to_markdown.ts

import {Node, Mark} from "prosemirror-model"

type MarkSerializerSpec = {
  /// The string that should appear before a piece of content marked
  /// by this mark, either directly or as a function that returns an
  /// appropriate string.
  open: string | ((state: MarkdownSerializerState, mark: Mark, parent: Node, index: number) => string),
  /// The string that should appear after a piece of content marked by
  /// this mark.
  close: string | ((state: MarkdownSerializerState, mark: Mark, parent: Node, index: number) => string),
  /// When `true`, this indicates that the order in which the mark's
  /// opening and closing syntax appears relative to other mixable
  /// marks can be varied. (For example, you can say `**a *b***` and
  /// `*a **b***`, but not `` `a *b*` ``.)
  mixable?: boolean,
  /// When enabled, causes the serializer to move enclosing whitespace
  /// from inside the marks to outside the marks. This is necessary
  /// for emphasis marks as CommonMark does not permit enclosing
  /// whitespace inside emphasis marks, see:
  /// http:///spec.commonmark.org/0.26/#example-330
  expelEnclosingWhitespace?: boolean,
  /// Can be set to `false` to disable character escaping in a mark. A
  /// non-escaping mark has to have the highest precedence (must
  /// always be the innermost mark).
  escape?: boolean
}

const blankMark: MarkSerializerSpec = {open: "", close: "", mixable: true}

/// A specification for serializing a ProseMirror document as
/// Markdown/CommonMark text.
export class MarkdownSerializer {
  /// Construct a serializer with the given configuration. The `nodes`
  /// object should map node names in a given schema to function that
  /// take a serializer state and such a node, and serialize the node.
  constructor(
    /// The node serializer functions for this serializer.
    readonly nodes: {[node: string]: (state: MarkdownSerializerState, node: Node, parent: Node, index: number) => void},
    /// The mark serializer info.
    readonly marks: {[mark: string]: MarkSerializerSpec},
    readonly options: {
      /// Extra characters can be added for escaping. This is passed
      /// directly to String.replace(), and the matching characters are
      /// preceded by a backslash.
      escapeExtraCharacters?: RegExp,
      /// Specify the node name of hard breaks.
      /// Defaults to "hard_break"
      hardBreakNodeName?: string,
      /// By default, the serializer raises an error when it finds a
      /// node or mark type for which no serializer is defined. Set
      /// this to `false` to make it just ignore such elements,
      /// rendering only their content.
      strict?: boolean
    } = {}
  ) {}

  /// Serialize the content of the given node to
  /// [CommonMark](http://commonmark.org/).
  serialize(content: Node, options: {
    /// Whether to render lists in a tight style. This can be overridden
    /// on a node level by specifying a tight attribute on the node.
    /// Defaults to false.
    tightLists?: boolean
  } = {}) {
    options = Object.assign({}, this.options, options)
    let state = new MarkdownSerializerState(this.nodes, this.marks, options)
    state.renderContent(content)
    return state.out
  }
}

/// A serializer for the [basic schema](#schema).
export const defaultMarkdownSerializer = new MarkdownSerializer({
  blockquote(state, node) {
    state.wrapBlock("> ", null, node, () => state.renderContent(node))
  },
  code_block(state, node) {
    // Make sure the front matter fences are longer than any dash sequence within it
    const backticks = node.textContent.match(/`{3,}/gm)
    const fence = backticks ? (backticks.sort().slice(-1)[0] + "`") : "```"

    state.write(fence + (node.attrs.params || "") + "\n")
    state.text(node.textContent, false)
    // Add a newline to the current content before adding closing marker
    state.write("\n")
    state.write(fence)
    state.closeBlock(node)
  },
  heading(state, node) {
    state.write(state.repeat("#", node.attrs.level) + " ")
    state.renderInline(node, false)
    state.closeBlock(node)
  },
  horizontal_rule(state, node) {
    state.write(node.attrs.markup || "---")
    state.closeBlock(node)
  },
  bullet_list(state, node) {
    state.renderList(node, "  ", () => (node.attrs.bullet || "*") + " ")
  },
  ordered_list(state, node) {
    let start = node.attrs.order || 1
    let maxW = String(start + node.childCount - 1).length
    let space = state.repeat(" ", maxW + 2)
    state.renderList(node, space, i => {
      let nStr = String(start + i)
      return state.repeat(" ", maxW - nStr.length) + nStr + ". "
    })
  },
  list_item(state, node) {
    state.renderContent(node)
  },
  paragraph(state, node) {
    state.renderInline(node)
    state.closeBlock(node)
  },

