MZC/KnowMathBot/frontend/node_modules/mdast-util-from-markdown/lib/index.js

/**
 * @import {
 *   Break,
 *   Blockquote,
 *   Code,
 *   Definition,
 *   Emphasis,
 *   Heading,
 *   Html,
 *   Image,
 *   InlineCode,
 *   Link,
 *   ListItem,
 *   List,
 *   Nodes,
 *   Paragraph,
 *   PhrasingContent,
 *   ReferenceType,
 *   Root,
 *   Strong,
 *   Text,
 *   ThematicBreak
 * } from 'mdast'
 * @import {
 *   Encoding,
 *   Event,
 *   Token,
 *   Value
 * } from 'micromark-util-types'
 * @import {Point} from 'unist'
 * @import {
 *   CompileContext,
 *   CompileData,
 *   Config,
 *   Extension,
 *   Handle,
 *   OnEnterError,
 *   Options
 * } from './types.js'
 */

import { toString } from 'mdast-util-to-string';
import { parse, postprocess, preprocess } from 'micromark';
import { decodeNumericCharacterReference } from 'micromark-util-decode-numeric-character-reference';
import { decodeString } from 'micromark-util-decode-string';
import { normalizeIdentifier } from 'micromark-util-normalize-identifier';
import { decodeNamedCharacterReference } from 'decode-named-character-reference';
import { stringifyPosition } from 'unist-util-stringify-position';
const own = {}.hasOwnProperty;

/**
 * Turn markdown into a syntax tree.
 *
 * @overload
 * @param {Value} value
 * @param {Encoding | null | undefined} [encoding]
 * @param {Options | null | undefined} [options]
 * @returns {Root}
 *
 * @overload
 * @param {Value} value
 * @param {Options | null | undefined} [options]
 * @returns {Root}
 *
 * @param {Value} value
 *   Markdown to parse.
 * @param {Encoding | Options | null | undefined} [encoding]
 *   Character encoding for when `value` is `Buffer`.
 * @param {Options | null | undefined} [options]
 *   Configuration.
 * @returns {Root}
 *   mdast tree.
 */
export function fromMarkdown(value, encoding, options) {
  if (encoding && typeof encoding === 'object') {
    options = encoding;
    encoding = undefined;
  }
  return compiler(options)(postprocess(parse(options).document().write(preprocess()(value, encoding, true))));
}

