// The document's first heading doubles as its name: it becomes the browser tab
// title and, more usefully, the filename the browser offers by default when
// the document is printed to PDF.

const MAX_LENGTH = 80

// Strip the inline markdown that would otherwise end up in a filename, along
// with the characters that operating systems disallow there.
function clean(s: string): string {
  return s
    .replace(/\[([^\]]*)\]\([^)]*\)/g, '$1')
    .replace(/[`*_~]/g, '')
    .replace(/\s+#+\s*$/, '')
    .replace(/[\\/:*?"<>|]/g, '-')
    .replace(/\s+/g, ' ')
    .trim()
    .slice(0, MAX_LENGTH)
    .trim()
}

// The first ATX heading outside of a fenced code block, falling back to the
// first non-empty line. Fences are tracked because a comment line such as
// `# load the data` inside a python block is not a heading.
export function documentTitle(markdown: string): string {
  let inFence = false
  let firstLine = ''
  for (const raw of markdown.split('\n')) {
    const line = raw.trim()
    if (/^(```|~~~)/.test(line)) {
      inFence = !inFence
      continue
    }
    if (inFence || !line) continue
    const heading = /^#{1,6}\s+(.+)$/.exec(line)
    if (heading) return clean(heading[1])
    if (!firstLine) firstLine = line
  }
  return clean(firstLine)
}