// FIXME: Nested blockquotes require blank line
// TODO: HTML tags probably need better handling. Consider whether interior of matched tags should be interpreted as markdown.
// TODO: {.class #cssid lang=fr}
//     [link](url){.class}
// TODO: Test broken/incomplete syntax thoroughly
// TODO: Sanity checks on loops/recursion?
// TODO: Tolerate whitespace between tokens (e.g. [click here] [urlref])
// TODO: Spreadsheet functions in tables

class _MDHAlign {
	static Left = new _MDHAlign('Left');
	static Center = new _MDHAlign('Center');
	static Right = new _MDHAlign('Right');

	/** @var {String} */
	name;

	constructor(name) {
		this.name = name;
	}

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

	static toHTMLAttribute(align) {
		switch (align) {
			case _MDHAlign.Left: return ' align="left"';
			case _MDHAlign.Center: return ' align="center"';
			case _MDHAlign.Right: return ' align="right"';
		}
		return '';
	}
}

class _MDTokenType {
	static Text = new _MDTokenType('Text');
	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 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'); // content

	static HTMLTag = new _MDTokenType('HTMLTag'); // content=tag string, tag=_MDHTMLTag

	static META_AnyNonWhitespace = new _MDTokenType('METAAnyNonWhitespace');

	/** @var {String} */
	name;

	constructor(name) {
		this.name = name;
	}

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

class _MDToken {
	/**
	 * The original token string.
	 * @var {String}
	 */
	original;
	/** @var {_MDTokenType} */
	type;
	/** @var {String|null} */
	content;
	/** @var {String|null} */
	extra;
	/** @var {_MDHTMLTag|null} */
	tag;
	/** @var {_MDTagModifier|null} */
	modifier;

	/**
	 * @param {String} original
	 * @param {_MDTokenType} type
	 * @param {String|_MDTagModifier|null} content
	 * @param {String|null} extra
	 * @param {_MDHTMLTag|null} tag
	 */
	constructor(original, type, content=null, extra=null, tag=null) {
		this.original = original;
		this.type = type;
		if (content instanceof _MDTagModifier) {
			this.content = null;
			this.modifier = content;
		} else {
			this.content = content;
			this.modifier = null;
		}
		this.extra = extra;
		this.tag = tag;
	}
}


// -- Spans -----------------------------------------------------------------


class _MDSpan {
	/** @var {String[]} */
	cssClasses = [];
	/** @var {String|null} */
	id = null;
	/** @var {Object} */
	attributes = {};

	/**
	 * @param {_MDState} state
	 * @returns {String} HTML
	 */
	toHTML(state) {
		throw new Error(self.constructor.name + ".toHTML not implemented");
	}

	htmlAttributes() {
		var html = '';
		if (this.cssClasses.length > 0) {
			html += ` class="${this.cssClasses.join(' ')}"`;
		}
		if (this.id !== null) {
			html += ` id="${this.id}"`;
		}
		for (const name in this.attributes) {
			let value = this.attributes[name];
			html += ` ${name}="${value.replace('"', '"')}"`;
		}
		return html;
	}

	/**
	 * @param {_MDSpan[]} spans
	 * @param {_MDState} state
	 */
	static toHTML(spans, state) {
		return spans.map((span) => span.toHTML(state)).join("");
	}
}

class _MDMultiSpan extends _MDSpan {
	/** @var {_MDSpan[]} */
	content;

	/**
	 * @param {_MDSpan[]} content
	 */
	constructor(content) {
		super();
		this.content = content;
	}

	toHTML(state) {
		return _MDSpan.toHTML(this.content, state);
	}
}

class _MDTextSpan extends _MDSpan {
	/** @param {String} text */
	text;

	/**
	 * @param {String} text
	 */
	constructor(text) {
		super();
		this.text = text;
	}

	toHTML(state) {
		let html = this.text.replace('<', '&lt;');
		let abbrevs = state.abbreviations;
		let regexes = state.abbreviationRegexes;
		for (const abbrev in abbrevs) {
			let def = abbrevs[abbrev];
			let regex = regexes[abbrev];
			let escapedDef = def.replace('"', '&quot;');
			html = html.replace(regex, `<abbr title="${escapedDef}">$1</abbr>`);
		}
		return html;
	}
}

class _MDHTMLSpan extends _MDSpan {
	/** @param {String} html */
	html;

	/**
	 * @param {String} html
	 */
	constructor(html) {
		super();
		this.html = html;
	}

	toHTML(state) {
		return this.html;
	}
}

class _MDLinkSpan extends _MDSpan {
	/** @var {String} */
	link;
	/** @var {String|null} */
	target = null;
	/** @var {_MDSpan} */
	content;
	/** @var {String|null} */
	title = null;

	/**
	 * @param {String} link
	 * @param {_MDSpan} content
	 */
	constructor(link, content, title=null) {
		super();
		this.link = link;
		this.content = content;
		this.title = title;
	}

	toHTML(state) {
		let escapedLink = this.link.replace('"', '&quot;');
		var html = `<a href="${escapedLink}"`;
		if (this.target) {
			let escapedTarget = this.target.replace('"', '&quot;');
			html += ` target="${escapedTarget}"`;
		}
		if (this.title) {
			html += ` title="${this.title.replace('"', '&quot;')}"`;
		}
		html += this.htmlAttributes();
		html += '>' + this.content.toHTML(state) + '</a>';
		return html;
	}
}

class _MDReferencedLinkSpan extends _MDLinkSpan {
	/** @var {String} id */
	id;

	constructor(id, content) {
		super(null, content);
		this.id = id;
	}

	/**
	 * @param {_MDState} state
	 */
	toHTML(state) {
		if (!this.link) {
			let url = state.urls[this.id.toLowerCase()];
			let title = state.urlTitles[this.id.toLowerCase()];
			this.link = url;
			this.title = title || this.title;
		}
		if (this.link) {
			return super.toHTML(state);
		} else {
			let contentHTML = this.content.toHTML(state);
			return `[${contentHTML}][${this.id}]`;
		}
	}
}

class _MDEmphasisSpan extends _MDSpan {
	/** @var {_MDSpan} */
	#content;