/**
 * Note this compiler only understand complete buffering, not streaming.
 *
 * @param {Options | null | undefined} [options]
 */
function compiler(options) {
  /** @type {Config} */
  const config = {
    transforms: [],
    canContainEols: ['emphasis', 'fragment', 'heading', 'paragraph', 'strong'],
    enter: {
      autolink: opener(link),
      autolinkProtocol: onenterdata,
      autolinkEmail: onenterdata,
      atxHeading: opener(heading),
      blockQuote: opener(blockQuote),
      characterEscape: onenterdata,
      characterReference: onenterdata,
      codeFenced: opener(codeFlow),
      codeFencedFenceInfo: buffer,
      codeFencedFenceMeta: buffer,
      codeIndented: opener(codeFlow, buffer),
      codeText: opener(codeText, buffer),
      codeTextData: onenterdata,
      data: onenterdata,
      codeFlowValue: onenterdata,
      definition: opener(definition),
      definitionDestinationString: buffer,
      definitionLabelString: buffer,
      definitionTitleString: buffer,
      emphasis: opener(emphasis),
      hardBreakEscape: opener(hardBreak),
      hardBreakTrailing: opener(hardBreak),
      htmlFlow: opener(html, buffer),
      htmlFlowData: onenterdata,
      htmlText: opener(html, buffer),
      htmlTextData: onenterdata,
      image: opener(image),
      label: buffer,
      link: opener(link),
      listItem: opener(listItem),
      listItemValue: onenterlistitemvalue,
      listOrdered: opener(list, onenterlistordered),
      listUnordered: opener(list),
      paragraph: opener(paragraph),
      reference: onenterreference,
      referenceString: buffer,
      resourceDestinationString: buffer,
      resourceTitleString: buffer,
      setextHeading: opener(heading),
      strong: opener(strong),
      thematicBreak: opener(thematicBreak)
    },
    exit: {
      atxHeading: closer(),
      atxHeadingSequence: onexitatxheadingsequence,
      autolink: closer(),
      autolinkEmail: onexitautolinkemail,
      autolinkProtocol: onexitautolinkprotocol,
      blockQuote: closer(),
      characterEscapeValue: onexitdata,
      characterReferenceMarkerHexadecimal: onexitcharacterreferencemarker,
      characterReferenceMarkerNumeric: onexitcharacterreferencemarker,
      characterReferenceValue: onexitcharacterreferencevalue,
      characterReference: onexitcharacterreference,
      codeFenced: closer(onexitcodefenced),
      codeFencedFence: onexitcodefencedfence,
      codeFencedFenceInfo: onexitcodefencedfenceinfo,
      codeFencedFenceMeta: onexitcodefencedfencemeta,
      codeFlowValue: onexitdata,
      codeIndented: closer(onexitcodeindented),
      codeText: closer(onexitcodetext),
      codeTextData: onexitdata,
      data: onexitdata,
      definition: closer(),
      definitionDestinationString: onexitdefinitiondestinationstring,
      definitionLabelString: onexitdefinitionlabelstring,
      definitionTitleString: onexitdefinitiontitlestring,
      emphasis: closer(),
      hardBreakEscape: closer(onexithardbreak),
      hardBreakTrailing: closer(onexithardbreak),
      htmlFlow: closer(onexithtmlflow),
      htmlFlowData: onexitdata,
      htmlText: closer(onexithtmltext),
      htmlTextData: onexitdata,
      image: closer(onexitimage),
      label: onexitlabel,
      labelText: onexitlabeltext,
      lineEnding: onexitlineending,
      link: closer(onexitlink),
      listItem: closer(),
      listOrdered: closer(),
      listUnordered: closer(),
      paragraph: closer(),
      referenceString: onexitreferencestring,
      resourceDestinationString: onexitresourcedestinationstring,
      resourceTitleString: onexitresourcetitlestring,
      resource: onexitresource,
      setextHeading: closer(onexitsetextheading),
      setextHeadingLineSequence: onexitsetextheadinglinesequence,
      setextHeadingText: onexitsetextheadingtext,
      strong: closer(),
      thematicBreak: closer()
    }
  };
  configure(config, (options || {}).mdastExtensions || []);

  /** @type {CompileData} */
  const data = {};
  return compile;

  /**
   * Turn micromark events into an mdast tree.
   *
   * @param {Array<Event>} events
   *   Events.
   * @returns {Root}
   *   mdast tree.
   */
  function compile(events) {
    /** @type {Root} */
    let tree = {
      type: 'root',
      children: []
    };
    /** @type {Omit<CompileContext, 'sliceSerialize'>} */
    const context = {
      stack: [tree],
      tokenStack: [],
      config,
      enter,
      exit,
      buffer,
      resume,
      data
    };
    /** @type {Array<number>} */
    const listStack = [];
    let index = -1;
    while (++index < events.length) {
      // We preprocess lists to add `listItem` tokens, and to infer whether
      // items the list itself are spread out.
      if (events[index][1].type === "listOrdered" || events[index][1].type === "listUnordered") {
        if (events[index][0] === 'enter') {
          listStack.push(index);
        } else {
          const tail = listStack.pop();
          index = prepareList(events, tail, index);
        }
      }
    }
    index = -1;
    while (++index < events.length) {
      const handler = config[events[index][0]];
      if (own.call(handler, events[index][1].type)) {
        handler[events[index][1].type].call(Object.assign({
          sliceSerialize: events[index][2].sliceSerialize
        }, context), events[index][1]);
      }
    }

    // Handle tokens still being open.
    if (context.tokenStack.length > 0) {
      const tail = context.tokenStack[context.tokenStack.length - 1];
      const handler = tail[1] || defaultOnError;
      handler.call(context, undefined, tail[0]);
    }

    // Figure out `root` position.
    tree.position = {
      start: point(events.length > 0 ? events[0][1].start : {
        line: 1,
        column: 1,
        offset: 0
      }),
      end: point(events.length > 0 ? events[events.length - 2][1].end : {
        line: 1,
        column: 1,
        offset: 0
      })
    };

