import { collect } from '../parser/util';
import { push } from 'spica/array';
import { html } from 'typed-dom/dom';

export function toc(source: DocumentFragment | HTMLElement | ShadowRoot): HTMLUListElement {
  const hs: HTMLHeadingElement[] = [];
  for (let es = collect(source, ':is(h1, h2, h3, h4, h5, h6, aside.aside)[id]'),
           len = es.length, i = 0; i < len; ++i) {
    const el = es[i];
    assert(el.parentNode === source);
    switch (el.tagName) {
      case 'ASIDE':
        hs.push(html(el.firstElementChild!.tagName.toLowerCase() as 'h1', { id: el.id, class: 'aside' }, el.firstElementChild!.cloneNode(true).childNodes));
        continue;
      default:
        hs.push(el as HTMLHeadingElement);
        continue;
    }
  }
  return parse(cons(hs));
}

type Tree = readonly [HTMLHeadingElement, Tree][];

function parse(node: Tree, index: string = ''): HTMLUListElement {
  let i = 0;
  return html('ul', node.map(([el, cs]) => {
    const isHeading = !el.classList.contains('aside');
    const idx = isHeading
      ? index === ''
        ? `${++i}`
        : `${index}.${++i}`
      : index === ''
        ? `${i}`
        : `${index}.${i}`;
    return html('li',
      push(
        [html('a', { href: `#${el.id}`, 'data-index': isHeading ? idx : undefined }, unlink(el.cloneNode(true)))],
        cs.length > 0 ? [parse(cs, idx)] : []));
  }));
}

function cons(hs: HTMLHeadingElement[]): Tree {
  return hs
    .reduce<HTMLHeadingElement[][]>((hss, h) => {
      const hs = hss.pop() ?? [];
      return hs.length === 0 || level(h) > level(hs[0])
        ? push(hss, [push(hs, [h])])
        : push(hss, [hs, [h]]);
    }, [])
    .map(hs =>
      [hs.shift()!, cons(hs)]);
}

function level(h: HTMLHeadingElement): number {
  assert(isFinite(+h.tagName[1]));
  return +h.tagName[1];
}

function unlink(h: HTMLHeadingElement): Iterable<Node> {
  for (let es = h.getElementsByTagName('a'), len = es.length, i = 0; i < len; ++i) {
    const el = es[i];
    assert(el.childNodes.length <= 1);
    el.firstChild
      ? el.replaceWith(el.firstChild)
      : el.remove();
  }
  return h.childNodes;
}