app/node_modules/markdown-it/lib/rules_block/heading.mjs

// heading (#, ##, ...)

import { isSpace } from '../common/utils.mjs'

export default function heading (state, startLine, endLine, silent) {
  let pos = state.bMarks[startLine] + state.tShift[startLine]
  let max = state.eMarks[startLine]

  // if it's indented more than 3 spaces, it should be a code block
  if (state.sCount[startLine] - state.blkIndent >= 4) { return false }

  let ch  = state.src.charCodeAt(pos)

  if (ch !== 0x23/* # */ || pos >= max) { return false }

  // count heading level
  let level = 1
  ch = state.src.charCodeAt(++pos)
  while (ch === 0x23/* # */ && pos < max && level <= 6) {
    level++
    ch = state.src.charCodeAt(++pos)
  }

  if (level > 6 || (pos < max && !isSpace(ch))) { return false }

  if (silent) { return true }

  // Let's cut tails like '    ###  ' from the end of string

  max = state.skipSpacesBack(max, pos)
  const tmp = state.skipCharsBack(max, 0x23, pos) // #
  if (tmp > pos && isSpace(state.src.charCodeAt(tmp - 1))) {
    max = tmp
  }

  state.line = startLine + 1

  const token_o  = state.push('heading_open', 'h' + String(level), 1)
  token_o.markup = '########'.slice(0, level)
  token_o.map    = [startLine, state.line]

  const token_i    = state.push('inline', '', 0)
  token_i.content  = state.src.slice(pos, max).trim()
  token_i.map      = [startLine, state.line]
  token_i.children = []

  const token_c  = state.push('heading_close', 'h' + String(level), -1)
  token_c.markup = '########'.slice(0, level)

  return true
}