += ch;
						} else {
							return null;
						}
					} else if (isWhitespace) {
						expect = expectEqualsOrAttributeOrEnd;
					} else if (ch == '/') {
						endAttribute();
						expect = expectCloseBracket;
					} else if (ch == '>') {
						endAttribute();
						fullTag = line.substring(0, p + 1);
						break;
					} else if (ch == '=') {
						expect = expectAttributeValue;
					} else if (this.#htmlAttributeNameMedialRegex.exec(ch)) {
						attributeName += ch;
					} else {
						return null;
					}
					break;
				case expectEqualsOrAttributeOrEnd:
					if (ch == '=') {
						expect = expectAttributeValue;
					} else if (isWhitespace) {
						// skip whitespace
					} else if (ch == '/') {
						expect = expectCloseBracket;
					} else if (ch == '>') {
						fullTag = line.substring(0, p + 1);
						break;
					} else if (this.#htmlAttributeNameFirstRegex.exec(ch)) {
						endAttribute();
						expect = expectAttributeNameOrEnd;
						p--;
					}
					break;
				case expectAttributeValue:
					if (attributeValue.length == 0) {
						if (attributeQuote === null) {
							if (isWhitespace) {
								// skip whitespace
							} else if (ch == '"' || ch == "'") {
								attributeQuote = ch;
							} else {
								attributeQuote = ''; // explicitly unquoted
								p--;
							}
						} else {
							if (ch === attributeQuote) {
								// Empty string
								endAttribute(attributeQuote != '');
								expect = expectAttributeNameOrEnd;
							} else if (attributeQuote === '' && (ch == '/' || ch == '>')) {
								return null;
							} else {
								attributeValue += ch;
							}
						}
					} else {
						if (ch === attributeQuote) {
							endAttribute(attributeQuote != '');
							expect = expectAttributeNameOrEnd;
						} else if (attributeQuote === '' && isWhitespace) {
							endAttribute();
							expect = expectAttributeNameOrEnd;
						} else {
							attributeValue += ch;
						}
					}
					break;
				case expectCloseBracket:
					if (isWhitespace) {
						// ignore whitespace
					} else if (ch == '>') {
						fullTag = line.substring(0, p + 1);
						break;
					}
					break;
			}
		}
		if (fullTag === null) return null;
		endAttribute();
		return new MDHTMLTag(fullTag, tagName, isCloser, attributes);
	}
}

/**
 * Represents HTML modifications to a node, such as CSS classes to add or
 * additional attributes. See `MDHTMLFilter.scrubModifier()` to remove disallowed
 * values.
 */
class MDTagModifier {
	/**
	 * Verbatim markdown syntax. Unmodified by changes to other properties.
	 * @type {string}
	 */
	original;
	/** @type {string[]} */
	cssClasses = [];
	/** @type {string|null} */
	cssId = null;
	/** @type {object} */
	cssStyles = {};
	/** @type {object} */
	attributes = {};

	static #baseClassRegex = /\.([a-z_\-][a-z0-9_\-]*?)/i;
	static #baseIdRegex = /#([a-z_\-][a-z0-9_\-]*?)/i;
	static #baseAttributeRegex = /([a-z0-9]+?)=([^\s\}]+?)/i;
	static #baseRegex = /\{([^}]+?)}/i;
	static #leadingClassRegex = new RegExp('^' + this.#baseRegex.source, 'i');
	static #trailingClassRegex = new RegExp('^(.*?)\\s*' + this.#baseRegex.source + '\\s*$', 'i');
	static #classRegex = new RegExp('^' + this.#baseClassRegex.source + '$', 'i');  // 1=classname
	static #idRegex = new RegExp('^' + this.#baseIdRegex.source + '$', 'i');  // 1=id
	static #attributeRegex = new RegExp('^' + this.#baseAttributeRegex.source + '$', 'i');  // 1=attribute name, 2=attribute value

	/**
	 * @param {MDNode} node
	 */
	applyTo(node) {
		if (node instanceof MDNode) {
			for (const cssClass of this.cssClasses) {
				node.addClass(cssClass);
			}
			if (this.cssId) node.cssId = this.cssId;
			for (const name in this.attributes) {
				node.attributes[name] = this.attributes[name];
			}
			for (const name in this.cssStyles) {
				node.cssStyles[name] = this.cssStyles[name];
			}
		}
	}

	/**
	 * Adds a CSS class. If already present it will not be duplicated.
	 *
	 * @param {string} cssClass
	 * @returns {boolean} whether the class was added
	 */
	addClass(cssClass) {
		if (this.cssClasses.indexOf(cssClass) >= 0) return false;
		this.cssClasses.push(cssClass);
		return true;
	}

	/**
	 * Removes a CSS class.
	 *
	 * @param {string} cssClass
	 * @returns {boolean} whether the class was present and removed
	 */
	removeClass(cssClass) {
		const beforeLength = this.cssClasses.length;
		this.cssClasses = this.cssClasses.filter((val) => val !== cssClass);
		return this.cssClasses.length != beforeLength;
	}

	equals(other) {
		if (!(other instanceof MDTagModifier)) return false;
		if (!MDUtils.equal(other.cssClasses, this.cssClasses)) return false;
		if (other.cssId !== this.cssId) return false;
		if (!MDUtils.equal(other.attributes, this.attributes)) return false;
		return true;
	}

	toString() {
		return this.original;
	}

	static #styleToObject(styleValue) {
		const pairs = styleValue.split(';');
		var styles = {};
		for (const pair of pairs) {
			const keyAndValue = pair.split(':');
			if (keyAndValue.length != 2) continue;
			styles[keyAndValue[0]] = keyAndValue[1];
		}
		return styles;
	}

	static #fromContents(contents) {
		let modifierTokens = contents.split(/\s+/);
		let mod = new MDTagModifier();
		mod.original = `{${contents}}`;
		var groups;
		for (const token of modifierTokens) {
			if (token.trim() == '') continue;
			if (groups = this.#classRegex.exec(token)) {
				mod.addClass(groups[1]);
			} else if (groups = this.#idRegex.exec(token)) {
				mod.cssId = groups[1];
			} else if (groups = this.#attributeRegex.exec(token)) {
				if (groups[1] == 'style') {
					mod.cssStyles = this.#styleToObject(groups[2]);
				} else {
					mod.attributes[groups[1]] = groups[2];
				}
			} else {
				return null;
			}
		}
		return mod;
	}

	/**
	 * Extracts block modifier from end of a line. Always returns a 2-element
	 * tuple array:
	 * - `0`: the line without the modifier
	 * - `1`: an `MDTagModifier` if found or `null` if not
	 *
	 * @param {string} line
	 * @param {MDState} state
	 * @returns {Array} tuple with remaining line and `MDTagModifier` or `null`
	 */
	static fromLine(line, state) {
		if (state) {
			var found = false;
			for (const reader of state.root.readersByBlockPriority) {
				if (reader instanceof MDModifierReader) {
					found = true;
					break;
				}
			}
			if (!found) return [ line, null ];
		}
		let groups = this.#trailingClassRegex.exec(line);
		if (groups === null) return [ line, null ];
		let bareLine = groups[1];
		let mod = this.#fromContents(groups[2]);
		return [ bareLine, mod ];
	}

	/**
	 * Attempts to extract modifier from head of string.
	 *
	 * @param {string} line
	 * @returns {MDTagModifier|null}
	 */
	static fromStart(line) {
		let groups = this.#leadingClassRegex.exec(line);
		if (groups === null) return null;
		return this.#fromContents(groups[1]);
	}

	/**
	 * Discards any modifiers from a line and returns what remains.
	 *
	 * @param {string} line
	 * @returns {string}
	 */
	static strip(line) {
		let groups = this.#trailingClassRegex.exec(line);
		if (groups === null) return line;
		return groups[1];
	}
}


// -- Readers ---------------------------------------------------------------


/**
 * Base class for readers of various markdown syntax. A `Markdown` instance can
 * be created with any combination of subclasses of these to customize the
 * flavor of markdown parsed.
 * 
 * @see {@link custom.md} for details on subclassing
 */
class MDReader {
	/**
	 * Called before processing begins. `state.lines` is populated and the
	 * line pointer `state.p` will be at `0`.
	 *
	 * Default implementation does nothing.
	 *
	 * @param {MDState} state
	 */
	preProcess(state) {}

	/**
	 * Attempts to read an `MDBlockNode` subclass at the current line pointer
	 * `state.p`. Only matches if the block pattern starts at the line pointer,
	 * not elsewhere in the `state.lines` array. If a block is found, `state.p`
	 * should be incremented to the next line _after_ the block structure and
	 * a `MDBlockNode` subclass instance is returned. If no block is found,
	 * returns `null`.
	 *
	 * Default implementation always returns `null`.
	 *
	 * @param {MDState} state
	 * @returns {MDBlockNode|null} found block, or `null` if not found
	 */
	readBlock(state) { return null; }

	/**
	 * Attempts to read an inline token from the beginning of `line`. Only the
	 * start of the given `line` is considered. If a matching token is found, an
	 * `MDToken` is returned. Otherwise `null` is returned.
	 *
	 * Default implementation always returns `null`.
	 *
	 * @param {MDState} state
	 * @param {string} line - string to check for a leading token
	 * @returns {MDToken|null} found token, or `null` if not found
	 */
	readToken(state, line) { return null; }

	/**
	 * Attempts to find a pattern anywhere in `tokens` and perform a _single_
	 * in-place substitution with one or more `MDNode` subclass instances.
	 * If a substitution is performed, must return `true`, otherwise `false`.
	 *
	 * Default implementation always returns `false`.
	 *
	 * @param {MDState} state
	 * @param {number} pass - what substitution pass this is, starting with 1
	 * @param {Array} tokens - mixed array of `MDToken` and `MDInlineNode` elements
	 * @returns {boolean} `true` if a substitution was performed, `false` if not
	 */
	substituteTokens(state, pass, tokens) { return false; }

	/**
	 * Called after all parsing has completed. An array `blocks` is passed of
	 * all the top-level `MDBlockNode` elements in the document which this
	 * method can traverse or alter in-place via `.splice` operations if
	 * necessary.
	 *
	 * `MDNode.visitChildren` is useful for recursively looking for certain
	 * `MDNode` instances. `MDNode.replaceNodes` is useful for swapping in
	 * replacements.
	 *
	 * Default implementation does nothing.
	 *
	 * @param {MDState} state
	 * @param {MDBlockNode[]} blocks
	 */
	postProcess(state, blocks) {}