	/**
	 * @param {_MDSpan} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		let contentHTML = this.#content.toHTML(state);
		return `<em${this.htmlAttributes()}>${contentHTML}</em>`;
	}
}

class _MDStrongSpan extends _MDSpan {
	/** @var {_MDSpan} content */
	#content;

	/**
	 * @param {_MDSpan} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		let contentHTML = this.#content.toHTML(state);
		return `<strong${this.htmlAttributes()}>${contentHTML}</strong>`;
	}
}

class _MDStrikethroughSpan extends _MDSpan {
	/** @var {_MDSpan} content */
	#content;

	/**
	 * @param {_MDSpan} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		let contentHTML = this.#content.toHTML(state);
		return `<strike${this.htmlAttributes()}>${contentHTML}</strike>`;
	}
}

class _MDCodeSpan extends _MDSpan {
	/** @var {_MDSpan} content */
	#content;

	/**
	 * @param {_MDSpan} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		let contentHTML = this.#content.toHTML(state);
		return `<code${this.htmlAttributes()}>${contentHTML}</code>`;
	}
}

class _MDImageSpan extends _MDSpan {
	/** @var {String} */
	source;
	/** @var {String|null} */
	alt;
	/** @var {String|null} */
	title;

	/**
	 * @param {String} source
	 */
	constructor(source, alt, title=null) {
		super();
		this.source = source;
		this.alt = alt;
		this.title = title;
	}

	toHTML(state) {
		let escapedSource = this.source.replace('"', '&quot;');
		let html = `<img src="${escapedSource}"`;
		if (this.alt) {
			let altEscaped = this.alt.replace('"', '&quot');
			html += ` alt="${altEscaped}"`;
		}
		if (this.title) {
			let titleEscaped = this.title.replace('"', '&quot;');
			html += ` title="${titleEscaped}"`;
		}
		html += this.htmlAttributes();
		html += '>';
		return html;
	}
}

class _MDReferencedImageSpan extends _MDImageSpan {
	/** @var {String} */
	id;

	/**
	 * @param {String} id
	 */
	constructor(id, alt) {
		super(null, alt);
		this.id = id;
	}

	toHTML(state) {
		if (!this.source) {
			let url = state.urls[this.id.toLowerCase()];
			let title = state.urlTitles[this.id.toLowerCase()];
			this.source = url;
			this.title = title || this.title;
		}
		if (this.source) {
			return super.toHTML(state);
		} else {
			let altEscaped = this.alt.replace('"', '&quot;');
			let idEscaped = this.id.replace('"', '&quot;');
			return `![${altEscaped}][${idEscaped}]`;
		}
	}
}

class _MDFootnoteReferenceSpan extends _MDSpan {
	/** @var {String} */
	symbol;

	/**
	 * @param {String} symbol
	 */
	constructor(symbol) {
		super();
		this.symbol = symbol;
	}

	toHTML(state) {
		return `<!--FNREF:{${this.symbol}}-->`;
	}
}


// -- Blocks ----------------------------------------------------------------


class _MDBlock {
	/** @var {String[]} */
	cssClasses = [];
	/** @var {String|null} */
	id = null;
	/** @var {Object} */
	attributes = {};

	/**
	 * @param {_MDState} state
	 */
	toHTML(state) {
		throw new Error(self.constructor.name + ".toHTML not implemented");
	}

	htmlAttributes() {
		var html = '';
		if (this.cssClasses.length > 0) {
			html += ` class="${this.cssClasses.join(' ')}"`;
		}
		if (this.id !== null) {
			html += ` id="${this.id}"`;
		}
		for (const name in this.attributes) {
			let value = this.attributes[name];
			html += ` ${name}="${value.replace('"', '&quot;')}"`;
		}
		return html;
	}

	/**
	 * @param {_MDBlock[]} blocks
	 * @param {_MDState} state
	 * @returns {String}
	 */
	static toHTML(blocks, state) {
		return blocks.map((block) => block.toHTML(state)).join("\n");
	}
}

class _MDMultiBlock extends _MDBlock {
	/** @var {_MDBlock[]} */
	#blocks;
	/**
	 * @param {_MDBlock[]} blocks
	 */
	constructor(blocks) {
		super();
		this.#blocks = blocks;
	}
	toHTML(state) {
		return _MDBlock.toHTML(this.#blocks, state);
	}
}

class _MDParagraphBlock extends _MDBlock {
	/** @var {_MDBlock} */
	content;

	/**
	 * @param {_MDBlock} content
	 */
	constructor(content) {
		super();
		this.content = content;
	}

	toHTML(state) {
		let contentHTML = this.content.toHTML(state);
		return `<p${this.htmlAttributes()}>${contentHTML}</p>\n`;
	}
}

class _MDHeaderBlock extends _MDBlock {
	/** @var {number} */
	level;
	/** @var {_MDBlock} */
	content;

	/**
	 * @param {number} level
	 * @param {_MDBlock} content
	 */
	constructor(level, content) {
		super();
		this.level = level;
		this.content = content;
	}

	toHTML(state) {
		let contentHTML = this.content.toHTML(state);
		return `<h${this.level}${this.htmlAttributes()}>${contentHTML}</h${this.level}>\n`;
	}
}

class _MDBlockquoteBlock extends _MDBlock {
	/** @var {_MDBlock[]} */
	content;

	/**
	 * @param {_MDBlock[]} content
	 */
	constructor(content) {
		super();
		this.content = content;
	}

	toHTML(state) {
		let contentHTML = _MDBlock.toHTML(this.content, state);
		return `<blockquote${this.htmlAttributes()}>\n${contentHTML}\n</blockquote>`;
	}
}

class _MDUnorderedListBlock extends _MDBlock {
	/** @var {_MDListItemBlock[]} */
	items;

	/**
	 * @param {_MDListItemBlock[]} items
	 */
	constructor(items) {
		super();
		this.items = items;
	}

	toHTML(state) {
		let contentHTML = _MDBlock.toHTML(this.items, state);
		return `<ul${this.htmlAttributes()}>\n${contentHTML}\n</ul>`;
	}
}

class _MDOrderedListBlock extends _MDBlock {
	/** @var {_MDListItemBlock[]} */
	items;