  image(state, node) {
    state.write("![" + state.esc(node.attrs.alt || "") + "](" + node.attrs.src.replace(/[\(\)]/g, "\\$&") +
                (node.attrs.title ? ' "' + node.attrs.title.replace(/"/g, '\\"') + '"' : "") + ")")
  },
  hard_break(state, node, parent, index) {
    for (let i = index + 1; i < parent.childCount; i++)
      if (parent.child(i).type != node.type) {
        state.write("\\\n")
        return
      }
  },
  text(state, node) {
    state.text(node.text!, !state.inAutolink)
  }
}, {
  em: {open: "*", close: "*", mixable: true, expelEnclosingWhitespace: true},
  strong: {open: "**", close: "**", mixable: true, expelEnclosingWhitespace: true},
  link: {
    open(state, mark, parent, index) {
      state.inAutolink = isPlainURL(mark, parent, index)
      return state.inAutolink ? "<" : "["
    },
    close(state, mark, parent, index) {
      let {inAutolink} = state
      state.inAutolink = undefined
      return inAutolink ? ">"
        : "](" + mark.attrs.href.replace(/[\(\)"]/g, "\\$&") + (mark.attrs.title ? ` "${mark.attrs.title.replace(/"/g, '\\"')}"` : "") + ")"
    },
    mixable: true
  },
  code: {open(_state, _mark, parent, index) { return backticksFor(parent.child(index), -1) },
         close(_state, _mark, parent, index) { return backticksFor(parent.child(index - 1), 1) },
         escape: false}
})

function backticksFor(node: Node, side: number) {
  let ticks = /`+/g, m, len = 0
  if (node.isText) while (m = ticks.exec(node.text!)) len = Math.max(len, m[0].length)
  let result = len > 0 && side > 0 ? " `" : "`"
  for (let i = 0; i < len; i++) result += "`"
  if (len > 0 && side < 0) result += " "
  return result
}

function isPlainURL(link: Mark, parent: Node, index: number) {
  if (link.attrs.title || !/^\w+:/.test(link.attrs.href)) return false
  let content = parent.child(index)
  if (!content.isText || content.text != link.attrs.href || content.marks[content.marks.length - 1] != link) return false
  return index == parent.childCount - 1 || !link.isInSet(parent.child(index + 1).marks)
}

/// This is an object used to track state and expose
/// methods related to markdown serialization. Instances are passed to
/// node and mark serialization methods (see `toMarkdown`).
export class MarkdownSerializerState {
  /// @internal
  delim: string = ""
  /// @internal
  out: string = ""
  /// @internal
  closed: Node | null = null
  /// @internal
  inAutolink: boolean | undefined = undefined
  /// @internal
  atBlockStart: boolean = false
  /// @internal
  inTightList: boolean = false

  /// @internal
  constructor(
    /// @internal
    readonly nodes: {[node: string]: (state: MarkdownSerializerState, node: Node, parent: Node, index: number) => void},
    /// @internal
    readonly marks: {[mark: string]: MarkSerializerSpec},
    /// The options passed to the serializer.
    readonly options: {tightLists?: boolean, escapeExtraCharacters?: RegExp, hardBreakNodeName?: string, strict?: boolean}
  ) {
    if (typeof this.options.tightLists == "undefined")
      this.options.tightLists = false
    if (typeof this.options.hardBreakNodeName == "undefined")
      this.options.hardBreakNodeName = "hard_break"
  }

  /// @internal
  flushClose(size: number = 2) {
    if (this.closed) {
      if (!this.atBlank()) this.out += "\n"
      if (size > 1) {
        let delimMin = this.delim
        let trim = /\s+$/.exec(delimMin)
        if (trim) delimMin = delimMin.slice(0, delimMin.length - trim[0].length)
        for (let i = 1; i < size; i++)
          this.out += delimMin + "\n"
      }
      this.closed = null
    }
  }

  /// @internal
  getMark(name: string) {
    let info = this.marks[name]
    if (!info) {
      if (this.options.strict !== false)
        throw new Error(`Mark type \`${name}\` not supported by Markdown renderer`)
      info = blankMark
    }
    return info
  }

  /// Render a block, prefixing each line with `delim`, and the first
  /// line in `firstDelim`. `node` should be the node that is closed at
  /// the end of the block, and `f` is a function that renders the
  /// content of the block.
  wrapBlock(delim: string, firstDelim: string | null, node: Node, f: () => void) {
    let old = this.delim
    this.write(firstDelim != null ? firstDelim : delim)
    this.delim += delim
    f()
    this.delim = old
    this.closeBlock(node)
  }