	/**
	 * Can be overridden to influence ordering of this reader with respect to
	 * another during the block parsing phase. Return `-1` to be ordered before
	 * the given reader, `1` to be ordered after it, or `0` for no preference.
	 * Only return non-`0` values to resolve specific conflicts.
	 *
	 * Default implementation always returns `0` (no preference).
	 *
	 * @param {MDReader} other
	 * @returns {number} a negative, positive, or 0 value to be ordered before,
	 *   after, or anwhere relative to `other`, respectively
	 */
	compareBlockOrdering(other) {
		return 0;
	}

	/**
	 * Can be overridden to influence ordering of this reader with respect to
	 * another during the tokenizing phase. Return `-1` to be ordered before
	 * the given reader, `1` to be ordered after it, or `0` for no preference.
	 * Only return non-`0` values to resolve specific conflicts.
	 *
	 * Default implementation always returns `0` (no preference).
	 *
	 * @param {MDReader} other
	 * @returns {number} a negative, positive, or 0 value to be ordered before,
	 *   after, or anwhere relative to `other`, respectively
	 */
	compareTokenizeOrdering(other) {
		return 0;
	}

	/**
	 * Can be overridden to influence ordering of this reader with respect to
	 * another during the substitution phase. Return `-1` to be ordered before
	 * the given reader, `1` to be ordered after it, or `0` for no preference.
	 * Only return non-`0` values to resolve specific conflicts.
	 *
	 * Readers are sorted within each substitution pass. All pass 1 readers are
	 * processed first, then all pass 2 readers, etc. The number of passes this
	 * reader participates in is dictated by `substitionPassCount`.
	 *
	 * Default implementation always returns `0` (no preference).
	 *
	 * @param {MDReader} other
	 * @param {number} pass - substitution pass, with numbering starting at `1`
	 * @returns {number} a negative, positive, or 0 value to be ordered before,
	 *   after, or anwhere relative to `other`, respectively
	 */
	compareSubstituteOrdering(other, pass) {
		return 0;
	}

	/**
	 * How many substitution passes this reader requires. Substitution allows
	 * all pass 1 readers to process first, then all pass 2 readers, etc.
	 */
	get substitutionPassCount() { return 1; }

	/**
	 * For sorting readers with ordering preferences. The `compare` methods
	 * don't have the properties of normal sorting compares so need to sort
	 * differently.
	 *
	 * @param {MDReader[]} arr - array to sort
	 * @param {function} compareFn - comparison function, taking two array element
	 *   arguments and returning -1, 0, or 1 for a < b, a == b, and a > b,
	 *   respectively
	 * @param {function} idFn - function for returning a unique hashable id for
	 *   the array element
	 * @returns {MDReader[]} sorted array
	 */
	static #kahnTopologicalSort(arr, compareFn, idFn) {
		const graph = {};
		const inDegrees = {};
		const valuesById = {};

		// Build the graph and compute in-degrees
		for (const elem of arr) {
			const id = idFn(elem);
			graph[id] = [];
			inDegrees[id] = 0;
			valuesById[id] = elem;
		}

		for (let i = 0; i < arr.length; i++) {
			const elemA = arr[i];
			const idA = idFn(elemA);
			for (let j = 0; j < arr.length; j++) {
				if (i === j) continue;
				const elemB = arr[j];
				const idB = idFn(elemB);
				const comparisonResult = compareFn(elemA, elemB);
				if (comparisonResult < 0) {
					graph[idA].push(idB);
					inDegrees[idB]++;
				} else if (comparisonResult > 0) {
					graph[idB].push(idA);
					inDegrees[idA]++;
				}
			}
		}

		// Initialize the queue with zero-inDegree nodes
		const queue = [];
		for (const elemId in inDegrees) {
			if (inDegrees[elemId] === 0) {
				queue.push(elemId);
			}
		}

		// Process the queue and build the topological order list
		const sorted = [];
		while (queue.length > 0) {
			const elemId = queue.shift();
			sorted.push(valuesById[elemId]);
			delete valuesById[elemId];

			for (const neighbor of graph[elemId]) {
				inDegrees[neighbor]--;
				if (inDegrees[neighbor] === 0) {
					queue.push(neighbor);
				}
			}
		}
		// Anything left over can go at the end. No ordering dependencies.
		for (const elemId in valuesById) {
			sorted.push(valuesById[elemId]);
		}

		return sorted;
	}

	/**
	 * Returns a sorted array of readers by their block priority preferences.
	 *
	 * @param {MDReader[]} readers
	 * @returns {MDReader[]} sorted readers
	 */
	static sortReaderForBlocks(readers) {
		const sorted = readers.slice();
		return MDReader.#kahnTopologicalSort(sorted, (a, b) => {
			return a.compareBlockOrdering(b);
		}, (elem) => elem.constructor.name);
	}

	/**
	 * Returns a sorted array of readers by their tokenization priority preferences.
	 *
	 * @param {MDReader[]} readers
	 * @returns {MDReader[]} sorted readers
	 */
	static sortReadersForTokenizing(readers) {
		const sorted = readers.slice();
		return MDReader.#kahnTopologicalSort(sorted, (a, b) => {
			return a.compareTokenizeOrdering(b);
		}, (elem) => elem.constructor.name);
	}

	/**
	 * Returns a sorted array of tuples (arrays) containing the substitution
	 * pass number and reader instance, sorted by their substitution priority
	 * preferences.
	 *
	 * For readers with `substitutionPassCount` > `1`, the same reader will
	 * appear multiple times in the resulting array, one per pass.
	 *
	 * @param {MDReader[]} readers
	 * @returns {MDReader[]} sorted array of tuples with the pass number and
	 *   reader instance in each
	 */
	static sortReadersForSubstitution(readers) {
		var tuples = [];
		var maxPass = 1;
		for (const reader of readers) {
			const passCount = reader.substitutionPassCount;
			for (var pass = 1; pass <= passCount; pass++) {
				tuples.push([ pass, reader ]);
			}
			maxPass = Math.max(maxPass, pass);
		}
		var result = [];
		for (var pass = 1; pass <= maxPass; pass++) {
			var readersThisPass = tuples.filter((tup) => tup[0] == pass);
			const passResult = MDReader.#kahnTopologicalSort(readersThisPass, (a, b) => {
				const aReader = a[1];
				const bReader = b[1];
				return aReader.compareSubstituteOrdering(bReader, pass);
			}, (elem) => `${elem[1].constructor.name}:${elem[0]}`);
			result = result.concat(passResult);
		}
		return result;
	}
}

/**
 * Reads markdown blocks for headings denoted with the underline syntax.
 *
 * Supports `MDTagModifier` suffixes.
 */
class MDUnderlinedHeadingReader extends MDReader {
	readBlock(state) {
		var p = state.p;
		if (!state.hasLines(2)) return null;
		var modifier;
		let contentLine = state.lines[p++].trim();
		[contentLine, modifier] = MDTagModifier.fromLine(contentLine, state);
		let underLine = state.lines[p++].trim();
		if (contentLine == '') return null;
		if (/^=+$/.exec(underLine)) {
			state.p = p;
			let block = new MDHeadingNode(1, state.inlineMarkdownToNodes(contentLine));
			if (modifier) modifier.applyTo(block);
			return block;
		}
		if (/^\-+$/.exec(underLine)) {
			state.p = p;
			let block = new MDHeadingNode(2, state.inlineMarkdownToNodes(contentLine));
			if (modifier) modifier.applyTo(block);
			return block;
		}
		return null;
	}
}

/**
 * Reads markdown blocks for headings denoted with hash marks. Heading levels 1
 * to 6 are supported.
 *
 * Supports `MDTagModifier` suffixes.
 */
class MDHashHeadingReader extends MDReader {
	static #hashHeadingRegex = /^(#{1,6})\s*([^#].*?)\s*\#*\s*$/;  // 1=hashes, 2=content

	readBlock(state) {
		var p = state.p;
		let line = state.lines[p++];
		var modifier;
		[line, modifier] = MDTagModifier.fromLine(line, state);
		var groups = MDHashHeadingReader.#hashHeadingRegex.exec(line);
		if (groups === null) return null;
		state.p = p;
		const level = groups[1].length;
		const content = groups[2];
		let block = new MDHeadingNode(level, state.inlineMarkdownToNodes(content));
		if (modifier) modifier.applyTo(block);
		return block;
	}
}

/**
 * Reads subtext blocks. Subtext is smaller, fainter text for things like
 * disclaimers or sources.
 *
 * Supports `MDTagModifier` suffixes.
 */
class MDSubtextReader extends MDReader {
	static #subtextRegex = /^\-#\s*(.*?)\s*$/;  // 1=content

	readBlock(state) {
		var p = state.p;
		let line = state.lines[p++];
		var modifier;
		[line, modifier] = MDTagModifier.fromLine(line, state);
		var groups = MDSubtextReader.#subtextRegex.exec(line);
		if (groups === null) return null;
		state.p = p;
		const content = groups[1];
		let block = new MDSubtextNode(state.inlineMarkdownToNodes(content));
		if (modifier) modifier.applyTo(block);
		return block;
	}

	compareBlockOrdering(other) {
		if (other instanceof MDUnorderedListReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Reads markdown blocks for blockquoted text.
 */
class MDBlockQuoteReader extends MDReader {
	readBlock(state) {
		var blockquoteLines = [];
		var p = state.p;
		while (p < state.lines.length) {
			let line = state.lines[p++];
			if (line.startsWith(">")) {
				blockquoteLines.push(line);
			} else {
				break;
			}
		}
		if (blockquoteLines.length == 0) return null;
		let contentLines = blockquoteLines.map(function(line) {
			return line.substring(1).replace(/^ {0,3}\t?/, '');
		});
		let substate = state.copy(contentLines);
		let quotedBlocks = substate.readBlocks();
		state.p = p;
		return new MDBlockquoteNode(quotedBlocks);
	}
}

/**
 * Internal abstract base class for ordered and unordered lists.
 */
class _MDListReader extends MDReader {
	#readItemLines(state, firstLineStartPos) {
		var p = state.p;
		var lines = [];
		var seenBlankLine = false;
		var stripTrailingBlankLines = true;
		while (state.hasLines(1, p)) {
			const isFirstLine = p == state.p;
			var line = state.lines[p++];
			if (isFirstLine) {
				line = line.substring(firstLineStartPos);
			}
			if (/^(?:\*|\+|\-|\d+\.)\s+/.exec(line)) {
				// Found next list item
				stripTrailingBlankLines = false; // because this signals extra spacing intended
				break;
			}
			const isBlankLine = line.trim().length == 0;
			const isIndented = /^\s+\S/.exec(line) !== null;
			if (isBlankLine) {
				seenBlankLine = true;
			} else if (!isIndented && seenBlankLine) {
				// Post-list content
				break;
			}
			lines.push(line);
		}
		lines = MDUtils.withoutTrailingBlankLines(lines);
		return MDUtils.stripIndent(lines);
	}