	/** @var {Number|null} */
	startOrdinal;

	/**
	 * @param {_MDListItemBlock[]} items
	 */
	constructor(items, startOrdinal=null) {
		super();
		this.items = items;
		this.startOrdinal = startOrdinal;
	}

	htmlAttributes() {
		var html = super.htmlAttributes();
		if (this.startOrdinal !== null) {
			html += ` start="${this.startOrdinal}"`;
		}
		return html;
	}

	toHTML(state) {
		let contentHTML = _MDBlock.toHTML(this.items, state);
		return `<ol${this.htmlAttributes()}>\n${contentHTML}\n</ol>`;
	}
}

class _MDListItemBlock extends _MDBlock {
	/** @var {_MDBlock} */
	content;
	/** @var {Number|null} */
	ordinal;

	/**
	 * @param {_MDBlock} content
	 */
	constructor(content, ordinal=null) {
		super();
		this.content = content;
		this.ordinal = ordinal;
	}

	toHTML(state) {
		let contentHTML = this.content.toHTML(state);
		return `<li${this.htmlAttributes()}>${contentHTML}</li>`;
	}
}

class _MDCodeBlock extends _MDBlock {
	/** @var {String} */
	#code;

	/**
	 * @param {String} code
	 */
	constructor(code) {
		super();
		this.#code = code;
	}

	toHTML(state) {
		return `<pre${this.htmlAttributes()}><code>${this.#code}</code></pre>`;
	}
}

class _MDHorizontalRuleBlock extends _MDBlock {
	toHTML(state) {
		return `<hr${this.htmlAttributes()}>\n`;
	}
}

class _MDTableCellBlock extends _MDBlock {
	/** @var {_MDBlock} */
	#content;
	/** @var {_MDHAlign|null} */
	align = null;

	/**
	 * @param {_MDBlock} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	htmlAttributes() {
		var html = super.htmlAttributes();
		html += _MDHAlign.toHTMLAttribute(this.align);
		return html;
	}

	toHTML(state) {
		let contentHTML = this.#content.toHTML(state);
		return `<td${this.htmlAttributes()}>${contentHTML}</td>`;
	}
}

class _MDTableHeaderCellBlock extends _MDTableCellBlock {
	toHTML(state) {
		let html = super.toHTML(state);
		let groups = /^<td(.*)td>$/.exec(html);
		return `<th${groups[1]}th>`;
	}
}

class _MDTableRowBlock extends _MDBlock {
	/** @var {_MDTableCellBlock[]|_MDTableHeaderCellBlock[]} */
	#cells;

	/**
	 * @param {_MDTableCellBlock[]|_MDTableHeaderCellBlock[]} cells
	 */
	constructor(cells) {
		super();
		this.#cells = cells;
	}

	/**
	 * @param {_MDHAlign[]} alignments
	 */
	applyAlignments(alignments) {
		for (var i = 0; i < this.#cells.length; i++) {
			let cell = this.#cells[i];
			let align = i < alignments.length ? alignments[i] : null;
			cell.align = align;
		}
	}

	toHTML(state) {
		let cellsHTML = _MDBlock.toHTML(this.#cells, state);
		return `<tr${this.htmlAttributes()}>\n${cellsHTML}\n</tr>`;
	}
}

class _MDTableBlock extends _MDBlock {
	/** @var {_MDTableRowBlock} */
	#headerRow;
	/** @var {_MDTableRowBlock[]} */
	#bodyRows;

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

	toHTML(state) {
		let headerRowHTML = this.#headerRow.toHTML(state);
		let bodyRowsHTML = _MDBlock.toHTML(this.#bodyRows, state);
		return `<table${this.htmlAttributes()}>\n<thead>\n${headerRowHTML}\n</thead>\n<tbody>\n${bodyRowsHTML}\n</tbody>\n</table>`;
	}
}

class _MDDefinitionListBlock extends _MDBlock {
	/** @var {_MDBlock[]} */
	#content;

	/**
	 * @param {_MDBlock[]} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		let contentHTML = _MDBlock.toHTML(this.#content, state);
		return `<dl${this.htmlAttributes()}>\n${contentHTML}\n</dl>`;
	}
}

class _MDDefinitionTermBlock extends _MDBlock {
	/** @var {_MDBlock} */
	#content;

	/**
	 * @param {_MDBlock} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		let contentHTML = this.#content.toHTML(state);
		return `<dt${this.htmlAttributes()}>${contentHTML}</dt>`;
	}
}

class _MDDefinitionDefinitionBlock extends _MDBlock {
	/** @var {_MDBlock} */
	#content;

	/**
	 * @param {_MDBlock} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		let contentHTML = this.#content.toHTML(state);
		return `<dd${this.htmlAttributes()}>${contentHTML}</dd>`;
	}
}

class _MDInlineBlock extends _MDBlock {
	/** @var {_MDSpan[]} */
	#content;

	/**
	 * @param {_MDSpan[]} content
	 */
	constructor(content) {
		super();
		this.#content = content;
	}

	toHTML(state) {
		return _MDSpan.toHTML(this.#content, state);
	}
}

class _MDHTMLTag {
	/** @var {String} */
	fullTag;
	/** @var {String} */
	tagName;
	/** @var {Boolean} */
	isCloser;
	/** @var {Object} */
	attributes;

	/**
	 * @param {String} fullTag
	 * @param {String} tagName
	 * @param {Boolean} isCloser
	 * @param {Object} attributes
	 */
	constructor(fullTag, tagName, isCloser, attributes) {
		this.fullTag = fullTag;
		this.tagName = tagName;
		this.isCloser = isCloser;
		this.attributes = attributes;
	}
}

class _MDState {
	/** @var {String[]} */
	lines = [];

	/** @var {Object} */
	#abbreviations = {};

	/** @var {Object} */
	#abbreviationRegexes = {};

	/** @var {Object} */
	#footnotes = {};

	/** @var {Object} */
	#urlDefinitions = {};

	/** @var {Object} */
	#urlTitles = {};

	/** @var {number} */
	p = 0;

	/** @var {_MDState|null} */
	#parent = null;

	/** @var {Object} */
	get abbreviations() {
		return (this.#parent) ? this.#parent.abbreviations : this.#abbreviations;
	}