  /// @internal
  atBlank() {
    return /(^|\n)$/.test(this.out)
  }

  /// Ensure the current content ends with a newline.
  ensureNewLine() {
    if (!this.atBlank()) this.out += "\n"
  }

  /// Prepare the state for writing output (closing closed paragraphs,
  /// adding delimiters, and so on), and then optionally add content
  /// (unescaped) to the output.
  write(content?: string) {
    this.flushClose()
    if (this.delim && this.atBlank())
      this.out += this.delim
    if (content) this.out += content
  }

  /// Close the block for the given node.
  closeBlock(node: Node) {
    this.closed = node
  }

  /// Add the given text to the document. When escape is not `false`,
  /// it will be escaped.
  text(text: string, escape = true) {
    let lines = text.split("\n")
    for (let i = 0; i < lines.length; i++) {
      this.write()
      // Escape exclamation marks in front of links
      if (!escape && lines[i][0] == "[" && /(^|[^\\])\!$/.test(this.out))
        this.out = this.out.slice(0, this.out.length - 1) + "\\!"
      this.out += escape ? this.esc(lines[i], this.atBlockStart) : lines[i]
      if (i != lines.length - 1) this.out += "\n"
    }
  }

  /// Render the given node as a block.
  render(node: Node, parent: Node, index: number) {
    if (this.nodes[node.type.name]) {
      this.nodes[node.type.name](this, node, parent, index)
    } else {
      if (this.options.strict !== false) {
        throw new Error("Token type `" + node.type.name + "` not supported by Markdown renderer")
      } else if (!node.type.isLeaf) {
        if (node.type.inlineContent) this.renderInline(node)
        else this.renderContent(node)
        if (node.isBlock) this.closeBlock(node)
      }
    }
  }

  /// Render the contents of `parent` as block nodes.
  renderContent(parent: Node) {
    parent.forEach((node, _, i) => this.render(node, parent, i))
  }

  /// Render the contents of `parent` as inline content.
  renderInline(parent: Node, fromBlockStart = true) {
    this.atBlockStart = fromBlockStart
    let active: Mark[] = [], trailing = ""
    let progress = (node: Node | null, offset: number, index: number) => {
      let marks = node ? node.marks : []

      // Remove marks from `hard_break` that are the last node inside
      // that mark to prevent parser edge cases with new lines just
      // before closing marks.
      if (node && node.type.name === this.options.hardBreakNodeName)
        marks = marks.filter(m => {
          if (index + 1 == parent.childCount) return false
          let next = parent.child(index + 1)
          return m.isInSet(next.marks) && (!next.isText || /\S/.test(next.text!))
        })

      let leading = trailing
      trailing = ""
      // If whitespace has to be expelled from the node, adjust
      // leading and trailing accordingly.
      if (node && node.isText && marks.some(mark => {
        let info = this.getMark(mark.type.name)
        return info && info.expelEnclosingWhitespace && !mark.isInSet(active)
      })) {
        let [_, lead, rest] = /^(\s*)(.*)$/m.exec(node.text!)!
        if (lead) {
          leading += lead
          node = rest ? (node as any).withText(rest) : null
          if (!node) marks = active
        }
      }
      if (node && node.isText && marks.some(mark => {
        let info = this.getMark(mark.type.name)
        return info && info.expelEnclosingWhitespace &&
          (index == parent.childCount - 1 || !mark.isInSet(parent.child(index + 1).marks))
      })) {
        let [_, rest, trail] = /^(.*?)(\s*)$/m.exec(node.text!)!
        if (trail) {
          trailing = trail
          node = rest ? (node as any).withText(rest) : null
          if (!node) marks = active
        }
      }
      let inner = marks.length ? marks[marks.length - 1] : null
      let noEsc = inner && this.getMark(inner.type.name).escape === false
      let len = marks.length - (noEsc ? 1 : 0)