	/**
	 * @param {MDState} state
	 * @param {number} firstLineStart
	 * @return {MDBlockNode}
	 */
	_readListItemContent(state, firstLineStartPos) {
		const itemLines = this.#readItemLines(state, firstLineStartPos);
		state.p += Math.max(itemLines.length, 1);

		if (itemLines.length == 1) {
			return state.inlineMarkdownToNode(itemLines[0]);
		}

		const hasBlankLines = itemLines.filter((line) => line.trim().length == 0).length > 0;
		if (hasBlankLines) {
			const substate = state.copy(itemLines);
			const blocks = substate.readBlocks();
			return (blocks.length == 1) ? blocks[0] : new MDNode(blocks);
		}

		// Multiline content with no blank lines. Search for new block
		// boundaries without the benefit of a blank line to demarcate it.
		for (var p = 1; p < itemLines.length; p++) {
			const line = itemLines[p];
			if (/^(?:\*|\-|\+|\d+\.)\s+/.exec(line)) {
				// Nested list found
				const firstBlock = state.inlineMarkdownToNode(itemLines.slice(0, p).join("\n"));
				const substate = state.copy(itemLines.slice(p));
				const blocks = substate.readBlocks();
				return new MDNode([ firstBlock, ...blocks ]);
			}
		}

		// Ok, give up and just do a standard block read
		{
			const substate = state.copy(itemLines);
			const blocks = substate.readBlocks();
			return (blocks.length == 1) ? blocks[0] : new MDNode(blocks);
		}
	}

	readBlock(state) {
		throw new Error(`Abstract readBlock must be overridden in ${this.constructor.name}`);
	}
}

/**
 * Block reader for unordered (bulleted) lists.
 */
class MDUnorderedListReader extends _MDListReader {
	static #unorderedListRegex = /^([\*\+\-]\s+)(.*)$/;  // 1=bullet, 2=content

	/**
	 * @param {MDState} state
	 * @returns {MDListItemNode|null}
	 */
	#readUnorderedListItem(state) {
		var p = state.p;
		let line = state.lines[p];
		let groups = MDUnorderedListReader.#unorderedListRegex.exec(line);
		if (groups === null) return null;
		const firstLineOffset = groups[1].length;
		return new MDListItemNode(this._readListItemContent(state, firstLineOffset));
	}

	readBlock(state) {
		var items = [];
		var item = null;
		do {
			item = this.#readUnorderedListItem(state);
			if (item) items.push(item);
		} while (item);
		if (items.length == 0) return null;
		return new MDUnorderedListNode(items);
	}
}

/**
 * Block reader for ordered (numbered) lists. The number of the first item is
 * used to begin counting. The subsequent items increase by 1, regardless of
 * their value.
 */
class MDOrderedListReader extends _MDListReader {
	static #orderedListRegex = /^(\d+)(\.\s+)(.*)$/;  // 1=number, 2=dot, 3=content

	/**
	 * @param {MDState} state
	 * @returns {MDListItemNode|null}
	 */
	#readOrderedListItem(state) {
		var p = state.p;
		let line = state.lines[p];
		let groups = MDOrderedListReader.#orderedListRegex.exec(line);
		if (groups === null) return null;
		const ordinal = parseInt(groups[1]);
		const firstLineOffset = groups[1].length + groups[2].length;
		return new MDListItemNode(this._readListItemContent(state, firstLineOffset), ordinal);
	}

	readBlock(state) {
		var items = [];
		var item = null;
		do {
			item = this.#readOrderedListItem(state);
			if (item) items.push(item);
		} while (item);
		if (items.length == 0) return null;
		return new MDOrderedListNode(items, items[0].ordinal);
	}
}

/**
 * Block reader for code blocks denoted by pairs of triple tickmarks. If
 * a programming language name, _xyz_, immediately follows the backticks, a
 * `language-xyz` CSS class will be added to the resulting `<code>`
 * element.
 *
 * Supports `MDTagModifier` suffix.
 */
class MDFencedCodeBlockReader extends MDReader {
	readBlock(state) {
		if (!state.hasLines(2)) return null;
		var p = state.p;
		let openFenceLine = state.lines[p++];
		var modifier;
		[openFenceLine, modifier] = MDTagModifier.fromLine(openFenceLine, state);
		const match = /^```\s*([a-z0-9]*)\s*$/.exec(openFenceLine);
		if (match === null) return null;
		const language = match[1].length > 0 ? match[1] : null;
		var codeLines = [];
		while (state.hasLines(1, p)) {
			let line = state.lines[p++];
			if (line.trim() == '```') {
				state.p = p;
				let block = new MDCodeBlockNode(codeLines.join("\n"), language);
				if (modifier) modifier.applyTo(block);
				return block;
			}
			codeLines.push(line);
		}
		return null;
	}
}

/**
 * Block reader for code blocks denoted by indenting text.
 */
class MDIndentedCodeBlockReader extends MDReader {
	readBlock(state) {
		var p = state.p;
		var codeLines = [];
		while (state.hasLines(1, p)) {
			let line = state.lines[p++];
			if (MDUtils.countIndents(line, true) < 1) {
				p--;
				break;
			}
			codeLines.push(MDUtils.stripIndent(line));
		}
		if (codeLines.length == 0) return null;
		state.p = p;
		return new MDCodeBlockNode(codeLines.join("\n"));
	}
}

/**
 * Block reader for horizontal rules. Composed of three or more hypens or
 * asterisks on a line by themselves, with or without intermediate whitespace.
 */
class MDHorizontalRuleReader extends MDReader {
	static #horizontalRuleRegex = /^\s*(?:\-(?:\s*\-){2,}|\*(?:\s*\*){2,})\s*$/;

	readBlock(state) {
		var p = state.p;
		let line = state.lines[p++];
		var modifier;
		[line, modifier] = MDTagModifier.fromLine(line, state);
		if (MDHorizontalRuleReader.#horizontalRuleRegex.exec(line)) {
			state.p = p;
			let block = new MDHorizontalRuleNode();
			if (modifier) modifier.applyTo(block);
			return block;
		}
		return null;
	}

	compareBlockOrdering(other) {
		if (other instanceof MDUnorderedListReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Block reader for tables.
 *
 * Supports `MDTagModifier` suffix.
 */
class MDTableReader extends MDReader {
	/**
	 * @param {MDState} state
	 * @param {boolean} isHeader
	 * @return {MDTableRowNode|null}
	 */
	#readTableRow(state, isHeader) {
		if (!state.hasLines(1)) return null;
		var p = state.p;
		let line = MDTagModifier.strip(state.lines[p++].trim());
		if (/.*\|.*/.exec(line) === null) return null;
		if (line.startsWith('|')) line = line.substring(1);
		if (line.endsWith('|')) line = line.substring(0, line.length - 1);
		let cellTokens = line.split('|');
		let cells = cellTokens.map(function(token) {
			let content = state.inlineMarkdownToNode(token.trim());
			return isHeader ? new MDTableHeaderCellNode(content) : new MDTableCellNode(content);
		});
		state.p = p;
		return new MDTableRowNode(cells);
	}

	/**
	 * @param {string} line
	 * @returns {string[]}
	 */
	#parseColumnAlignments(line) {
		line = line.trim();
		if (line.startsWith('|')) line = line.substring(1);
		if (line.endsWith('|')) line = line.substring(0, line.length - 1);
		return line.split(/\s*\|\s*/).map(function(token) {
			if (token.startsWith(':')) {
				if (token.endsWith(':')) {
					return 'center';
				}
				return 'left';
			} else if (token.endsWith(':')) {
				return 'right';
			}
			return null;
		});
	}

	static #tableDividerRegex = /^\s*[|]?\s*(?:[:]?-+[:]?)(?:\s*\|\s*[:]?-+[:]?)*\s*[|]?\s*$/;

	readBlock(state) {
		if (!state.hasLines(2)) return null;
		let startP = state.p;
		let firstLine = state.lines[startP];
		var modifier = MDTagModifier.fromLine(firstLine, state)[1];
		let headerRow = this.#readTableRow(state, true);
		if (headerRow === null) {
			state.p = startP;
			return null;
		}
		let dividerLine = state.lines[state.p++];
		let dividerGroups = MDTableReader.#tableDividerRegex.exec(dividerLine);
		if (dividerGroups === null) {
			state.p = startP;
			return null;
		}
		let columnAlignments = this.#parseColumnAlignments(dividerLine);
		var bodyRows = [];
		while (state.hasLines(1)) {
			let row = this.#readTableRow(state, false);
			if (row === null) break;
			bodyRows.push(row);
		}
		let table = new MDTableNode(headerRow, bodyRows);
		table.columnAlignments = columnAlignments;
		if (modifier) modifier.applyTo(table);
		return table;
	}
}

/**
 * Block reader for definition lists. Definitions go directly under terms starting
 * with a colon.
 */
class MDDefinitionListReader extends MDReader {
	readBlock(state) {
		var p = state.p;
		var groups;
		var termCount = 0;
		var definitionCount = 0;
		var defLines = [];
		while (state.hasLines(1, p)) {
			let line = state.lines[p++];
			if (line.trim().length == 0) {
				break;
			}
			if (/^\s+/.exec(line)) {
				if (defLines.length == 0) return null;
				defLines[defLines.length - 1] += "\n" + line;
			} else if (/^:\s+/.exec(line)) {
				defLines.push(line);
				definitionCount++;
			} else {
				defLines.push(line);
				termCount++;
			}
		}
		if (termCount == 0 || definitionCount == 0) return null;
		let blocks = defLines.map(function(line) {
			if (groups = /^:\s+(.*?)$/s.exec(line)) {
				return new MDDefinitionListDefinitionNode(state.inlineMarkdownToNodes(groups[1]));
			} else {
				return new MDDefinitionListTermNode(state.inlineMarkdownToNodes(line));
			}
		});
		state.p = p;
		return new MDDefinitionListNode(blocks);
	}
}