    // Call transforms.
    index = -1;
    while (++index < config.transforms.length) {
      tree = config.transforms[index](tree) || tree;
    }
    return tree;
  }

  /**
   * @param {Array<Event>} events
   * @param {number} start
   * @param {number} length
   * @returns {number}
   */
  function prepareList(events, start, length) {
    let index = start - 1;
    let containerBalance = -1;
    let listSpread = false;
    /** @type {Token | undefined} */
    let listItem;
    /** @type {number | undefined} */
    let lineIndex;
    /** @type {number | undefined} */
    let firstBlankLineIndex;
    /** @type {boolean | undefined} */
    let atMarker;
    while (++index <= length) {
      const event = events[index];
      switch (event[1].type) {
        case "listUnordered":
        case "listOrdered":
        case "blockQuote":
          {
            if (event[0] === 'enter') {
              containerBalance++;
            } else {
              containerBalance--;
            }
            atMarker = undefined;
            break;
          }
        case "lineEndingBlank":
          {
            if (event[0] === 'enter') {
              if (listItem && !atMarker && !containerBalance && !firstBlankLineIndex) {
                firstBlankLineIndex = index;
              }
              atMarker = undefined;
            }
            break;
          }
        case "linePrefix":
        case "listItemValue":
        case "listItemMarker":
        case "listItemPrefix":
        case "listItemPrefixWhitespace":
          {
            // Empty.

            break;
          }
        default:
          {
            atMarker = undefined;
          }
      }
      if (!containerBalance && event[0] === 'enter' && event[1].type === "listItemPrefix" || containerBalance === -1 && event[0] === 'exit' && (event[1].type === "listUnordered" || event[1].type === "listOrdered")) {
        if (listItem) {
          let tailIndex = index;
          lineIndex = undefined;
          while (tailIndex--) {
            const tailEvent = events[tailIndex];
            if (tailEvent[1].type === "lineEnding" || tailEvent[1].type === "lineEndingBlank") {
              if (tailEvent[0] === 'exit') continue;
              if (lineIndex) {
                events[lineIndex][1].type = "lineEndingBlank";
                listSpread = true;
              }
              tailEvent[1].type = "lineEnding";
              lineIndex = tailIndex;
            } else if (tailEvent[1].type === "linePrefix" || tailEvent[1].type === "blockQuotePrefix" || tailEvent[1].type === "blockQuotePrefixWhitespace" || tailEvent[1].type === "blockQuoteMarker" || tailEvent[1].type === "listItemIndent") {
              // Empty
            } else {
              break;
            }
          }
          if (firstBlankLineIndex && (!lineIndex || firstBlankLineIndex < lineIndex)) {
            listItem._spread = true;
          }

          // Fix position.
          listItem.end = Object.assign({}, lineIndex ? events[lineIndex][1].start : event[1].end);
          events.splice(lineIndex || index, 0, ['exit', listItem, event[2]]);
          index++;
          length++;
        }

        // Create a new list item.
        if (event[1].type === "listItemPrefix") {
          /** @type {Token} */
          const item = {
            type: 'listItem',
            _spread: false,
            start: Object.assign({}, event[1].start),
            // @ts-expect-error: we’ll add `end` in a second.
            end: undefined
          };
          listItem = item;
          events.splice(index, 0, ['enter', item, event[2]]);
          index++;
          length++;
          firstBlankLineIndex = undefined;
          atMarker = true;
        }
      }
    }
    events[start][1]._spread = listSpread;
    return length;
  }

  /**
   * Create an opener handle.
   *
   * @param {(token: Token) => Nodes} create
   *   Create a node.
   * @param {Handle | undefined} [and]
   *   Optional function to also run.
   * @returns {Handle}
   *   Handle.
   */
  function opener(create, and) {
    return open;

    /**
     * @this {CompileContext}
     * @param {Token} token
     * @returns {undefined}
     */
    function open(token) {
      enter.call(this, create(token), token);
      if (and) and.call(this, token);
    }
  }

  /**
   * @type {CompileContext['buffer']}
   */
  function buffer() {
    this.stack.push({
      type: 'fragment',
      children: []
    });
  }