	/** @var {Object} */
	get abbreviationRegexes() {
		return (this.#parent) ? this.#parent.abbreviationRegexes : this.#abbreviationRegexes;
	}

	/** @var {Object} */
	get footnotes() {
		return (this.#parent) ? this.#parent.footnotes : this.#footnotes;
	}

	/** @var {Object} */
	get urls() {
		return (this.#parent) ? this.#parent.urls : this.#urlDefinitions;
	}

	/** @var {Object} */
	get urlTitles() {
		return (this.#parent) ? this.#parent.urlTitles : this.#urlTitles;
	}

	/**
	 * @param {String[]} lines
	 */
	copy(lines) {
		let cp = new _MDState();
		cp.#parent = this;
		cp.lines = lines;
		cp.p = 0;
		return cp;
	}

	/**
	 * @param {String} abbreviation
	 * @param {String} definition
	 */
	defineAbbreviation(abbreviation, definition) {
		if (this.#parent) {
			this.#parent.defineAbbreviation(abbreviation, definition);
			return;
		}
		this.#abbreviations[abbreviation] = definition;
		let regex = new RegExp("\\b(" + abbreviation + ")\\b", "ig");
		this.#abbreviationRegexes[abbreviation] = regex;
	}

	/**
	 * @param {String} symbol
	 * @param {_MDBlock} footnote
	 */
	defineFootnote(symbol, footnote) {
		if (this.#parent) {
			this.#parent.defineFootnote(symbol, footnote);
		} else {
			this.#footnotes[symbol] = footnote;
		}
	}

	defineURL(symbol, url, title=null) {
		if (this.#parent) {
			this.#parent.defineURL(symbol, url, title);
		} else {
			this.#urlDefinitions[symbol.toLowerCase()] = url;
			if (title !== null) {
				this.#urlTitles[symbol.toLowerCase()] = title;
			}
		}
	}

	hasLines(minCount, p=-1) {
		let relativeTo = (p < 0) ? this.p : p;
		return relativeTo + minCount <= this.lines.length;
	}
}

class _MDTagModifier {
	/** @var {String} */
	original;
	/** @var {String[]} */
	cssClasses = [];
	/** @var {String|null} */
	id = null;
	/** @var {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 {_MDBlock|_MDSpan} elem
	 */
	applyTo(elem) {
		if (elem instanceof _MDBlock || elem instanceof _MDSpan) {
			elem.cssClasses = elem.cssClasses.concat(this.cssClasses);
			if (this.id) elem.id = this.id;
			for (const name in this.attributes) {
				elem.attributes[name] = this.attributes[name];
			}
		}
	}

	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.cssClasses.push(groups[1]);
			} else if (groups = this.#idRegex.exec(token)) {
				mod.id = groups[1];
			} else if (groups = this.#attributeRegex.exec(token)) {
				mod.attributes[groups[1]] = groups[2];
			} else {
				return null;
			}
		}
		return mod;
	}

	/**
	 * Extracts modifier from line.
	 * @param {String} line
	 * @returns {Array} Tuple with remaining line and _MDTagModifier.
	 */
	static fromLine(line) {
		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 ];
	}

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

	/**
	 * @param {String} line
	 * @returns {String}
	 */
	static strip(line) {
		let groups = this.#trailingClassRegex.exec(line);
		if (groups === null) return line;
		return groups[1];
	}
}

class Markdown {
	/**
	 * @param {String} line
	 */
	static #stripIndent(line, count=1) {
		let regex = new RegExp(`^(?: {1,4}|\\t){${count}}`);
		return line.replace(regex, '');
	}

	/**
	 * @param {String} line
	 * @param {Boolean} fullIndentsOnly
	 * @returns {Number} indent count
	 */
	static #countIndents(line, fullIndentsOnly=false) {
		var count = 0;
		var lastLine = line;
		while (line.length > 0) {
			line = (fullIndentsOnly)
				? line.replace(/^(?: {4}|\t)/, '')
				: line.replace(/^(?: {1,4}|\t)/, '');
			if (line != lastLine) {
				count++;
			} else {
				break;
			}
			lastLine = line;
		}
		return count;
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock[]}
	 */
	static #readBlocks(state) {
		var blocks = [];
		while (state.hasLines(1)) {
			let block = this.#readNextBlock(state);
			if (block) {
				blocks.push(block);
			} else {
				break;
			}
		}
		return blocks;
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock}
	 */
	static #readNextBlock(state) {
		while (state.hasLines(1) && state.lines[state.p].trim().length == 0) {
			state.p++;
		}
		var block;
		block = this.#readUnderlineHeader(state); if (block) return block;
		block = this.#readHashHeader(state); if (block) return block;
		block = this.#readBlockQuote(state); if (block) return block;
		block = this.#readUnorderedList(state); if (block) return block;
		block = this.#readOrderedList(state); if (block) return block;
		block = this.#readFencedCodeBlock(state); if (block) return block;
		block = this.#readIndentedCodeBlock(state); if (block) return block;
		block = this.#readHorizontalRule(state); if (block) return block;
		block = this.#readTable(state); if (block) return block;
		block = this.#readFootnoteDef(state); if (block) return block;
		block = this.#readAbbreviationDef(state); if (block) return block;
		block = this.#readURLDef(state); if (block) return block;
		block = this.#readDefinitionList(state); if (block) return block;
		block = this.#readParagraph(state); if (block) return block;
		return null;
	}

	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/;

	/**
	 * @param {String} line
	 * @returns {_MDHTMLTag|null} HTML tag if possible
	 */
	static #htmlTag(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() {
			if (attributeName.length > 0) {
				if (attributeValue.length > 0 || attributeQuote) {
					attributes[attributeName] = 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();
								expect = expectAttributeNameOrEnd;
							} else if (attributeQuote === '' && (ch == '/' || ch == '>')) {
								return null;
							} else {
								attributeValue += ch;
							}
						}
					} else {
						if (ch === attributeQuote) {
							endAttribute();
							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);
	}