/**
 * Block reader for defining footnote contents. Footnotes can be defined anywhere
 * in the document but will always be rendered at the end of a page or end of
 * the document.
 */
class MDFootnoteReader extends MDReader {
	static #footnoteWithTitleRegex = /^\[\^([^\s\[\]]+?)\s+"(.*?)"\]/;  // 1=symbol, 2=title
	static #footnoteRegex = /^\[\^([^\s\[\]]+?)\]/;  // 1=symbol

	/**
	 * @param {MDState} state
	 * @param {string} symbol
	 * @param {MDNode[]} content
	 */
	#defineFootnote(state, symbol, footnote) {
		var footnotes = state.root['footnotes'] ?? {};
		footnotes[symbol] = footnote;
		state.root['footnotes'] = footnotes;
	}

	/**
	 * @param {MDState} state
	 * @param {string} symbol
	 * @param {number} unique
	 */
	#registerUniqueInstance(state, symbol, unique) {
		var footnoteInstances = state.root['footnoteInstances'];
		var instances = footnoteInstances[symbol] ?? [];
		instances.push(unique);
		footnoteInstances[symbol] = instances;
	}

	#idForFootnoteSymbol(state, symbol) {
		var footnoteIds = state.root['footnoteIds'];
		const existing = footnoteIds[symbol];
		if (existing) return existing;
		var nextFootnoteId = state.root['nextFootnoteId'];
		const id = nextFootnoteId++;
		footnoteIds[symbol] = id;
		state.root['nextFootnoteId'] = nextFootnoteId;
		return id;
	}

	preProcess(state) {
		state.root['footnoteInstances'] = {};
		state.root['footnotes'] = {};
		state.root['footnoteIds'] = {};
		state.root['nextFootnoteId'] = 1;
	}

	/**
	 * @param {MDState} state
	 */
	readBlock(state) {
		var p = state.p;
		let groups = /^\s*\[\^\s*([^\]]+)\s*\]:\s+(.*)\s*$/.exec(state.lines[p++]);
		if (groups === null) return null;
		let symbol = groups[1];
		let def = groups[2];
		while (state.hasLines(1, p)) {
			let line = state.lines[p++];
			if (/^\s+/.exec(line)) {
				def += "\n" + line;
			} else {
				p--;
				break;
			}
		}
		let content = state.inlineMarkdownToNodes(def);
		this.#defineFootnote(state, symbol, content);
		state.p = p;
		return new MDNode(); // empty
	}

	readToken(state, line) {
		var groups;
		if (groups = MDFootnoteReader.#footnoteWithTitleRegex.exec(line)) {
			return new MDToken(groups[0], MDTokenType.Footnote, groups[1], groups[2]);
		}
		if (groups = MDFootnoteReader.#footnoteRegex.exec(line)) {
			return new MDToken(groups[0], MDTokenType.Footnote, groups[1]);
		}
		return null;
	}

	substituteTokens(state, pass, tokens) {
		var match;
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.Footnote ])) {
			let symbol = match.tokens[0].content;
			tokens.splice(match.index, 1, new MDFootnoteNode(symbol));
			return true;
		}
		return false;
	}

	/**
	 * @param {MDState} state
	 * @param {MDBlockNode[]} blocks
	 */
	postProcess(state, blocks) {
		var nextOccurrenceId = 1;
		for (const block of blocks) {
			const me = this;
			block.visitChildren((function(node) {
				if (!(node instanceof MDFootnoteNode)) return;
				node.footnoteId = me.#idForFootnoteSymbol(state, node.symbol);
				node.occurrenceId = nextOccurrenceId++;
				node.displaySymbol = `${node.footnoteId}`;
				me.#registerUniqueInstance(state, node.symbol, node.occurrenceId);
			}).bind(this));
		}
		if (Object.keys(state.footnotes).length == 0) return;
		blocks.push(new MDFootnoteListNode());
	}

	compareBlockOrdering(other) {
		if (other instanceof MDLinkReader || other instanceof MDImageReader) {
			return -1;
		}
		return 0;
	}

	compareTokenizeOrdering(other) {
		if (other instanceof MDLinkReader || other instanceof MDImageReader) {
			return -1;
		}
		return 0;
	}

	compareSubstituteOrdering(other, pass) {
		if (other instanceof MDLinkReader || other instanceof MDImageReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Block reader for abbreviation definitions. Anywhere the abbreviation appears
 * in plain text will have its definition available when hovering over it.
 * Definitions can appear anywhere in the document. Their content should only
 * contain simple text, not markdown.
 */
class MDAbbreviationReader extends MDReader {
	/**
	 * @param {MDState} state
	 * @param {string} abbreviation
	 * @param {string} definition
	 */
	#defineAbbreviation(state, abbreviation, definition) {
		state.abbreviations[abbreviation] = definition;
		const regex = new RegExp("\\b(" + MDUtils.escapeRegex(abbreviation) + ")\\b", "ig");
		state.abbreviationRegexes[abbreviation] = regex;
}

	preProcess(state) {
		state.root['abbreviations'] = {};
		state.root['abbreviationRegexes'] = {};
	}

	readBlock(state) {
		var p = state.p;
		let line = state.lines[p++];
		let groups = /^\s*\*\[([^\]]+?)\]:\s+(.*?)\s*$/.exec(line);
		if (groups === null) return null;
		let abbrev = groups[1];
		let def = groups[2];
		this.#defineAbbreviation(state, abbrev, def);
		state.p = p;
		return new MDNode(); // empty
	}

	/**
	 * @param {MDState} state
	 * @param {MDNode[]} blocks
	 */
	postProcess(state, blocks) {
		const abbreviations = state.root['abbreviations'];
		const regexes = state.root['abbreviationRegexes'];
		MDNode.replaceNodes(state, blocks, (original) => {
			if (!(original instanceof MDTextNode)) return null;
			var changed = false;
			var elems = [ original.text ]; // mix of strings and MDNodes
			for (var i = 0; i < elems.length; i++) {
				var text = elems[i];
				if (typeof text !== 'string') continue;
				for (const abbreviation in abbreviations) {
					const groups = regexes[abbreviation].exec(text);
					if (groups === null) continue;
					const definition = abbreviations[abbreviation];
					const prefix = text.substring(0, groups.index);
					const suffix = text.substring(groups.index + groups[0].length);
					elems.splice(i, 1, prefix, new MDAbbreviationNode(groups[0], definition), suffix);
					i = -1; // start over
					changed = true;
					break;
				}
			}
			if (!changed) return null;
			const nodes = elems.map((elem) => typeof elem === 'string' ? new MDTextNode(elem) : elem);
			return new MDNode(nodes);
		});
	}
}

/**
 * Block reader for simple paragraphs. Paragraphs are separated by a blank (or
 * whitespace-only) line. This reader is prioritized after every other reader
 * since there is no distinguishing syntax.
 */
class MDParagraphReader extends MDReader {
	readBlock(state) {
		var paragraphLines = [];
		var p = state.p;
		while (state.hasLines(1, $p)) {
			let line = state.lines[p++];
			if (line.trim().length == 0) {
				break;
			}
			paragraphLines.push(line);
		}
		if (state.p == 0 && p >= state.lines.length) {
			// If it's the entire document don't wrap it in a paragraph
			return null;
		}
		if (paragraphLines.length > 0) {
			state.p = p;
			let content = paragraphLines.join("\n");
			return new MDParagraphNode(state.inlineMarkdownToNodes(content));
		}
		return null;
	}

	compareBlockOrdering(other) {
		return 1; // always dead last
	}
}

/**
 * Abstract base class for readers that look for one or two delimiting tokens
 * on either side of some content. E.g. `**strong**`.
 */
class MDSimplePairInlineReader extends MDReader {
	// Passes:
	//   1. Syntaxes with two delimiting tokens, interior tokens of the same
	//      kind must be even in number
	//   2. Syntaxes with one delimiting token, interior tokens of the same
	//      kind must be even in number
	//   3. Syntaxes with two delimiting tokens, any tokens inside
	//   4. Syntaxes with one delimiting token, any tokens inside
	get substitutionPassCount() { return 4; }

	/**
	 * Attempts a substitution of a matched pair of delimiting token types.
	 * If successful, the substitution is performed on `tokens` and `true` is
	 * returned, otherwise `false` is returned and the array is untouched.
	 *
	 * If `this.substitutionPassCount` is greater than 1, the first pass
	 * will reject matches with the delimiting character inside the content
	 * tokens. If the reader uses a single pass or a subsequent pass is performed
	 * with multiple pass any contents will be accepted.
	 *
	 * @param {MDState} state
	 * @param {number} pass - pass number, starting with `1`
	 * @param {MDToken[]} tokens - tokens/nodes to perform substitution on
	 * @param {class} nodeClass - class of the node to return if matched
	 * @param {MDTokenType} delimiter - delimiting token
	 * @param {number} count - how many times the token is repeated to form the delimiter
	 * @param {boolean} plaintext - whether to invoke `nodeClass` with a verbatim
	 *   content string instead of parsed `MDNode`s
	 * @returns {boolean} `true` if substitution was performed, `false` if not
	 */
	attemptPair(state, pass, tokens, nodeClass, delimiter, count=1, plaintext=false) {
		// We do four passes. #1: doubles without inner tokens, #2: singles
		// without inner tokens, #3: doubles with paired inner tokens,
		// #4: singles with paired inner tokens
		if (count == 1 && pass != 2 && pass != 4) return false;
		if (count > 1 && pass != 1 && pass != 3) return false;
		let delimiters = Array(count).fill(delimiter);
		const isFirstOfMultiplePasses = this.substitutionPassCount > 1 && pass == 1;
		let match = MDToken.findPairedTokens(tokens, delimiters, delimiters, function(content) {
			const firstType = content[0] instanceof MDToken ? content[0].type : null;
			const lastType = content[content.length - 1] instanceof MDToken ? content[content.length - 1].type : null;
			if (firstType == MDTokenType.Whitespace) return false;
			if (lastType == MDTokenType.Whitespace) return false;
			for (const token of content) {
				// Don't allow nesting
				if (token.constructor == nodeClass) return false;
			}
			if (isFirstOfMultiplePasses) {
				var innerCount = 0;
				for (let token of content) {
					if (token instanceof MDToken && token.type == delimiter) innerCount++;
				}
				if ((innerCount % 2) != 0) return false;
			}
			return true;
		});
		if (match === null) return false;
		let content = (plaintext)
			? match.contentTokens.map((token) => token.original).join('')
			: state.tokensToNodes(match.contentTokens);
		tokens.splice(match.startIndex, match.totalLength, new nodeClass(content));
		return true;
	}
}