  /**
   * @type {CompileContext['enter']}
   */
  function enter(node, token, errorHandler) {
    const parent = this.stack[this.stack.length - 1];
    /** @type {Array<Nodes>} */
    const siblings = parent.children;
    siblings.push(node);
    this.stack.push(node);
    this.tokenStack.push([token, errorHandler || undefined]);
    node.position = {
      start: point(token.start),
      // @ts-expect-error: `end` will be patched later.
      end: undefined
    };
  }

  /**
   * Create a closer handle.
   *
   * @param {Handle | undefined} [and]
   *   Optional function to also run.
   * @returns {Handle}
   *   Handle.
   */
  function closer(and) {
    return close;

    /**
     * @this {CompileContext}
     * @param {Token} token
     * @returns {undefined}
     */
    function close(token) {
      if (and) and.call(this, token);
      exit.call(this, token);
    }
  }

  /**
   * @type {CompileContext['exit']}
   */
  function exit(token, onExitError) {
    const node = this.stack.pop();
    const open = this.tokenStack.pop();
    if (!open) {
      throw new Error('Cannot close `' + token.type + '` (' + stringifyPosition({
        start: token.start,
        end: token.end
      }) + '): it’s not open');
    } else if (open[0].type !== token.type) {
      if (onExitError) {
        onExitError.call(this, token, open[0]);
      } else {
        const handler = open[1] || defaultOnError;
        handler.call(this, token, open[0]);
      }
    }
    node.position.end = point(token.end);
  }

  /**
   * @type {CompileContext['resume']}
   */
  function resume() {
    return toString(this.stack.pop());
  }

  //
  // Handlers.
  //

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onenterlistordered() {
    this.data.expectingFirstListItemValue = true;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onenterlistitemvalue(token) {
    if (this.data.expectingFirstListItemValue) {
      const ancestor = this.stack[this.stack.length - 2];
      ancestor.start = Number.parseInt(this.sliceSerialize(token), 10);
      this.data.expectingFirstListItemValue = undefined;
    }
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitcodefencedfenceinfo() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.lang = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitcodefencedfencemeta() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.meta = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitcodefencedfence() {
    // Exit if this is the closing fence.
    if (this.data.flowCodeInside) return;
    this.buffer();
    this.data.flowCodeInside = true;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitcodefenced() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.value = data.replace(/^(\r?\n|\r)|(\r?\n|\r)$/g, '');
    this.data.flowCodeInside = undefined;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitcodeindented() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.value = data.replace(/(\r?\n|\r)$/g, '');
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitdefinitionlabelstring(token) {
    const label = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.label = label;
    node.identifier = normalizeIdentifier(this.sliceSerialize(token)).toLowerCase();
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitdefinitiontitlestring() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.title = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitdefinitiondestinationstring() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.url = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitatxheadingsequence(token) {
    const node = this.stack[this.stack.length - 1];
    if (!node.depth) {
      const depth = this.sliceSerialize(token).length;
      node.depth = depth;
    }
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitsetextheadingtext() {
    this.data.setextHeadingSlurpLineEnding = true;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitsetextheadinglinesequence(token) {
    const node = this.stack[this.stack.length - 1];
    node.depth = this.sliceSerialize(token).codePointAt(0) === 61 ? 1 : 2;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitsetextheading() {
    this.data.setextHeadingSlurpLineEnding = undefined;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onenterdata(token) {
    const node = this.stack[this.stack.length - 1];
    /** @type {Array<Nodes>} */
    const siblings = node.children;
    let tail = siblings[siblings.length - 1];
    if (!tail || tail.type !== 'text') {
      // Add a new text node.
      tail = text();
      tail.position = {
        start: point(token.start),
        // @ts-expect-error: we’ll add `end` later.
        end: undefined
      };
      siblings.push(tail);
    }
    this.stack.push(tail);
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitdata(token) {
    const tail = this.stack.pop();
    tail.value += this.sliceSerialize(token);
    tail.position.end = point(token.end);
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitlineending(token) {
    const context = this.stack[this.stack.length - 1];
    // If we’re at a hard break, include the line ending in there.
    if (this.data.atHardBreak) {
      const tail = context.children[context.children.length - 1];
      tail.position.end = point(token.end);
      this.data.atHardBreak = undefined;
      return;
    }
    if (!this.data.setextHeadingSlurpLineEnding && config.canContainEols.includes(context.type)) {
      onenterdata.call(this, token);
      onexitdata.call(this, token);
    }
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexithardbreak() {
    this.data.atHardBreak = true;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexithtmlflow() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.value = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexithtmltext() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.value = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitcodetext() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.value = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitlink() {
    const node = this.stack[this.stack.length - 1];
    // Note: there are also `identifier` and `label` fields on this link node!
    // These are used / cleaned here.