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

	// 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;

	static #footnoteWithTitleRegex = /^\[\^(\d+?)\s+"(.*?)"\]/;  // 1=symbol, 2=title
	static #footnoteRegex = /^\[\^(\d+?)\]/;  // 1=symbol
	// Note: label contents have to have matching pairs of [] and (). Handles images inside links.
	static #labelRegex = /^\[((?:[^\[\]]*?\[[^\[\]]*?\][^\[\]]*?|[^\(\)]*?\([^\(\)]*?\)[^\(\)]*?|[^\[\]\(\)]*?)*?)\]/;  // 1=content
	static #urlWithTitleRegex = /^\((\S+?)\s+"(.*?)"\)/i;  // 1=URL, 2=title
	static #urlRegex = /^\((\S+?)\)/i;  // 1=URL
	static #emailWithTitleRegex = new RegExp("^\\(\\s*(" + this.#baseEmailRegex.source + ")\\s+\"(.*?)\"\\s*\\)", "i");  // 1=email, 2=title
	static #emailRegex = new RegExp("^\\(\\s*(" + this.#baseEmailRegex.source + ")\\s*\\)", "i");  // 1=email
	static #simpleURLRegex = new RegExp("^<" + this.#baseURLRegex.source + ">", "i");  // 1=URL
	static #simpleEmailRegex = new RegExp("^<" + this.#baseEmailRegex.source + ">", "i");  // 1=email

	/**
	 * @param {String} line
	 * @returns {String[]}
	 */
	static #matchLabel(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 == '(') {
				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;
	}

	/**
	 * @param {String} line
	 * @returns {_MDToken[]} tokens
	 */
	static #tokenize(line) {
		var tokens = [];
		var text = '';
		var expectLiteral = false;
		var groups = null;
		var tag = null;
		var modifier = null;
		const endText = function() {
			if (text.length == 0) return;
			let textGroups = Markdown.#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++) {
			let ch = line.substring(p, p + 1);
			let remainder = line.substring(p);
			if (expectLiteral) {
				text += ch;
				expectLiteral = false;
				continue;
			}
			if (ch == '\\') {
				expectLiteral = true;
			} else if (ch == '*') {
				endText();
				tokens.push(new _MDToken(ch, _MDTokenType.Asterisk));
			} else if (ch == '_') {
				endText();
				tokens.push(new _MDToken(ch, _MDTokenType.Underscore));
			} else if (ch == '`') {
				endText();
				tokens.push(new _MDToken(ch, _MDTokenType.Backtick));
			} else if (ch == '~') {
				endText();
				tokens.push(new _MDToken(ch, _MDTokenType.Tilde));
			} else if (ch == '!') {
				endText();
				tokens.push(new _MDToken(ch, _MDTokenType.Bang));

			} else if (groups = this.#footnoteWithTitleRegex.exec(remainder)) {
				// Footnote with title   [^1 "Foo"]
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.Footnote, groups[1], groups[2]));
				p += groups[0].length - 1;
			} else if (groups = this.#footnoteRegex.exec(remainder)) {
				// Footnote without title   [^1]
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.Footnote, groups[1]));
				p += groups[0].length - 1;
			} else if (groups = this.#matchLabel(remainder)) {
				// Label/ref for link/image   [Foo]
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.Label, groups[1]));
				p += groups[0].length - 1;
			} else if (groups = this.#urlWithTitleRegex.exec(remainder)) {
				// URL with title   (https://foo "Bar")
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.URL, groups[1], groups[2]));
				p += groups[0].length - 1;
			} else if (groups = this.#emailWithTitleRegex.exec(remainder)) {
				// Email address with title   (user@example.com  "Foo")
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.Email, groups[1], groups[2]));
				p += groups[0].length - 1;
			} else if (groups = this.#urlRegex.exec(remainder)) {
				// URL   (https://example.com)
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.URL, groups[1]));
				p += groups[0].length - 1;
			} else if (groups = this.#emailRegex.exec(remainder)) {
				// Email   (user@example.com)
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.Email, groups[1]));
				p += groups[0].length - 1;
			} else if (groups = this.#simpleURLRegex.exec(remainder)) {
				// Simple URL   <https://example.com>
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.SimpleLink, groups[1]));
				p += groups[0].length - 1;
			} else if (groups = this.#simpleEmailRegex.exec(remainder)) {
				// Simple email   <user@example.com>
				endText();
				tokens.push(new _MDToken(groups[0], _MDTokenType.SimpleEmail, groups[1]));
				p += groups[0].length - 1;
			} else if (tag = this.#htmlTag(remainder)) {
				endText();
				tokens.push(new _MDToken(tag.fullTag, _MDTokenType.HTMLTag, tag.fullTag, null, tag));
				p += tag.fullTag.length - 1;
			} else if (modifier = _MDTagModifier.fromStart(remainder)) {
				endText();
				tokens.push(new _MDToken(modifier.original, _MDTokenType.Modifier, modifier));
				p += modifier.original.length - 1;
			} else {
				text += ch;
			}
		}
		endText();
		return tokens;
	}

	static #firstTokenIndex(tokens, pattern, startIndex=0) {
		for (var t = startIndex; t < tokens.length; t++) {
			var matchedAll = true;
			for (var p = 0; p < pattern.length; p++) {
				var t0 = t + p;
				if (t0 >= tokens.length) return null;
				let token = tokens[t0];
				let elem = pattern[p];
				if (elem == _MDTokenType.META_AnyNonWhitespace) {
					if (token instanceof _MDToken && token.type == _MDTokenType.Whitespace) {
						matchedAll = false;
						break;
					}
				} else {
					if (!(token instanceof _MDToken) || token.type != elem) {
						matchedAll = false;
						break;
					}
				}
			}
			if (matchedAll) {
				return t;
			}
		}
		return null;
	}

	/**
	 * @param {_MDState} state
	 * @param {String} line
	 * @returns {_MDBlock|null}
	 */
	static #readInline(state, line) {
		var tokens = this.#tokenize(line);
		return new _MDInlineBlock(this.#tokensToSpans(tokens, state));
	}

