markdown-it/lib/rules_inline/emphasis.js

// Process *this* and _that_

'use strict';


function isAlphaNum(code) {
  return (code >= 0x30 /* 0 */ && code <= 0x39 /* 9 */) ||
         (code >= 0x41 /* A */ && code <= 0x5A /* Z */) ||
         (code >= 0x61 /* a */ && code <= 0x7A /* z */);
}

// parse sequence of emphasis markers,
// "start" should point at a valid marker
function scanDelims(state, start) {
  var pos = start, lastChar, nextChar, count,
      can_open = true,
      can_close = true,
      max = state.posMax,
      marker = state.src.charCodeAt(start);

  lastChar = start > 0 ? state.src.charCodeAt(start - 1) : -1;

  while (pos < max && state.src.charCodeAt(pos) === marker) { pos++; }
  if (pos >= max) { can_open = false; }
  count = pos - start;

  if (count >= 4) {
    // sequence of four or more unescaped markers can't start/end an emphasis
    can_open = can_close = false;
  } else {
    nextChar = pos < max ? state.src.charCodeAt(pos) : -1;

    // check whitespace conditions
    if (nextChar === 0x20 || nextChar === 0x0A) { can_open = false; }
    if (lastChar === 0x20 || lastChar === 0x0A) { can_close = false; }

    if (marker === 0x5F /* _ */) {
      // check if we aren't inside the word
      if (isAlphaNum(lastChar)) { can_open = false; }
      if (isAlphaNum(nextChar)) { can_close = false; }
    }
  }

  return {
    can_open: can_open,
    can_close: can_close,
    delims: count
  };
}

module.exports = function emphasis(state, silent) {
  var startCount,
      count,
      found,
      oldCount,
      newCount,
      stack,
      res,
      max = state.posMax,
      start = state.pos,
      marker = state.src.charCodeAt(start);

  if (marker !== 0x5F/* _ */ && marker !== 0x2A /* * */) { return false; }
  if (silent) { return false; } // don't run any pairs in validation mode

  res = scanDelims(state, start);
  startCount = res.delims;
  if (!res.can_open) {
    state.pos += startCount;
    if (!silent) { state.pending += state.src.slice(start, state.pos); }
    return true;
  }

  if (state.level >= state.options.maxNesting) { return false; }

  state.pos = start + startCount;
  stack = [ startCount ];

  while (state.pos < max) {
    if (state.src.charCodeAt(state.pos) === marker) {
      res = scanDelims(state, state.pos);
      count = res.delims;
      if (res.can_close) {
        oldCount = stack.pop();
        newCount = count;

        while (oldCount !== newCount) {
          if (newCount < oldCount) {
            stack.push(oldCount - newCount);
            break;
          }

          // assert(newCount > oldCount)
          newCount -= oldCount;

          if (stack.length === 0) { break; }
          state.pos += oldCount;
          oldCount = stack.pop();
        }

        if (stack.length === 0) {
          startCount = oldCount;
          found = true;
          break;
        }
        state.pos += count;
        continue;
      }

      if (res.can_open) { stack.push(count); }
      state.pos += count;
      continue;
    }

    state.parser.skipToken(state);
  }

  if (!found) {
    // parser failed to find ending tag, so it's not valid emphasis
    state.pos = start;
    return false;
  }

  // found!
  state.posMax = state.pos;
  state.pos = start + startCount;

  if (!silent) {
    if (startCount === 2 || startCount === 3) {
      state.push({ type: 'strong_open', level: state.level++ });
    }
    if (startCount === 1 || startCount === 3) {
      state.push({ type: 'em_open', level: state.level++ });
    }

    state.parser.tokenize(state);

    if (startCount === 1 || startCount === 3) {
      state.push({ type: 'em_close', level: --state.level });
    }
    if (startCount === 2 || startCount === 3) {
      state.push({ type: 'strong_close', level: --state.level });
    }
  }

  state.pos = state.posMax + startCount;
  state.posMax = max;
  return true;
};