      // Try to reorder 'mixable' marks, such as em and strong, which
      // in Markdown may be opened and closed in different order, so
      // that order of the marks for the token matches the order in
      // active.
      outer: for (let i = 0; i < len; i++) {
        let mark = marks[i]
        if (!this.getMark(mark.type.name).mixable) break
        for (let j = 0; j < active.length; j++) {
          let other = active[j]
          if (!this.getMark(other.type.name).mixable) break
          if (mark.eq(other)) {
            if (i > j)
              marks = marks.slice(0, j).concat(mark).concat(marks.slice(j, i)).concat(marks.slice(i + 1, len))
            else if (j > i)
              marks = marks.slice(0, i).concat(marks.slice(i + 1, j)).concat(mark).concat(marks.slice(j, len))
            continue outer
          }
        }
      }

      // Find the prefix of the mark set that didn't change
      let keep = 0
      while (keep < Math.min(active.length, len) && marks[keep].eq(active[keep])) ++keep

      // Close the marks that need to be closed
      while (keep < active.length)
        this.text(this.markString(active.pop()!, false, parent, index), false)

      // Output any previously expelled trailing whitespace outside the marks
      if (leading) this.text(leading)

      // Open the marks that need to be opened
      if (node) {
        while (active.length < len) {
          let add = marks[active.length]
          active.push(add)
          this.text(this.markString(add, true, parent, index), false)
          this.atBlockStart = false
        }

        // Render the node. Special case code marks, since their content
        // may not be escaped.
        if (noEsc && node.isText)
          this.text(this.markString(inner!, true, parent, index) + node.text +
                    this.markString(inner!, false, parent, index + 1), false)
        else
          this.render(node, parent, index)
        this.atBlockStart = false
      }

      // After the first non-empty text node is rendered, the end of output
      // is no longer at block start.
      //
      // FIXME: If a non-text node writes something to the output for this
      // block, the end of output is also no longer at block start. But how
      // can we detect that?
      if (node?.isText && node.nodeSize > 0) {
        this.atBlockStart = false
      }
    }
    parent.forEach(progress)
    progress(null, 0, parent.childCount)
    this.atBlockStart = false
  }

  /// Render a node's content as a list. `delim` should be the extra
  /// indentation added to all lines except the first in an item,
  /// `firstDelim` is a function going from an item index to a
  /// delimiter for the first line of the item.
  renderList(node: Node, delim: string, firstDelim: (index: number) => string) {
    if (this.closed && this.closed.type == node.type)
      this.flushClose(3)
    else if (this.inTightList)
      this.flushClose(1)

    let isTight = typeof node.attrs.tight != "undefined" ? node.attrs.tight : this.options.tightLists
    let prevTight = this.inTightList
    this.inTightList = isTight
    node.forEach((child, _, i) => {
      if (i && isTight) this.flushClose(1)
      this.wrapBlock(delim, firstDelim(i), node, () => this.render(child, node, i))
    })
    this.inTightList = prevTight
  }

  /// Escape the given string so that it can safely appear in Markdown
  /// content. If `startOfLine` is true, also escape characters that
  /// have special meaning only at the start of the line.
  esc(str: string, startOfLine = false) {
    str = str.replace(
      /[`*\\~\[\]_]/g,
      (m, i) => m == "_" && i > 0 && i + 1 < str.length && str[i-1].match(/\w/) && str[i+1].match(/\w/) ?  m : "\\" + m
    )
    if (startOfLine) str = str.replace(/^(\+[ ]|[\-*>])/, "\\$&").replace(/^(\s*)(#{1,6})(\s|$)/, '$1\\$2$3').replace(/^(\s*\d+)\.\s/, "$1\\. ")
    if (this.options.escapeExtraCharacters) str = str.replace(this.options.escapeExtraCharacters, "\\$&")
    return str
  }

  /// @internal
  quote(str: string) {
    let wrap = str.indexOf('"') == -1 ? '""' : str.indexOf("'") == -1 ? "''" : "()"
    return wrap[0] + str + wrap[1]
  }

  /// Repeat the given string `n` times.
  repeat(str: string, n: number) {
    let out = ""
    for (let i = 0; i < n; i++) out += str
    return out
  }

  /// Get the markdown string for a given opening or closing mark.
  markString(mark: Mark, open: boolean, parent: Node, index: number) {
    let info = this.getMark(mark.type.name)
    let value = open ? info.open : info.close
    return typeof value == "string" ? value : value(this, mark, parent, index)
  }

  /// Get leading and trailing whitespace from a string. Values of
  /// leading or trailing property of the return object will be undefined
  /// if there is no match.
  getEnclosingWhitespace(text: string): {leading?: string, trailing?: string} {
    return {
      leading: (text.match(/^(\s+)/) || [undefined])[0],
      trailing: (text.match(/(\s+)$/) || [undefined])[0]
    }
  }
}