index.mjs

export default function ins_plugin (md) {
  // Insert each marker as a separate text token, and add it to delimiter list
  //
  function tokenize (state, silent) {
    const start = state.pos
    const marker = state.src.charCodeAt(start)

    if (silent) { return false }

    if (marker !== 0x3D/* = */) { return false }

    const scanned = state.scanDelims(state.pos, true)
    let len = scanned.length
    const ch = String.fromCharCode(marker)

    if (len < 2) { return false }

    if (len % 2) {
      const token = state.push('text', '', 0)
      token.content = ch
      len--
    }

    for (let i = 0; i < len; i += 2) {
      const token = state.push('text', '', 0)
      token.content = ch + ch

      if (!scanned.can_open && !scanned.can_close) { continue }

      state.delimiters.push({
        marker,
        length: 0,     // disable "rule of 3" length checks meant for emphasis
        jump: i / 2, // 1 delimiter = 2 characters
        token: state.tokens.length - 1,
        end: -1,
        open: scanned.can_open,
        close: scanned.can_close
      })
    }

    state.pos += scanned.length

    return true
  }

  // Walk through delimiter list and replace text tokens with tags
  //
  function postProcess (state, delimiters) {
    const loneMarkers = []
    const max = delimiters.length

    for (let i = 0; i < max; i++) {
      const startDelim = delimiters[i]

      if (startDelim.marker !== 0x3D/* = */) {
        continue
      }

      if (startDelim.end === -1) {
        continue
      }

      const endDelim = delimiters[startDelim.end]

      const token_o = state.tokens[startDelim.token]
      token_o.type = 'mark_open'
      token_o.tag = 'mark'
      token_o.nesting = 1
      token_o.markup = '=='
      token_o.content = ''

      const token_c = state.tokens[endDelim.token]
      token_c.type = 'mark_close'
      token_c.tag = 'mark'
      token_c.nesting = -1
      token_c.markup = '=='
      token_c.content = ''

      if (state.tokens[endDelim.token - 1].type === 'text' &&
          state.tokens[endDelim.token - 1].content === '=') {
        loneMarkers.push(endDelim.token - 1)
      }
    }

    // If a marker sequence has an odd number of characters, it's splitted
    // like this: `~~~~~` -> `~` + `~~` + `~~`, leaving one marker at the
    // start of the sequence.
    //
    // So, we have to move all those markers after subsequent s_close tags.
    //
    while (loneMarkers.length) {
      const i = loneMarkers.pop()
      let j = i + 1

      while (j < state.tokens.length && state.tokens[j].type === 'mark_close') {
        j++
      }

      j--

      if (i !== j) {
        const token = state.tokens[j]
        state.tokens[j] = state.tokens[i]
        state.tokens[i] = token
      }
    }
  }

  md.inline.ruler.before('emphasis', 'mark', tokenize)
  md.inline.ruler2.before('emphasis', 'mark', function (state) {
    let curr
    const tokens_meta = state.tokens_meta
    const max = (state.tokens_meta || []).length

    postProcess(state, state.delimiters)

    for (curr = 0; curr < max; curr++) {
      if (tokens_meta[curr] && tokens_meta[curr].delimiters) {
        postProcess(state, tokens_meta[curr].delimiters)
      }
    }
  })
};