	/**
	 * @param {Array} tokens
	 * @returns {_MDSpan[]} spans
	 */
	static #tokensToSpans(tokens, state) {
		var spans = tokens.slice(0, tokens.length);
		var anyChanges = false;
		var index, index0;
		// First pass - contiguous constructs
		do {
			anyChanges = false;
			// ![alt](image.jpg)
			if ((index = this.#firstTokenIndex(spans, [
				_MDTokenType.Bang,
				_MDTokenType.Label,
				_MDTokenType.URL,
			])) !== null) {
				let alt = spans[index + 1].content;
				let url = spans[index + 2].content;
				let title = spans[index + 2].extra;
				spans.splice(index, 3, new _MDImageSpan(url, alt, title));
				anyChanges = true;
			}
			
			// ![alt][ref]
			else if ((index = this.#firstTokenIndex(spans, [
				_MDTokenType.Bang,
				_MDTokenType.Label,
				_MDTokenType.Label,
			])) !== null) {
				let alt = spans[index + 1].content;
				let ref = spans[index + 2].content;
				spans.splice(index, 3, new _MDReferencedImageSpan(ref, alt));
				anyChanges = true;
			}
			
			// [text](link.html)
			else if ((index = this.#firstTokenIndex(spans, [
				_MDTokenType.Label,
				_MDTokenType.URL,
			])) !== null) {
				let text = spans[index + 0].content;
				let url = spans[index + 1].content;
				spans.splice(index, 2, new _MDLinkSpan(url, this.#readInline(state, text)));
				anyChanges = true;
			}

			// [text][ref]
			else if ((index = this.#firstTokenIndex(spans, [
				_MDTokenType.Label,
				_MDTokenType.Label,
			])) !== null) {
				let text = spans[index + 0].content;
				let ref = spans[index + 1].content;
				spans.splice(index, 2, new _MDReferencedLinkSpan(ref, this.#readInline(state, text)));
				anyChanges = true;
			}

			// [^1]
			else if ((index = this.#firstTokenIndex(spans, [
				_MDTokenType.Footnote,
			])) !== null) {
				let symbol = spans[index].content;
				spans.splice(index, 1, new _MDFootnoteReferenceSpan(symbol));
				anyChanges = true;
			}
		} while (anyChanges);

		/**
		 * @param {_MDTokenType[]} delimiter
		 * @param {Set<_MDTokenType>} disallowedInnerTokens
		 */
		const matchPair = function(delimiter, disallowedInnerTokens=new Set()) {
			var searchStart = 0;
			var hasNewStart = false;
			do {
				hasNewStart = false;
				let startIndex = Markdown.#firstTokenIndex(spans, delimiter.concat(_MDTokenType.META_AnyNonWhitespace), searchStart);
				if (startIndex === null) return null;
				let endIndex = Markdown.#firstTokenIndex(spans, [_MDTokenType.META_AnyNonWhitespace].concat(delimiter), startIndex + delimiter.length);
				if (endIndex === null) return null;
				let contentTokens = spans.slice(startIndex + delimiter.length, endIndex + 1);
				if (disallowedInnerTokens.size > 0) {
					for (const token of contentTokens) {
						if (token instanceof _MDToken && disallowedInnerTokens.has(token.type)) {
							searchStart = startIndex + 1;
							hasNewStart = true;
							break;
						}
					}
					if (hasNewStart) continue;
				}
				let contentSpans = Markdown.#tokensToSpans(contentTokens, state);
				return {
					startIndex: startIndex,
					toDelete: endIndex - startIndex + delimiter.length + 1,
					content: new _MDMultiSpan(contentSpans),
				};
			} while (hasNewStart);
			return null;
		};
		var spanMatch = null;

		// Second pass - paired constructs. Prioritize pairs with no other paired tokens inside.
		const delimiterTokens = new Set([
			_MDTokenType.Backtick,
			_MDTokenType.Tilde,
			_MDTokenType.Asterisk,
			_MDTokenType.Underscore
		]);
		for (let disallowed of [ delimiterTokens, new Set() ]) {
			do {
				anyChanges = false;
				// ``code``
				if (spanMatch = matchPair([ _MDTokenType.Backtick, _MDTokenType.Backtick ], disallowed)) {
					spans.splice(spanMatch.startIndex, spanMatch.toDelete, new _MDCodeSpan(spanMatch.content));
					anyChanges = true;
				}

				// ~~strike~~
				else if (spanMatch = matchPair([ _MDTokenType.Tilde, _MDTokenType.Tilde ], disallowed)) {
					spans.splice(spanMatch.startIndex, spanMatch.toDelete, new _MDStrikethroughSpan(spanMatch.content));
					anyChanges = true;
				}

				// **strong**  __strong__
				else if (spanMatch = (matchPair([ _MDTokenType.Asterisk, _MDTokenType.Asterisk ], disallowed) ||
						matchPair([ _MDTokenType.Underscore, _MDTokenType.Underscore ], disallowed))) {
					spans.splice(spanMatch.startIndex, spanMatch.toDelete, new _MDStrongSpan(spanMatch.content));
					anyChanges = true;
				}

				// `code`
				if (spanMatch = matchPair([ _MDTokenType.Backtick ], disallowed)) {
					spans.splice(spanMatch.startIndex, spanMatch.toDelete, new _MDCodeSpan(spanMatch.content));
					anyChanges = true;
				}

				// ~strike~
				else if (spanMatch = matchPair([ _MDTokenType.Tilde ], disallowed)) {
					spans.splice(spanMatch.startIndex, spanMatch.toDelete, new _MDStrikethroughSpan(spanMatch.content));
					anyChanges = true;
				}

				// *strong*  _strong_
				else if (spanMatch = (matchPair([ _MDTokenType.Asterisk ], disallowed) ||
						matchPair([ _MDTokenType.Underscore ], disallowed))) {
					spans.splice(spanMatch.startIndex, spanMatch.toDelete, new _MDEmphasisSpan(spanMatch.content));
					anyChanges = true;
				}
			} while (anyChanges);
		}
		var lastSpan = null;
		spans = spans.map(function(span) {
			if (span instanceof _MDToken) {
				if (span.type == _MDTokenType.Modifier && lastSpan) {
					span.modifier.applyTo(lastSpan);
					lastSpan = null;
					return new _MDTextSpan('');
				}
				lastSpan = null;
				return new _MDTextSpan(span.original);
			} else if (span instanceof _MDSpan) {
				lastSpan = (span instanceof _MDTextSpan) ? null : span;
				return span;
			} else {
				throw new Error(`Unexpected span type ${span.constructor.name}`);
			}
		});
		return spans;
	}

