markdown-it/lib/rules_inline/emphasis.mjs

// Process *this* and _that_
//

// Insert each marker as a separate text token, and add it to delimiter list
//
function emphasis_tokenize(state, silent) {
  var i, scanned, token,
      start = state.pos,
      marker = state.src.charCodeAt(start);

  if (silent) { return false; }

  if (marker !== 0x5F /* _ */ && marker !== 0x2A /* * */) { return false; }

  scanned = state.scanDelims(state.pos, marker === 0x2A);

  for (i = 0; i < scanned.length; i++) {
    token         = state.push('text', '', 0);
    token.content = String.fromCharCode(marker);

    state.delimiters.push({
      // Char code of the starting marker (number).
      //
      marker: marker,

      // Total length of these series of delimiters.
      //
      length: scanned.length,

      // A position of the token this delimiter corresponds to.
      //
      token:  state.tokens.length - 1,

      // If this delimiter is matched as a valid opener, `end` will be
      // equal to its position, otherwise it's `-1`.
      //
      end:    -1,

      // Boolean flags that determine if this delimiter could open or close
      // an emphasis.
      //
      open:   scanned.can_open,
      close:  scanned.can_close
    });
  }

  state.pos += scanned.length;

  return true;
}


function postProcess(state, delimiters) {
  var i,
      startDelim,
      endDelim,
      token,
      ch,
      isStrong,
      max = delimiters.length;

  for (i = max - 1; i >= 0; i--) {
    startDelim = delimiters[i];

    if (startDelim.marker !== 0x5F/* _ */ && startDelim.marker !== 0x2A/* * */) {
      continue;
    }

    // Process only opening markers
    if (startDelim.end === -1) {
      continue;
    }

    endDelim = delimiters[startDelim.end];

    // If the previous delimiter has the same marker and is adjacent to this one,
    // merge those into one strong delimiter.
    //
    // `<em><em>whatever</em></em>` -> `<strong>whatever</strong>`
    //
    isStrong = i > 0 &&
               delimiters[i - 1].end === startDelim.end + 1 &&
               // check that first two markers match and adjacent
               delimiters[i - 1].marker === startDelim.marker &&
               delimiters[i - 1].token === startDelim.token - 1 &&
               // check that last two markers are adjacent (we can safely assume they match)
               delimiters[startDelim.end + 1].token === endDelim.token + 1;

    ch = String.fromCharCode(startDelim.marker);

    token         = state.tokens[startDelim.token];
    token.type    = isStrong ? 'strong_open' : 'em_open';
    token.tag     = isStrong ? 'strong' : 'em';
    token.nesting = 1;
    token.markup  = isStrong ? ch + ch : ch;
    token.content = '';

    token         = state.tokens[endDelim.token];
    token.type    = isStrong ? 'strong_close' : 'em_close';
    token.tag     = isStrong ? 'strong' : 'em';
    token.nesting = -1;
    token.markup  = isStrong ? ch + ch : ch;
    token.content = '';

    if (isStrong) {
      state.tokens[delimiters[i - 1].token].content = '';
      state.tokens[delimiters[startDelim.end + 1].token].content = '';
      i--;
    }
  }
}


// Walk through delimiter list and replace text tokens with tags
//
function emphasis_post_process(state) {
  var curr,
      tokens_meta = state.tokens_meta,
      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);
    }
  }
}

export default {
  tokenize: emphasis_tokenize,
  postProcess: emphasis_post_process
};