/**
 * Reader for emphasis syntax. Denoted with a single underscore on either side of
 * some text (preferred) or a single asterisk on either side.
 */
class MDEmphasisReader extends MDSimplePairInlineReader {
	readToken(state, line) {
		if (line.startsWith('_')) return new MDToken('_', MDTokenType.Underscore);
		if (line.startsWith('*')) return new MDToken('*', MDTokenType.Asterisk);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		if (this.attemptPair(state, pass, tokens, MDEmphasisNode, MDTokenType.Underscore)) return true;
		if (this.attemptPair(state, pass, tokens, MDEmphasisNode, MDTokenType.Asterisk)) return true;
		return false;
	}

	compareSubstituteOrdering(other, pass) {
		if (other instanceof MDStrongReader) {
			return 1;
		}
		return 0;
	}
}

/**
 * Reader for strong syntax. Denoted with two asterisks on either side of some
 * text (preferred) or two underscores on either side. Note that if
 * `MDUnderlineReader` is in use, it will replace the double-underscore syntax.
 */
class MDStrongReader extends MDSimplePairInlineReader {
	readToken(state, line) {
		if (line.startsWith('*')) return new MDToken('*', MDTokenType.Asterisk);
		if (line.startsWith('_')) return new MDToken('_', MDTokenType.Underscore);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		if (this.attemptPair(state, pass, tokens, MDStrongNode, MDTokenType.Asterisk, 2)) return true;
		if (this.attemptPair(state, pass, tokens, MDStrongNode, MDTokenType.Underscore, 2)) return true;
		return false;
	}

	compareSubstituteOrdering(other, pass) {
		if (other instanceof MDEmphasisReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Reader for strikethrough syntax. Consists of two tildes on either side of
 * some text (preferred) or single tildes on either side. Note that if
 * `MDSubscriptReader` is in use, it will replace the single-tilde syntax.
 *
 * The number of recognized tildes can be configured.
 */
class MDStrikethroughReader extends MDSimplePairInlineReader {
	/** @type {boolean} */
	singleTildeEnabled = true;
	/** @type {boolean} */
	doubleTildeEnabled = true;

	readToken(state, line) {
		if (line.startsWith('~')) return new MDToken('~', MDTokenType.Tilde);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		if (this.singleTildeEnabled) {
			if (this.attemptPair(state, pass, tokens, MDStrikethroughNode, MDTokenType.Tilde, 2)) return true;
		}
		if (this.doubleTildeEnabled) {
			if (this.attemptPair(state, pass, tokens, MDStrikethroughNode, MDTokenType.Tilde)) return true;
		}
		return false;
	}
}

/**
 * Reader for underline syntax. Consists of two underscores on either side of
 * some text. If used with `MDStrongReader` which also looks for double
 * underscores, this reader will take priority.
 */
class MDUnderlineReader extends MDSimplePairInlineReader {
	readToken(state, line) {
		if (line.startsWith('_')) return new MDToken('_', MDTokenType.Underscore);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		return this.attemptPair(state, pass, tokens, MDUnderlineNode, MDTokenType.Underscore, 2);
	}

	compareSubstituteOrdering(other, pass) {
		if (other instanceof MDStrongReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Reader for highlight syntax. Consists of pairs of equal signs on either side
 * of some text.
 */
class MDHighlightReader extends MDSimplePairInlineReader {
	readToken(state, line) {
		if (line.startsWith('=')) return new MDToken('=', MDTokenType.Equal);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		return this.attemptPair(state, pass, tokens, MDHighlightNode, MDTokenType.Equal, 2);
	}
}

/**
 * Reader for inline code syntax. Consists of one or two delimiting backticks
 * around text. The contents between the backticks will be rendered verbatim,
 * ignoring any inner markdown syntax. To include a backtick inside, escape it
 * with a backslash.
 */
class MDCodeSpanReader extends MDSimplePairInlineReader {
	readToken(state, line) {
		if (line.startsWith('`')) return new MDToken('`', MDTokenType.Backtick);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		if (this.attemptPair(state, pass, tokens, MDCodeNode, MDTokenType.Backtick, 2, true)) return true;
		if (this.attemptPair(state, pass, tokens, MDCodeNode, MDTokenType.Backtick, 1, true)) return true;
	}
}

/**
 * Reader for subscript syntax. Consists of single tildes on either side of
 * some text. If used with `MDStrikethroughReader`, this reader will take
 * precedence, and strikethrough can only be done with double tildes.
 */
class MDSubscriptReader extends MDSimplePairInlineReader {
	readToken(state, line) {
		if (line.startsWith('~')) return new MDToken('~', MDTokenType.Tilde);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		return this.attemptPair(state, pass, tokens, MDSubscriptNode, MDTokenType.Tilde);
	}

	compareSubstituteOrdering(other, pass) {
		if (other instanceof MDStrikethroughReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Reader for superscript syntax. Consists of single caret characters on either
 * side of some text.
 */
class MDSuperscriptReader extends MDSimplePairInlineReader {
	readToken(state, line) {
		if (line.startsWith('^')) return new MDToken('^', MDTokenType.Caret);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		return this.attemptPair(state, pass, tokens, MDSuperscriptNode, MDTokenType.Caret);
	}
}

/**
 * Reads a hypertext link. Consists of link text between square brackets
 * followed immediately by a URL in parentheses.
 */
class MDLinkReader extends MDReader {
	static #simpleEmailRegex = new RegExp("^<(" + MDUtils.baseEmailRegex.source + ")>", "i");  // 1=email
	static #simpleURLRegex = new RegExp("^<(" + MDUtils.baseURLRegex.source + ")>", "i");  // 1=URL

	readToken(state, line) {
		var groups;
		if (groups = MDToken.tokenizeLabel(line)) {
			return new MDToken(groups[0], MDTokenType.Label, groups[1]);
		}
		if (groups = MDToken.tokenizeEmail(line)) {
			return new MDToken(groups[0], MDTokenType.Email, groups[1], groups[2]);
		}
		if (groups = MDToken.tokenizeURL(line)) {
			return new MDToken(groups[0], MDTokenType.URL, groups[1], groups[2]);
		}
		if (groups = MDLinkReader.#simpleEmailRegex.exec(line)) {
			return new MDToken(groups[0], MDTokenType.SimpleEmail, groups[1]);
		}
		if (groups = MDLinkReader.#simpleURLRegex.exec(line)) {
			return new MDToken(groups[0], MDTokenType.SimpleLink, groups[1]);
		}
		return null;
	}

	substituteTokens(state, pass, tokens) {
		var match;
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.Label, MDTokenType.META_OptionalWhitespace, MDTokenType.URL ])) {
			let text = match.tokens[0].content;
			let url = match.tokens[match.tokens.length - 1].content;
			let title = match.tokens[match.tokens.length - 1].extra;
			tokens.splice(match.index, match.tokens.length, new MDLinkNode(url, state.inlineMarkdownToNode(text), title));
			return true;
		}
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.Label, MDTokenType.META_OptionalWhitespace, MDTokenType.Email ])) {
			let text = match.tokens[0].content;
			let email = match.tokens[match.tokens.length - 1].content;
			let url = `mailto:${email}`;
			let title = match.tokens[match.tokens.length - 1].extra;
			tokens.splice(match.index, match.tokens.length, new MDLinkNode(url, state.inlineMarkdownToNodes(text), title));
			return true;
		}
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.SimpleEmail ])) {
			const token = match.tokens[0];
			const link = `mailto:${token.content}`;
			const node = new MDLinkNode(link, new MDObfuscatedTextNode(token.content));
			tokens.splice(match.index, 1, node);
			return true;
		}
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.SimpleLink ])) {
			const token = match.tokens[0];
			const link = token.content;
			const node = new MDLinkNode(link, new MDTextNode(link));
			tokens.splice(match.index, 1, node);
			return true;
		}
		return false;
	}
}

/**
 * Reader for referential URL definitions. Consists of link text between square
 * brackets followed immediately by a reference symbol also in square brackets.
 * The URL can be defined elsewhere on a line by itself with the symbol in square
 * brackets, colon, and the URL (and optional title in quotes).
 */
class MDReferencedLinkReader extends MDLinkReader {
	/**
	 * @param {MDState} state
	 */
	readBlock(state) {
		var p = state.p;
		let line = state.lines[p++];
		var symbol;
		var url;
		var title = null;
		let groups = /^\s*\[(.+?)]:\s*(\S+)\s+"(.*?)"\s*$/.exec(line);
		if (groups) {
			symbol = groups[1];
			url = groups[2];
			title = groups[3];
		} else {
			groups = /^\s*\[(.+?)]:\s*(\S+)\s*$/.exec(line);
			if (groups) {
				symbol = groups[1];
				url = groups[2];
			} else {
				return null;
			}
		}
		state.defineURL(symbol, url, title);
		state.p = p;
		return new MDNode([]); // empty
	}

	substituteTokens(state, pass, tokens) {
		var match;
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.Label, MDTokenType.META_OptionalWhitespace, MDTokenType.Label ])) {
			let text = match.tokens[0].content;
			let ref = match.tokens[match.tokens.length - 1].content;
			tokens.splice(match.index, match.tokens.length, new MDReferencedLinkNode(ref, state.inlineMarkdownToNodes(text)));
			return true;
		}
		return false;
	}
}

/**
 * Reader for images. Consists of an exclamation, alt text in square brackets,
 * and image URL in parentheses.
 */
class MDImageReader extends MDLinkReader {
	readToken(state, line) {
		const s = super.readToken(state, line);
		if (s) return s;
		if (line.startsWith('!')) return new MDToken('!', MDTokenType.Bang);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		var match;
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.Bang, MDTokenType.Label, MDTokenType.META_OptionalWhitespace, MDTokenType.URL ])) {
			let alt = match.tokens[1].content;
			let url = match.tokens[match.tokens.length - 1].content;
			let title = match.tokens[match.tokens.length - 1].extra;
			const node = new MDImageNode(url, alt);
			if (title !== null) {
				node.attributes['title'] = title;
			}
			tokens.splice(match.index, match.tokens.length, node);
			return true;
		}
		return false;
	}

	compareSubstituteOrdering(other, pass) {
		if (other.constructor === MDLinkReader || other.constructor === MDReferencedLinkReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Reader for images with referential URL definitions. Consists of an
 * exclamation, alt text in square brackets, and link symbol in square brackets.
 * URL is defined the same as for `MDReferencedLinkReader`.
 */
class MDReferencedImageReader extends MDReferencedLinkReader {
	readToken(state, line) {
		const s = super.readToken(state, line);
		if (s) return s;
		if (line.startsWith('!')) return new MDToken('!', MDTokenType.Bang);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		var match;
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.Bang, MDTokenType.Label, MDTokenType.META_OptionalWhitespace, MDTokenType.Label ])) {
			let alt = match.tokens[1].content;
			let ref = match.tokens[match.tokens.length - 1].content;
			tokens.splice(match.index, match.tokens.length, new MDReferencedImageNode(ref, alt));
			return true;
		}
		return false;
	}

	compareSubstituteOrdering(other, pass) {
		if (other.constructor === MDLinkReader || other.constructor === MDReferencedLinkReader) {
			return -1;
		}
		return 0;
	}
}

