app/node_modules/markdown-it/lib/rules_inline/strikethrough.mjs

// ~~strike through~~
//

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

  if (silent) { return false }

  if (marker !== 0x7E/* ~ */) { return false }

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

  if (len < 2) { return false }

  let token

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

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

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

  state.pos += scanned.length

  return true
}

function postProcess (state, delimiters) {
  let token
  const loneMarkers = []
  const max = delimiters.length

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

    if (startDelim.marker !== 0x7E/* ~ */) {
      continue
    }

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

    const endDelim = delimiters[startDelim.end]

    token         = state.tokens[startDelim.token]
    token.type    = 's_open'
    token.tag     = 's'
    token.nesting = 1
    token.markup  = '~~'
    token.content = ''

    token         = state.tokens[endDelim.token]
    token.type    = 's_close'
    token.tag     = 's'
    token.nesting = -1
    token.markup  = '~~'
    token.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 === 's_close') {
      j++
    }

    j--

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

// Walk through delimiter list and replace text tokens with tags
//
function strikethrough_postProcess (state) {
  const tokens_meta = state.tokens_meta
  const max = state.tokens_meta.length

  postProcess(state, state.delimiters)

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

export default {
  tokenize: strikethrough_tokenize,
  postProcess: strikethrough_postProcess
}