markdown-it/lib/rules_inline/balance_pairs.mjs

// For each opening emphasis-like marker find a matching closing one
//

function processDelimiters (delimiters) {
  const openersBottom = {}
  const max = delimiters.length

  if (!max) return

  // headerIdx is the first delimiter of the current (where closer is) delimiter run
  let headerIdx = 0
  let lastTokenIdx = -2 // needs any value lower than -1
  const jumps = []

  for (let closerIdx = 0; closerIdx < max; closerIdx++) {
    const closer = delimiters[closerIdx]

    jumps.push(0)

    // markers belong to same delimiter run if:
    //  - they have adjacent tokens
    //  - AND markers are the same
    //
    if (delimiters[headerIdx].marker !== closer.marker || lastTokenIdx !== closer.token - 1) {
      headerIdx = closerIdx
    }

    lastTokenIdx = closer.token

    // Length is only used for emphasis-specific "rule of 3",
    // if it's not defined (in strikethrough or 3rd party plugins),
    // we can default it to 0 to disable those checks.
    //
    closer.length = closer.length || 0

    if (!closer.close) continue

    // Previously calculated lower bounds (previous fails)
    // for each marker, each delimiter length modulo 3,
    // and for whether this closer can be an opener;
    // https://github.com/commonmark/cmark/commit/34250e12ccebdc6372b8b49c44fab57c72443460
    /* eslint-disable-next-line no-prototype-builtins */
    if (!openersBottom.hasOwnProperty(closer.marker)) {
      openersBottom[closer.marker] = [-1, -1, -1, -1, -1, -1]
    }

    const minOpenerIdx = openersBottom[closer.marker][(closer.open ? 3 : 0) + (closer.length % 3)]

    let openerIdx = headerIdx - jumps[headerIdx] - 1

    let newMinOpenerIdx = openerIdx

    for (; openerIdx > minOpenerIdx; openerIdx -= jumps[openerIdx] + 1) {
      const opener = delimiters[openerIdx]

      if (opener.marker !== closer.marker) continue

      if (opener.open && opener.end < 0) {
        let isOddMatch = false

        // from spec:
        //
        // If one of the delimiters can both open and close emphasis, then the
        // sum of the lengths of the delimiter runs containing the opening and
        // closing delimiters must not be a multiple of 3 unless both lengths
        // are multiples of 3.
        //
        if (opener.close || closer.open) {
          if ((opener.length + closer.length) % 3 === 0) {
            if (opener.length % 3 !== 0 || closer.length % 3 !== 0) {
              isOddMatch = true
            }
          }
        }

        if (!isOddMatch) {
          // If previous delimiter cannot be an opener, we can safely skip
          // the entire sequence in future checks. This is required to make
          // sure algorithm has linear complexity (see *_*_*_*_*_... case).
          //
          const lastJump = openerIdx > 0 && !delimiters[openerIdx - 1].open
            ? jumps[openerIdx - 1] + 1
            : 0

          jumps[closerIdx] = closerIdx - openerIdx + lastJump
          jumps[openerIdx] = lastJump

          closer.open  = false
          opener.end   = closerIdx
          opener.close = false
          newMinOpenerIdx = -1
          // treat next token as start of run,
          // it optimizes skips in **<...>**a**<...>** pathological case
          lastTokenIdx = -2
          break
        }
      }
    }

    if (newMinOpenerIdx !== -1) {
      // If match for this delimiter run failed, we want to set lower bound for
      // future lookups. This is required to make sure algorithm has linear
      // complexity.
      //
      // See details here:
      // https://github.com/commonmark/cmark/issues/178#issuecomment-270417442
      //
      openersBottom[closer.marker][(closer.open ? 3 : 0) + ((closer.length || 0) % 3)] = newMinOpenerIdx
    }
  }
}

export default function link_pairs (state) {
  const tokens_meta = state.tokens_meta
  const max = state.tokens_meta.length

  processDelimiters(state.delimiters)

  for (let curr = 0; curr < max; curr++) {
    if (tokens_meta[curr] && tokens_meta[curr].delimiters) {
      processDelimiters(tokens_meta[curr].delimiters)
    }
  }
}