/**
 * Converts line breaks within blocks into line breaks in the HTML. Not
 * included in any of the default reader sets since most flavors ignore
 * line breaks within blocks.
 */
class MDLineBreakReader extends MDReader {
	postProcess(state, blocks) {
		MDNode.replaceNodes(state, blocks, (original) => {
			if (!(original instanceof MDTextNode)) return null;
			const lines = original.text.split("\n");
			if (lines.length == 1) return null;
			var nodes = [];
			for (const [i, line] of lines.entries()) {
				if (i > 0) {
					nodes.push(new MDLineBreakNode());
				}
				nodes.push(new MDTextNode(line));
			}
			return new MDNode(nodes);
		});
	}
}

/**
 * Reads a verbatim HTML tag, and if it passes validation by `MDState.tagFilter`,
 * will be rendered in the final HTML document. Disallowed tags will be rendered
 * as plain text in the resulting document.
 */
class MDHTMLTagReader extends MDReader {
	readToken(state, line) {
		const tag = MDHTMLTag.fromLineStart(line, state);
		if (tag === null) return null;
		if (!state.root.tagFilter.isValidTagName(tag.tagName)) return null;
		state.root.tagFilter.scrubTag(tag);
		return new MDToken(tag.original, MDTokenType.HTMLTag, tag);
	}

	substituteTokens(state, pass, tokens) {
		var match;
		if (match = MDToken.findFirstTokens(tokens, [ MDTokenType.HTMLTag ])) {
			const tag = match.tokens[0].tag;
			tokens.splice(match.index, match.tokens.length, new MDHTMLTagNode(tag));
			return true;
		}
		return false;
	}
}

/**
 * Reads tag modifiers. Consists of curly braces with one or more CSS classes,
 * IDs, or custom attributes separated by spaces to apply to the preceding
 * node. Validation is performed on modifiers and only acceptable values are
 * applied.
 */
class MDModifierReader extends MDReader {
	readToken(state, line) {
		var modifier = MDTagModifier.fromStart(line);
		if (modifier) return new MDToken(modifier.original, MDTokenType.Modifier, modifier);
		return null;
	}

	substituteTokens(state, pass, tokens) {
		// Modifiers are applied elsewhere, and if they're not it's fine if they're
		// rendered as the original syntax.
		return false;
	}
}


// -- Document nodes --------------------------------------------------------


/**
 * Base class for nodes in the assembled document tree.
 */
class MDNode {
	/**
	 * Array of CSS classes to add to the node when rendered as HTML.
	 * @type {string[]}
	 */
	cssClasses = [];

	/** @type {string|null} */
	cssId = null;

	/**
	 * Mapping of CSS attributes to values.
	 * @type {object}
	 */
	cssStyles = {};

	/**
	 * Mapping of arbitrary attributes and values to add to this node's top-level
	 * tag when rendered as HTML. For `class`, `id`, and `style` attributes, use
	 * `cssClasses`, `cssId`, and `cssStyles` instead.
	 * @type {object}
	 */
	attributes = {};

	/**
	 * All child nodes in this node.
	 * @type {MDNode[]}
	 */
	children;

	/**
	 * @param {MDNode[]} children
	 */
	constructor(children=[]) {
		if (children instanceof Array) {
			for (const elem of children) {
				if (!(elem instanceof MDNode)) {
					throw new Error(`${this.constructor.name} expects children of type MDNode[] or MDNode, got array with ${MDUtils.typename(elem)} element`);
				}
			}
			this.children = children;
		} else if (children instanceof MDNode) {
			this.children = [ children ];
		} else {
			throw new Error(`${this.constructor.name} expects children of type MDNode[] or MDNode, got ${MDUtils.typename(children)}`);
		}
	}

	/**
	 * Adds a CSS class. If already present it will not be duplicated.
	 *
	 * @param {string} cssClass
	 * @returns {boolean} whether the class was added
	 */
	addClass(cssClass) {
		if (this.cssClasses.indexOf(cssClass) >= 0) return false;
		this.cssClasses.push(cssClass);
		return true;
	}

	/**
	 * Removes a CSS class.
	 *
	 * @param {string} cssClass
	 * @returns {boolean} whether the class was present and removed
	 */
	removeClass(cssClass) {
		const beforeLength = this.cssClasses.length;
		this.cssClasses = this.cssClasses.filter((val) => val !== cssClass);
		return this.cssClasses.length != beforeLength;
	}

	/**
	 * Renders this node and any children as an HTML string. If the node has no
	 * content an empty string should be returned.
	 *
	 * @param {MDState} state
	 * @returns {string} HTML string
	 */
	toHTML(state) {
		return MDNode.toHTML(this.children, state);
	}

	/**
	 * Renders this node and any children as a plain text string. The conversion
	 * should only render ordinary text, not attempt markdown-like formatting
	 * (e.g. list items should not be prefixed with asterisks, only have their
	 * content text returned). If the node has no renderable content an empty
	 * string should be returned.
	 *
	 * @param {MDState} state
	 * @returns {string} plaintext string
	 */
	toPlaintext(state) {
		return MDNode.toPlaintext(this.children, state);
	}

	/**
	 * Protected helper method that renders an HTML fragment of the attributes
	 * to apply to the root HTML tag representation of this node.
	 *
	 * Example result with a couple `cssClasses`, a `cssId`, and a custom
	 * `attributes` key-value pair:
	 *
	 * ```
	 *  class="foo bar" id="baz" lang="en"
	 * ```
	 *
	 * The value includes a leading space if it's non-empty so that it can be
	 * concatenated directly after the tag name and before the closing `>`.
	 *
	 * @returns {string} HTML fragment
	 */
	_htmlAttributes() {
		var html = '';
		if (this.cssClasses.length > 0) {
			html += ` class="${this.cssClasses.join(' ')}"`;
		}
		if (this.cssId !== null && this.cssId.length > 0) {
			html += ` id="${this.cssId}"`;
		}
		var styles = [];
		for (const key in this.cssStyles) {
			styles.push(`${key}: ${this.cssStyles[key]};`)
		}
		if (styles.length > 0) {
			html += ` style="${MDUtils.escapeHTML(styles.join(' '))}"`;
		}
		for (const key in this.attributes) {
			if (key == 'class' || key == 'id' || key == 'style') continue;
			const value = `${this.attributes[key]}`;
			const cleanKey = MDUtils.scrubAttributeName(key);
			if (cleanKey.length == 0) continue;
			const cleanValue = MDUtils.escapeHTML(value);
			html += ` ${cleanKey}="${cleanValue}"`;
		}
		return html;
	}

	/**
	 * Protected helper that renders and concatenates the HTML of all children
	 * of this node. Mostly for use by subclasses in their `toHTML`
	 * implementations.
	 *
	 * @param {MDState} state
	 * @returns {string} concatenated HTML
	 */
	_childHTML(state) {
		return MDNode.toHTML(this.children, state);
	}

	/**
	 * Protected helper that renders and concatenates the plaintext of all
	 * children of this node.
	 *
	 * @param {MDState} state
	 * @returns {string} concatenated plaintext
	 */
	_childPlaintext(state) {
		return MDNode.toPlaintext(this.children, state);
	}

	/**
	 * Protected helper for rendering nodes represented by simple paired HTML
	 * tags. Custom CSS classes and attributes will be included in the result,
	 * and child content will be rendered between the tags.
	 *
	 * @param {MDState} state
	 * @param {string} tagName - HTML tag name, without angle braces
	 * @returns {string} HTML string
	 */
	_simplePairedTagHTML(state, tagName) {
		const openTagSuffix = this.children[0] instanceof MDBlockNode ? '\n' : ''
		const closeTagPrefix = this.children[this.children.length - 1] instanceof MDBlockNode ? '\n' : '';
		const closeTagSuffix = this instanceof MDBlockNode ? '\n' : '';
		return `<${tagName}${this._htmlAttributes()}>${openTagSuffix}${this._childHTML(state)}${closeTagPrefix}</${tagName}>${closeTagSuffix}`;
	}

	/**
	 * Calls the given callback function with every child node, recursively.
	 * Nodes are visited depth-first.
	 *
	 * @param {function} fn - callback that accepts one `MDNode` argument
	 */
	visitChildren(fn) {
		if (this.children === undefined || !Array.isArray(this.children)) {
			return;
		}
		for (const child of this.children) {
			fn(child);
			child.visitChildren(fn);
		}
	}

	/**
	 * Helper for rendering and concatenating HTML from an array of `MDNode`s.
	 *
	 * @param {MDNode[]} nodes
	 * @param {MDState} state
	 * @returns {string} HTML string
	 */
	static toHTML(nodes, state) {
		return nodes.map((node) => node.toHTML(state) + (node instanceof MDBlockNode ? '\n' : '')).join('');
	}

	/**
	 * Helper for rendering and concatenating plaintext from an array of `MDNode`s.
	 *
	 * @param {MDNode[]} nodes
	 * @param {MDState} state
	 * @returns {string} plaintext
	 */
	static toPlaintext(nodes, state) {
		return nodes.map((node) => node.toPlaintext(state)).join('');
	}

	/**
	 * Recursively searches and replaces nodes in a tree. The given `replacer`
	 * is passed every node in the tree. If `replacer` returns a new `MDNode`
	 * the original will be replaced with it. If the function returns `null` no
	 * change will be made to that node. Traversal is depth-first.
	 *
	 * @param {MDState} state
	 * @param {MDNode[]} nodes
	 * @param {function} replacer - takes a node as an argument, returns either
	 *   a new node or `null` to leave it unchanged
	 */
	static replaceNodes(state, nodes, replacer) {
		for (var i = 0; i < nodes.length; i++) {
			var originalNode = nodes[i];
			const replacement = replacer(originalNode);
			if (replacement instanceof MDNode) {
				nodes.splice(i, 1, replacement);
			} else {
				this.replaceNodes(state, originalNode.children, replacer);
			}
		}
	}
}