	/**
	 * Reads the contents of something like a list item
	 * @param {_MDState} state
	 * @param {number} firstLineStartPos
	 * @param {RegExp} stopRegex
	 * @param {Boolean} inList
	 * @returns {_MDBlock}
	 */
	static #readInteriorContent(state, firstLineStartPos, stopRegex, inList=false) {
		var p = state.p;
		var seenBlankLine = false;
		var needsBlocks = false;
		var lines = [];
		var hasNestedList = false;
		var firstNestedListLine = -1;
		while (state.hasLines(1, p)) {
			let line = state.lines[p++];
			if (p == state.p + 1) {
				line = line.substring(firstLineStartPos);
			}
			let isBlank = line.trim().length == 0;
			let isIndented = /^\s+/.exec(line) !== null;
			if (isBlank) {
				seenBlankLine = true;
				lines.push(line.trim());
			} else if (stopRegex && stopRegex.exec(line)) {
				p--;
				break;
			} else if (isIndented) {
				if (seenBlankLine) {
					needsBlocks = true;
				}
				if (inList && /^\s*(?:\*|\+|\-|\d+\.)\s+/.exec(line)) {
					hasNestedList = true;
					if (firstNestedListLine < 0) {
						firstNestedListLine = lines.length;
					}
				}
				lines.push(this.#stripIndent(line));
			} else {
				if (seenBlankLine) {
					p--;
					break;
				}
				lines.push(this.#stripIndent(line));
			}
		}
		while (lines.length > 0 && lines[lines.length - 1].trim().length == 0) {
			lines.pop();
		}
		if (inList && hasNestedList) {
			let parentLines = lines.slice(0, firstNestedListLine);
			let parentContent = this.#readInline(state, parentLines.join("\n"));
			let nestedLines = lines.slice(firstNestedListLine);
			let substate = state.copy(nestedLines);
			let nestedContent = this.#readBlocks(substate);
			state.p = p;
			return new _MDMultiBlock([parentContent].concat(nestedContent));
		}
		if (needsBlocks) {
			let substate = state.copy(lines);
			let blocks = this.#readBlocks(substate);
			state.p = p;
			return new _MDMultiBlock(blocks);
		} else {
			state.p = p;
			return this.#readInline(state, lines.join("\n"));
		}
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readUnderlineHeader(state) {
		var p = state.p;
		if (!state.hasLines(2)) return null;
		var modifier;
		let contentLine = state.lines[p++].trim();
		[contentLine, modifier] = _MDTagModifier.fromLine(contentLine);
		let underLine = state.lines[p++].trim();
		if (contentLine == '') return null;
		if (/^=+$/.exec(underLine)) {
			state.p = p;
			let block = new _MDHeaderBlock(1, this.#readInline(state, contentLine));
			if (modifier) modifier.applyTo(block);
			return block;
		}
		if (/^\-+$/.exec(underLine)) {
			state.p = p;
			let block = new _MDHeaderBlock(2, this.#readInline(state, contentLine));
			if (modifier) modifier.applyTo(block);
			return block;
		}
		return null;
	}

	static #hashHeaderRegex = /^(#{1,6})\s*([^#].*?)\s*\#*\s*$/;  // 1=hashes, 2=content

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readHashHeader(state) {
		var p = state.p;
		let line = state.lines[p++];
		var modifier;
		[line, modifier] = _MDTagModifier.fromLine(line);
		var groups = this.#hashHeaderRegex.exec(line);
		if (groups === null) return null;
		state.p = p;
		let block = new _MDHeaderBlock(groups[1].length, this.#readInline(state, groups[2]));
		if (modifier) modifier.applyTo(block);
		return block;
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readBlockQuote(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) {
			let contentLines = blockquoteLines.map(function(line) {
				return line.substring(1).replace(/^ {0,3}\t?/, '');
			});
			let substate = state.copy(contentLines);
			let quotedBlocks = this.#readBlocks(substate);
			state.p = p;
			return new _MDBlockquoteBlock(quotedBlocks);
		}
		return null;
	}

	static #unorderedListRegex = /^([\*\+\-]\s+)(.*)$/;  // 1=bullet, 2=content
	static #unorderedListItemRegex = /^[\*\+\-]\s+/;

	/**
	 * @param {_MDState} state
	 * @returns {_MDListItemBlock|null}
	 */
	static #readUnorderedListItem(state) {
		var p = state.p;
		let line = state.lines[p];
		let groups = this.#unorderedListRegex.exec(line);
		if (groups === null) return null;
		return new _MDListItemBlock(this.#readInteriorContent(state, groups[1].length, this.#unorderedListItemRegex, true));
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readUnorderedList(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 _MDUnorderedListBlock(items);
	}

	static #orderedListRegex = /^(\d+)(\.\s+)(.*)$/;  // 1=number, 2=dot, 3=content
	static #orderedListItemRegex = /^\d+\.\s+/;

	/**
	 * @param {_MDState} state
	 * @returns {_MDListItemBlock|null}
	 */
	static #readOrderedListItem(state) {
		var p = state.p;
		let line = state.lines[p];
		let groups = this.#orderedListRegex.exec(line);
		if (groups === null) return null;
		let ordinal = parseInt(groups[1]);
		let content = this.#readInteriorContent(state, groups[1].length + groups[2].length, this.#orderedListItemRegex, true);
		return new _MDListItemBlock(content, ordinal);
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readOrderedList(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 _MDOrderedListBlock(items, items[0].ordinal);
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readFencedCodeBlock(state) {
		if (!state.hasLines(2)) return null;
		var p = state.p;
		let openFenceLine = state.lines[p++];
		var modifier;
		[openFenceLine, modifier] = _MDTagModifier.fromLine(openFenceLine);
		if (openFenceLine.trim() != '```') return null;
		var codeLines = [];
		while (state.hasLines(1, p)) {
			let line = state.lines[p++];
			if (line.trim() == '```') {
				state.p = p;
				let block = new _MDCodeBlock(codeLines.join("\n"));
				if (modifier) modifier.applyTo(block);
				return block;
			}
			codeLines.push(line);
		}
		return null;
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readIndentedCodeBlock(state) {
		var p = state.p;
		var codeLines = [];
		while (state.hasLines(1, p)) {
			let line = state.lines[p++];
			if (this.#countIndents(line, true) < 1) {
				p--;
				break;
			}
			codeLines.push(this.#stripIndent(line));
		}
		if (codeLines.length == 0) return null;
		state.p = p;
		return new _MDCodeBlock(codeLines.join("\n"));
	}

	static #horizontalRuleRegex = /^\s*(?:\-(?:\s*\-){2,}|\*(?:\s*\*){2,})\s*$/;