    // To do: clean.
    if (this.data.inReference) {
      /** @type {ReferenceType} */
      const referenceType = this.data.referenceType || 'shortcut';
      node.type += 'Reference';
      // @ts-expect-error: mutate.
      node.referenceType = referenceType;
      // @ts-expect-error: mutate.
      delete node.url;
      delete node.title;
    } else {
      // @ts-expect-error: mutate.
      delete node.identifier;
      // @ts-expect-error: mutate.
      delete node.label;
    }
    this.data.referenceType = undefined;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitimage() {
    const node = this.stack[this.stack.length - 1];
    // Note: there are also `identifier` and `label` fields on this link node!
    // These are used / cleaned here.

    // To do: clean.
    if (this.data.inReference) {
      /** @type {ReferenceType} */
      const referenceType = this.data.referenceType || 'shortcut';
      node.type += 'Reference';
      // @ts-expect-error: mutate.
      node.referenceType = referenceType;
      // @ts-expect-error: mutate.
      delete node.url;
      delete node.title;
    } else {
      // @ts-expect-error: mutate.
      delete node.identifier;
      // @ts-expect-error: mutate.
      delete node.label;
    }
    this.data.referenceType = undefined;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitlabeltext(token) {
    const string = this.sliceSerialize(token);
    const ancestor = this.stack[this.stack.length - 2];
    // @ts-expect-error: stash this on the node, as it might become a reference
    // later.
    ancestor.label = decodeString(string);
    // @ts-expect-error: same as above.
    ancestor.identifier = normalizeIdentifier(string).toLowerCase();
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitlabel() {
    const fragment = this.stack[this.stack.length - 1];
    const value = this.resume();
    const node = this.stack[this.stack.length - 1];
    // Assume a reference.
    this.data.inReference = true;
    if (node.type === 'link') {
      /** @type {Array<PhrasingContent>} */
      const children = fragment.children;
      node.children = children;
    } else {
      node.alt = value;
    }
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitresourcedestinationstring() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.url = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitresourcetitlestring() {
    const data = this.resume();
    const node = this.stack[this.stack.length - 1];
    node.title = data;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitresource() {
    this.data.inReference = undefined;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onenterreference() {
    this.data.referenceType = 'collapsed';
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitreferencestring(token) {
    const label = this.resume();
    const node = this.stack[this.stack.length - 1];
    // @ts-expect-error: stash this on the node, as it might become a reference
    // later.
    node.label = label;
    // @ts-expect-error: same as above.
    node.identifier = normalizeIdentifier(this.sliceSerialize(token)).toLowerCase();
    this.data.referenceType = 'full';
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */

  function onexitcharacterreferencemarker(token) {
    this.data.characterReferenceType = token.type;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitcharacterreferencevalue(token) {
    const data = this.sliceSerialize(token);
    const type = this.data.characterReferenceType;
    /** @type {string} */
    let value;
    if (type) {
      value = decodeNumericCharacterReference(data, type === "characterReferenceMarkerNumeric" ? 10 : 16);
      this.data.characterReferenceType = undefined;
    } else {
      const result = decodeNamedCharacterReference(data);
      value = result;
    }
    const tail = this.stack[this.stack.length - 1];
    tail.value += value;
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitcharacterreference(token) {
    const tail = this.stack.pop();
    tail.position.end = point(token.end);
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitautolinkprotocol(token) {
    onexitdata.call(this, token);
    const node = this.stack[this.stack.length - 1];
    node.url = this.sliceSerialize(token);
  }

  /**
   * @this {CompileContext}
   * @type {Handle}
   */
  function onexitautolinkemail(token) {
    onexitdata.call(this, token);
    const node = this.stack[this.stack.length - 1];
    node.url = 'mailto:' + this.sliceSerialize(token);
  }