/**
 * Marker subclass that indicates a node represents block syntax.
 */
class MDBlockNode extends MDNode {}

/**
 * Paragraph block.
 */
class MDParagraphNode extends MDBlockNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'p');
	}
}

/**
 * A heading block with a level from 1 to 6.
 */
class MDHeadingNode extends MDBlockNode {
	/** @type {number} */
	level;

	constructor(level, children) {
		super(children);
		if (typeof level !== 'number' || (level < 1 || level > 6)) {
			throw new Error(`${this.constructor.name} requires heading level 1 to 6`);
		}
		this.level = level;
	}

	toHTML(state) {
		return this._simplePairedTagHTML(state, `h${this.level}`);
	}
}

/**
 * A sub-text block with smaller, less prominent text.
 */
class MDSubtextNode extends MDBlockNode {
	toHTML(state) {
		this.addClass('subtext');
		return this._simplePairedTagHTML(state, 'div');
	}
}

/**
 * Node for a horizontal dividing line.
 */
class MDHorizontalRuleNode extends MDBlockNode {
	toHTML(state) {
		return `<hr${this._htmlAttributes()}>`;
	}
}

/**
 * A block quote, usually rendered indented from other text.
 */
class MDBlockquoteNode extends MDBlockNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'blockquote');
	}
}

/**
 * A bulleted list. Contains `MDListItemNode` children.
 */
class MDUnorderedListNode extends MDBlockNode {
	/** @type {MDListItemNode[]} children */

	/**
	 * @param {MDListItemNode[]} children
	 */
	constructor(children) {
		super(children);
	}

	toHTML(state) {
		return this._simplePairedTagHTML(state, 'ul');
	}
}

/**
 * A numbered list. Contains `MDListItemNode` children.
 */
class MDOrderedListNode extends MDBlockNode {
	/** @type {MDListItemNode[]} children */

	/** @type {number|null} */
	startOrdinal;

	/**
	 * @param {MDListItemNode[]} children
	 * @param {number|null} startOrdinal
	 */
	constructor(children, startOrdinal=null) {
		super(children);
		this.startOrdinal = startOrdinal;
	}

	toHTML(state) {
		if (this.startOrdinal !== null && this.startOrdinal != 1) this.attributes['start'] = this.startOrdinal;
		return this._simplePairedTagHTML(state, 'ol');
	}
}

/**
 * An item in a bulleted or numbered list.
 */
class MDListItemNode extends MDBlockNode {
	/** @type {number|null} */
	ordinal;

	/**
	 * @param {MDNode|MDNode[]} children
	 * @param {number|null} ordinal
	 */
	constructor(children, ordinal=null) {
		super(children);
		this.ordinal = ordinal;
	}

	toHTML(state) {
		return this._simplePairedTagHTML(state, 'li');
	}
}

/**
 * A block of preformatted computer code. Inner markdown is ignored.
 */
class MDCodeBlockNode extends MDBlockNode {
	/** @type {string} */
	text;

	/**
	 * The programming language of the content.
	 * @type {string|null}
	 */
	language;

	/**
	 * @param {string} text
	 * @param {string|null} language
	 */
	constructor(text, language=null) {
		super([]);
		this.text = text;
		this.language = language;
	}

	toHTML(state) {
		const languageModifier = (this.language !== null) ? ` class="language-${this.language}"` : '';
		return `<pre${this._htmlAttributes()}><code${languageModifier}>` +
			`${MDUtils.escapeHTML(this.text)}</code></pre>\n`;
	}
}

/**
 * A table node with a single header row and any number of body rows.
 *
 * If modifying the rows, use the `headerRow` and `bodyRows` accessors,
 * otherwise `children` may get out of sync.
 */
class MDTableNode extends MDBlockNode {
	/** @param {MDTableRowNode[]} children */

	/** @type {MDTableRowNode} */
	get headerRow() { return this.#headerRow; }
	set headerRow(newValue) {
		this.#headerRow = newValue;
		this.#recalculateChildren();
	}
	#headerRow;

	/** @type {MDTableRowNode[]} */
	get bodyRows() { return this.#bodyRows; }
	set bodyRows(newValue) {
		this.#bodyRows = newValue;
		this.#recalculateChildren();
	}
	#bodyRows;

	/**
	 * How to align each column. Columns beyond the length of the array or with
	 * corresponding `null` elements will have no alignment set. Values should
	 * be valid CSS `text-align` values.
	 *
	 * @type {string[]}
	 */
	columnAlignments = [];

	/**
	 * @param {MDTableRowNode} headerRow
	 * @param {MDTableRowNode[]} bodyRows
	 */
	constructor(headerRow, bodyRows) {
		super([ headerRow, ...bodyRows ]);
		this.#headerRow = headerRow;
		this.#bodyRows = bodyRows;
	}

	#recalculateChildren() {
		this.children = [ this.#headerRow, ...this.#bodyRows ];
	}

	#applyAlignments() {
		this.children.forEach((child) => this.#applyAlignmentsToRow(child));
	}

	/**
	 * @param {MDTableRowNode} row
	 */
	#applyAlignmentsToRow(row) {
		for (const [columnIndex, cell] of row.children.entries()) {
			const alignment = columnIndex < this.columnAlignments.length ? this.columnAlignments[columnIndex] : null;
			this.#applyAlignmentToCell(cell, alignment);
		}
	}

	/**
	 * @param {MDTableCellNode} cell
	 * @param {string|null} alignment
	 */
	#applyAlignmentToCell(cell, alignment) {
		if (alignment) {
			cell.cssStyles['text-align'] = alignment;
		} else {
			delete cell.cssStyles['text-align'];
		}
	}

	toHTML(state) {
		this.#applyAlignments();
		var html = '';
		html += `<table${this._htmlAttributes()}>\n`;
		html += '<thead>\n';
		html += this.headerRow.toHTML(state) + '\n';
		html += '</thead>\n';
		html += '<tbody>\n';
		html += MDNode.toHTML(this.bodyRows, state) + '\n';
		html += '</tbody>\n';
		html += '</table>\n';
		return html;
	}
}

/**
 * Node for one row (header or body) in a table.
 */
class MDTableRowNode extends MDBlockNode {
	/** @type {MDTableCellNode[]} children */

	toHTML(state) {
		return this._simplePairedTagHTML(state, 'tr');
	}
}

/**
 * Node for one cell in a table row.
 */
class MDTableCellNode extends MDBlockNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'td');
	}
}

/**
 * Node for a header cell in a header table row.
 */
class MDTableHeaderCellNode extends MDBlockNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'th');
	}
}

/**
 * Definition list with `MDDefinitionListTermNode` and
 * `MDDefinitionListDefinitionNode` children.
 */
class MDDefinitionListNode extends MDBlockNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'dl');
	}
}

/**
 * A word or term in a definition list.
 */
class MDDefinitionListTermNode extends MDBlockNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'dt');
	}
}

/**
 * The definition of a word or term in a definition list. Should follow a
 * definition term, or another definition to serve as an alternate.
 */
class MDDefinitionListDefinitionNode extends MDBlockNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'dd');
	}
}

/**
 * Block at the bottom of a document listing all the footnotes with their
 * content.
 */
class MDFootnoteListNode extends MDBlockNode {
	/**
	 * @param {MDState} state
	 * @param {string} symbol
	 * @return {number}
	 */
	#footnoteId(state, symbol) {
		const lookup = state.root['footnoteIds'];
		if (!lookup) return null;
		return lookup[symbol] ?? null;
	}

	toHTML(state) {
		const footnotes = state.footnotes;
		var symbolOrder = Object.keys(footnotes);
		if (Object.keys(footnotes).length == 0) return '';
		const footnoteUniques = state.root.footnoteInstances;
		var html = '';
		html += '<div class="footnotes">';
		html += '<ol>';
		for (const symbol of symbolOrder) {
			/** @type {MDNode[]} */
			let content = footnotes[symbol];
			if (!content) continue;
			let footnoteId = this.#footnoteId(state, symbol);
			const contentHTML = MDNode.toHTML(content, state);
			html += `<li value="${footnoteId}" id="${state.root.elementIdPrefix}footnote_${footnoteId}">${contentHTML}`;
			const uniques = footnoteUniques[symbol];
			if (uniques) {
				for (const unique of uniques) {
					html += ` <a href="#${state.root.elementIdPrefix}footnoteref_${unique}" class="footnote-backref">↩︎</a>`;
				}
			}
			html += `</li>\n`;
		}
		html += '</ol>';
		html += '</div>';
		return html;
	}

	toPlaintext(state) {
		const footnotes = state.footnotes;
		var symbolOrder = Object.keys(footnotes);
		if (Object.keys(footnotes).length == 0) return '';
		var text = '';
		for (const symbol of symbolOrder) {
			let content = footnotes[symbol];
			if (!content) continue;
			text += `${symbol}. ${this._childPlaintext(state)}\n`;
		}
		return text.trim();
	}
}

/**
 * Marker subclass that indicates a node represents inline syntax.
 */
class MDInlineNode extends MDNode {}

/**
 * Contains plain text. Special HTML characters are escaped when rendered.
 */
class MDTextNode extends MDInlineNode {
	text;

	constructor(text) {
		super([]);
		this.text = text;
	}

	toHTML(state) {
		return MDUtils.escapeHTML(this.text);
	}

	toPlaintext(state) {
		return this.text;
	}
}

/**
 * Contains plain text which is rendered with HTML entities when rendered to
 * be marginally more difficult for web scapers to decipher. Used for
 * semi-sensitive info like email addresses.
 */
class MDObfuscatedTextNode extends MDTextNode {
	toHTML(state) {
		return MDUtils.escapeObfuscated(this.text);
	}
}

/**
 * Emphasized (italicized) content.
 */
class MDEmphasisNode extends MDInlineNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'em');
	}
}

/**
 * Strong (bold) content.
 */
class MDStrongNode extends MDInlineNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'strong');
	}
}

/**
 * Content rendered with a line through it.
 */
class MDStrikethroughNode extends MDInlineNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 's');
	}
}

/**
 * Underlined content.
 */
class MDUnderlineNode extends MDInlineNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'u');
	}
}

/**
 * Highlighted content. Usually rendered with a bright colored background.
 */
class MDHighlightNode extends MDInlineNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'mark');
	}
}

/**
 * Superscripted content.
 */
class MDSuperscriptNode extends MDInlineNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'sup');
	}
}

/**
 * Subscripted content.
 */
class MDSubscriptNode extends MDInlineNode {
	toHTML(state) {
		return this._simplePairedTagHTML(state, 'sub');
	}
}