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readHorizontalRule(state) {
		var p = state.p;
		let line = state.lines[p++];
		var modifier;
		[line, modifier] = _MDTagModifier.fromLine(line);
		if (this.#horizontalRuleRegex.exec(line)) {
			state.p = p;
			let block = new _MDHorizontalRuleBlock();
			if (modifier) modifier.applyTo(block);
			return block;
		}
		return null;
	}

	/**
	 * @param {_MDState} state
	 * @param {Boolean} isHeader
	 * @return {_MDTableRowBlock|null}
	 */
	static #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 = Markdown.#readInline(state, token);
			return isHeader ? new _MDTableHeaderCellBlock(content) : new _MDTableCellBlock(content);
		});
		state.p = p;
		return new _MDTableRowBlock(cells);
	}

	/**
	 * @param {String} line
	 * @returns {_MDHAlign[]}
	 */
	static #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('|').map(function(token) {
			token = token.trim();
			if (token.startsWith(':')) {
				if (token.endsWith(':')) {
					return _MDHAlign.Center;
				}
				return _MDHAlign.Left;
			} else if (token.endsWith(':')) {
				return _MDHAlign.Right;
			}
			return null;
		});
	}

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

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readTable(state) {
		if (!state.hasLines(2)) return null;
		let startP = state.p;
		let firstLine = state.lines[startP];
		var ignore, modifier;
		[ignore, modifier] = _MDTagModifier.fromLine(firstLine);
		let headerRow = this.#readTableRow(state, true);
		if (headerRow === null) {
			state.p = startP;
			return null;
		}
		let dividerLine = state.lines[state.p++];
		let dividerGroups = this.#tableDividerRegex.exec(dividerLine);
		if (dividerGroups === null) {
			state.p = startP;
			return null;
		}
		let columnAlignments = this.#parseColumnAlignments(dividerLine);
		headerRow.applyAlignments(columnAlignments);
		var bodyRows = [];
		while (state.hasLines(1)) {
			let row = this.#readTableRow(state, false);
			if (row === null) break;
			row.applyAlignments(columnAlignments);
			bodyRows.push(row);
		}
		let table = new _MDTableBlock(headerRow, bodyRows);
		if (modifier) modifier.applyTo(table);
		return table;
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readDefinitionList(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) {
				p--;
				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+(.*)$/.exec(line)) {
				return new _MDDefinitionDefinitionBlock(Markdown.#readInline(state, groups[1]));
			} else {
				return new _MDDefinitionTermBlock(Markdown.#readInline(state, line));
			}
		});
		state.p = p;
		return new _MDDefinitionListBlock(blocks);
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readFootnoteDef(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;
			}
		}
		state.p = p;
		let content = this.#readInline(state, def);
		state.defineFootnote(symbol, content);
		state.p = p;
		return new _MDMultiBlock([]);
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readAbbreviationDef(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];
		state.defineAbbreviation(abbrev, def);
		state.p = p;
		return new _MDMultiBlock([]);
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readURLDef(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 _MDInlineBlock([]);
	}

	/**
	 * @param {_MDState} state
	 * @returns {_MDBlock|null}
	 */
	static #readParagraph(state) {
		var paragraphLines = [];
		var p = state.p;
		while (p < state.lines.length) {
			let line = state.lines[p++];
			if (line.trim().length == 0) {
				break;
			}
			paragraphLines.push(line);
		}
		if (paragraphLines.length > 0) {
			state.p = p;
			let content = paragraphLines.join("\n");
			return new _MDParagraphBlock(this.#readInline(state, content));
		}
		return null;
	}

	/**
	 * @param {String} html
	 * @param {_MDState} state
	 * @returns {String}
	 */
	static #postProcessFootnotes(html, state) {
		let footnotes = state.footnotes;
		if (Object.keys(footnotes).length == 0) return html;
		var symbolOrder = [];
		var footnoteOccurrences = {};
		var footnoteIndex = 0;
		html = html.replace(/<!--FNREF:{(\d+)}-->/g, function(match, symbol) {
			footnoteIndex++;
			symbol = symbol.toLowerCase();
			if (!symbolOrder.includes(symbol)) {
				symbolOrder.push(symbol);
			}
			var occurrences = footnoteOccurrences[symbol] || [];
			occurrences.push(footnoteIndex);
			footnoteOccurrences[symbol] = occurrences;
			return `<sup id="footnoteref_${footnoteIndex}"><a href="#footnote_${symbol}">${symbol}</a></sup>`;
		});
		if (footnoteIndex == 0) return html;
		html += '<div class="footnotes"><hr/>';
		html += '<ol>';
		for (const symbol of symbolOrder) {
			let content = state.footnotes[symbol];
			if (!content) continue;
			html += `<li value="${symbol}" id="footnote_${symbol}">${content.toHTML(state)}`;
			for (const ref of footnoteOccurrences[symbol]) {
				html += ` <a href="#footnoteref_${ref}" class="footnote-backref" role="doc-backlink">↩︎</a>`;
			}
			html += `</li>\n`;
		}
		html += '</ol>';
		html += '</div>';
		return html;
	}

	/**
	 * @param  {String} markdown
	 * @returns {String} HTML
	 */
	static toHTML(markdown) {
		var state = new _MDState();
		let lines = markdown.split(/(?:\n|\r|\r\n)/);
		state.lines = lines;
		let blocks = this.#readBlocks(state);
		let html = _MDBlock.toHTML(blocks, state);
		html = this.#postProcessFootnotes(html, state);
		return html;
	}
}