  //
  // Creaters.
  //

  /** @returns {Blockquote} */
  function blockQuote() {
    return {
      type: 'blockquote',
      children: []
    };
  }

  /** @returns {Code} */
  function codeFlow() {
    return {
      type: 'code',
      lang: null,
      meta: null,
      value: ''
    };
  }

  /** @returns {InlineCode} */
  function codeText() {
    return {
      type: 'inlineCode',
      value: ''
    };
  }

  /** @returns {Definition} */
  function definition() {
    return {
      type: 'definition',
      identifier: '',
      label: null,
      title: null,
      url: ''
    };
  }

  /** @returns {Emphasis} */
  function emphasis() {
    return {
      type: 'emphasis',
      children: []
    };
  }

  /** @returns {Heading} */
  function heading() {
    return {
      type: 'heading',
      // @ts-expect-error `depth` will be set later.
      depth: 0,
      children: []
    };
  }

  /** @returns {Break} */
  function hardBreak() {
    return {
      type: 'break'
    };
  }

  /** @returns {Html} */
  function html() {
    return {
      type: 'html',
      value: ''
    };
  }

  /** @returns {Image} */
  function image() {
    return {
      type: 'image',
      title: null,
      url: '',
      alt: null
    };
  }

  /** @returns {Link} */
  function link() {
    return {
      type: 'link',
      title: null,
      url: '',
      children: []
    };
  }

  /**
   * @param {Token} token
   * @returns {List}
   */
  function list(token) {
    return {
      type: 'list',
      ordered: token.type === 'listOrdered',
      start: null,
      spread: token._spread,
      children: []
    };
  }

  /**
   * @param {Token} token
   * @returns {ListItem}
   */
  function listItem(token) {
    return {
      type: 'listItem',
      spread: token._spread,
      checked: null,
      children: []
    };
  }

  /** @returns {Paragraph} */
  function paragraph() {
    return {
      type: 'paragraph',
      children: []
    };
  }

  /** @returns {Strong} */
  function strong() {
    return {
      type: 'strong',
      children: []
    };
  }

  /** @returns {Text} */
  function text() {
    return {
      type: 'text',
      value: ''
    };
  }

  /** @returns {ThematicBreak} */
  function thematicBreak() {
    return {
      type: 'thematicBreak'
    };
  }
}

/**
 * Copy a point-like value.
 *
 * @param {Point} d
 *   Point-like value.
 * @returns {Point}
 *   unist point.
 */
function point(d) {
  return {
    line: d.line,
    column: d.column,
    offset: d.offset
  };
}

/**
 * @param {Config} combined
 * @param {Array<Array<Extension> | Extension>} extensions
 * @returns {undefined}
 */
function configure(combined, extensions) {
  let index = -1;
  while (++index < extensions.length) {
    const value = extensions[index];
    if (Array.isArray(value)) {
      configure(combined, value);
    } else {
      extension(combined, value);
    }
  }
}

/**
 * @param {Config} combined
 * @param {Extension} extension
 * @returns {undefined}
 */
function extension(combined, extension) {
  /** @type {keyof Extension} */
  let key;
  for (key in extension) {
    if (own.call(extension, key)) {
      switch (key) {
        case 'canContainEols':
          {
            const right = extension[key];
            if (right) {
              combined[key].push(...right);
            }
            break;
          }
        case 'transforms':
          {
            const right = extension[key];
            if (right) {
              combined[key].push(...right);
            }
            break;
          }
        case 'enter':
        case 'exit':
          {
            const right = extension[key];
            if (right) {
              Object.assign(combined[key], right);
            }
            break;
          }
        // No default
      }
    }
  }
}

/** @type {OnEnterError} */
function defaultOnError(left, right) {
  if (left) {
    throw new Error('Cannot close `' + left.type + '` (' + stringifyPosition({
      start: left.start,
      end: left.end
    }) + '): a different token (`' + right.type + '`, ' + stringifyPosition({
      start: right.start,
      end: right.end
    }) + ') is open');
  } else {
    throw new Error('Cannot close document, a token (`' + right.type + '`, ' + stringifyPosition({
      start: right.start,
      end: right.end
    }) + ') is still open');
  }
}