/**
 * Inline plaintext indicating computer code.
 */
class MDCodeNode extends MDInlineNode {
	/** @type {string} */
	text;

	constructor(text) {
		super([]);
		this.text = text;
	}

	toHTML(state) {
		return `<code${this._htmlAttributes()}>${MDUtils.escapeHTML(this.text)}</code>`;
	}
}

/**
 * A footnote symbol in a document. Denoted as a superscripted number that can
 * be clicked to go to its content at the bottom of the document.
 */
class MDFootnoteNode extends MDInlineNode {
	/**
	 * Symbol the author used to match up the footnote to its content definition.
	 * @type {string}
	 */
	symbol;

	/**
	 * The superscript symbol rendered in HTML. May be the same or different
	 * than `symbol`.
	 * @type {string} display symbol
	 */
	displaySymbol = null;

	/**
	 * Unique ID for the footnote definition.
	 * @type {number|null}
	 */
	footnoteId = null;

	/**
	 * Unique number for backlinking to a footnote occurrence. Populated by
	 * `MDFootnoteReader.postProcess`.
	 * @type {number|null}
	 */
	occurrenceId = null;

	/**
	 * @param {string} symbol
	 * @param {string|null} title
	 */
	constructor(symbol, title=null) {
		super([]);
		this.symbol = symbol;
		if (title) this.attributes['title'] = title;
	}

	toHTML(state) {
		if (this.differentiator !== null) {
			return `<sup class="footnote" id="${state.root.elementIdPrefix}footnoteref_${this.occurrenceId}"${this._htmlAttributes()}>` +
				`<a href="#${state.root.elementIdPrefix}footnote_${this.footnoteId}">${MDUtils.escapeHTML(this.displaySymbol ?? this.symbol)}</a></sup>`;
		}
		return `<!--FNREF:{${this.symbol}}-->`;
	}
}

/**
 * A clickable hypertext link.
 */
class MDLinkNode extends MDInlineNode {
	/** @type {string} */
	href;

	/**
	 * @param {string} href
	 * @param {MDNode[]|MDNode} children
	 */
	constructor(href, children, title=null) {
		super(children);
		this.href = href;
		if (title !== null) this.attributes['title'] = title;
	}

	toHTML(state) {
		var escapedLink;
		if (this.href.startsWith('mailto:')) {
			escapedLink = MDUtils.escapeObfuscated(this.href);
		} else {
			escapedLink = MDUtils.escapeHTML(this.href);
		}
		return `<a href="${escapedLink}"${this._htmlAttributes()}>${this._childHTML(state)}</a>`;
	}
}

/**
 * A clickable hypertext link where the URL is defined elsewhere by reference.
 */
class MDReferencedLinkNode extends MDLinkNode {
	/** @type {string} */
	reference;

	constructor(reference, children) {
		super('', children);
		this.reference = reference;
	}

	/**
	 * @param {MDState} state
	 */
	toHTML(state) {
		if (this.href === '') {
			const url = state.urlForReference(this.reference);
			if (url) this.href = url;
			const title = state.urlTitleForReference(this.reference);
			if (title) this.attributes['title'] = title;
		}
		return super.toHTML(state);
	}
}

/**
 * An inline image.
 */
class MDImageNode extends MDInlineNode {
	/** @type {string} */
	src;

	/** @type {string|null} */
	alt;

	/**
	 * @param {string} src
	 * @param {string|null} alt
	 */
	constructor(src, alt) {
		super([]);
		this.src = src;
		this.alt = alt;
	}

	toHTML(state) {
		var html = `<img src="${MDUtils.escapeHTML(this.src)}"`;
		if (this.alt) html += ` alt="${MDUtils.escapeHTML(this.alt)}"`;
		html += `${this._htmlAttributes()}>`;
		return html;
	}
}

/**
 * An inline image where the URL is defined elsewhere by reference.
 */
class MDReferencedImageNode extends MDImageNode {
	/** @type {string} */
	reference;

	/**
	 * @param {string} reference
	 * @param {string|null} alt
	 */
	constructor(reference, alt='') {
		super('', alt, []);
		this.reference = reference;
	}

	toHTML(state) {
		if (this.src === '') {
			const url = state.urlForReference(this.reference);
			if (url !== null) this.src = url;
			const title = state.urlTitleForReference(this.reference);
			if (title !== null) this.attributes['title'] = title;
		}
		return super.toHTML(state);
	}
}

/**
 * An abbreviation that can be hovered over to see its full expansion.
 */
class MDAbbreviationNode extends MDInlineNode {
	/** @type {string} */
	abbreviation;

	/** @type {string} */
	get definition() { return this.attributes['title'] ?? null; }
	set definition(newValue) { this.attributes['title'] = newValue; }

	/**
	 * @param {string} abbreviation
	 * @param {string} definition
	 */
	constructor(abbreviation, definition) {
		super([]);
		this.abbreviation = abbreviation;
		this.attributes['title'] = definition;
	}

	toHTML(state) {
		return `<abbr${this._htmlAttributes()}>${MDUtils.escapeHTML(this.abbreviation)}</abbr>`;
	}
}

/**
 * A line break that is preserved when rendered to HTML.
 */
class MDLineBreakNode extends MDInlineNode {
	toHTML(state) {
		return '<br>';
	}

	toPlaintext(state) {
		return '\n';
	}
}

/**
 * A verbatim HTML tag. May be altered to strip out disallowed attributes or
 * CSS values.
 */
class MDHTMLTagNode extends MDInlineNode {
	/** @type {MDHTMLTag} */
	tag;

	constructor(tag) {
		super([]);
		this.tag = tag;
	}

	toHTML(state) {
		return this.tag.toString();
	}
}


// -- Main class ------------------------------------------------------------


/**
 * Markdown parser.
 */
class Markdown {
	/**
	 * Set of standard readers to handle common syntax.
	 * @type {MDReader[]}
	 */
	static standardReaders = [
		new MDUnderlinedHeadingReader(),
		new MDHashHeadingReader(),
		new MDBlockQuoteReader(),
		new MDHorizontalRuleReader(),
		new MDUnorderedListReader(),
		new MDOrderedListReader(),
		new MDFencedCodeBlockReader(),
		new MDIndentedCodeBlockReader(),
		new MDParagraphReader(),

		new MDStrongReader(),
		new MDEmphasisReader(),
		new MDCodeSpanReader(),
		new MDImageReader(),
		new MDLinkReader(),
		new MDHTMLTagReader(),
	];

	/**
	 * All supported readers except `MDLineBreakReader`.
	 * @type {MDReader[]}
	 */
	static allReaders = [
		...this.standardReaders,
		new MDSubtextReader(),
		new MDTableReader(),
		new MDDefinitionListReader(),
		new MDFootnoteReader(),
		new MDAbbreviationReader(),

		new MDUnderlineReader(),
		new MDSubscriptReader(),
		new MDStrikethroughReader(),
		new MDHighlightReader(),
		new MDSuperscriptReader(),
		new MDReferencedImageReader(),
		new MDReferencedLinkReader(),
		new MDModifierReader(),
	];

	/**
	 * Shared instance of a parser with standard syntax.
	 */
	static standardParser = new Markdown(this.standardReaders);

	/**
	 * Shared instance of a parser with all supported syntax.
	 */
	static completeParser = new Markdown(this.allReaders);

	/**
	 * Filter for what non-markdown HTML is permitted. HTML generated as a
	 * result of markdown is unaffected.
	 */
	tagFilter = new MDHTMLFilter();

	/** @type {MDReader[]} */
	#readers;

	/** @type {MDReader[]} */
	#readersByBlockPriority;
	/** @type {MDReader[]} */
	#readersByTokenPriority;
	/** @type {Array} */
	#readersBySubstitutePriority;

	/**
	 * Creates a Markdown parser with the given syntax readers.
	 *
	 * @param {MDReader[]} readers
	 */
	constructor(readers=Markdown.allReaders) {
		this.#readers = readers;
		this.#readersByBlockPriority = MDReader.sortReaderForBlocks(readers);
		this.#readersByTokenPriority = MDReader.sortReadersForTokenizing(readers);
		this.#readersBySubstitutePriority = MDReader.sortReadersForSubstitution(readers);
	}

	/**
	 * Converts a markdown string to an HTML string.
	 *
	 * @param {string} markdown
	 * @param {string} elementIdPrefix - Optional prefix for generated element
	 *   `id`s and links to them. For differentiating multiple markdown docs in
	 *   the same HTML page.
	 * @returns {string} HTML
	 */
	toHTML(markdown, elementIdPrefix='') {
		const lines = markdown.split(/(?:\n|\r|\r\n)/);
		try {
			return this.#parse(lines, elementIdPrefix);
		} catch (e) {
			this.#investigateException(lines, elementIdPrefix);
			throw e;
		}
	}

	/**
	 * @param {string[]} lines
	 * @param {string} elementIdPrefix
	 */
	#parse(lines, elementIdPrefix) {
		const state = new MDState(lines);
		state.readersByBlockPriority = this.#readersByBlockPriority;
		state.readersByTokenPriority = this.#readersByTokenPriority
		state.readersBySubstitutePriority = this.#readersBySubstitutePriority
		state.tagFilter = this.tagFilter;
		state.elementIdPrefix = elementIdPrefix;
		for (const reader of this.#readers) {
			reader.preProcess(state);
		}
		const nodes = state.readBlocks();
		for (const reader of this.#readers) {
			reader.postProcess(state, nodes);
		}
		return MDNode.toHTML(nodes, state);
	}

	/**
	 * Keeps removing first and last lines of markdown to locate the source of
	 * an exception and prints the minimal snippet to `console.error`.
	 *
	 * @param {string[]} lines
	 * @param {string} elementIdPrefix
	 */
	#investigateException(lines, elementIdPrefix) {
		var startIndex = 0;
		var endIndex = lines.length;
		// Keep stripping away first line until an exception stops being thrown
		for (var i = 0; i < lines.length; i++) {
			try {
				this.#parse(lines.slice(i, endIndex), elementIdPrefix);
				break;
			} catch (e0) {
				startIndex = i;
			}
		}
		// Keep stripping away last line until an exception stops being thrown
		for (var i = lines.length; i > startIndex; i--) {
			try {
				this.#parse(lines.slice(startIndex, i), elementIdPrefix);
				break;
			} catch (e0) {
				endIndex = i;
			}
		}
		const problematicMarkdown = lines.slice(startIndex, endIndex).join("\n");
		console.error(`This portion of markdown caused an unexpected exception: ${problematicMarkdown}`);
	}
}