const slugify = (s) => encodeURIComponent(String(s).trim().toLowerCase().replace(/\s+/g, '-'));

const position = {
  'false': 'push',
  'true': 'unshift'
};

const permalinkHref = slug => `#${slug}`;
const permalinkAttrs = slug => ({});

function renderPermalink(slug, opts, state, idx) {
  const space = () => Object.assign(new state.Token('text', '', 0), { content: ' ' });

  const linkTokens = [
    Object.assign(new state.Token('link_open', 'a', 1), {
      attrs: [
        ...(opts.permalinkClass ? [ [ 'class', opts.permalinkClass ] ] : []),
        [ 'href', opts.permalinkHref(slug, state) ],
        ...Object.entries(opts.permalinkAttrs(slug, state))
      ]
    }),
    Object.assign(new state.Token('html_block', '', 0), { content: opts.permalinkSymbol }),
    new state.Token('link_close', 'a', -1)
  ];

  // `push` or `unshift` according to position option.
  // Space is at the opposite side.
  if (opts.permalinkSpace) {
    linkTokens[position[!opts.permalinkBefore]](space());
  }

  for (let j = idx + 1, iK = state.tokens.length; j < iK; j++) {
    const token = state.tokens[j];
    if (token.type === 'heading_close') { break; }
    if (!token.children) { continue; }
    token.children[position[opts.permalinkBefore]](...linkTokens);
    break;
  }
}

function uniqueSlug(slug, slugs, failOnNonUnique, startIndex) {
  // If first slug, return as is.
  let key = slug;
  let n = startIndex;
  if (slugs.has(key) && failOnNonUnique) {
    throw new Error(`The ID attribute '${slug}' defined by user or other markdown-it plugin is not unique. Please fix it in your markdown to continue.`);
  }

  while (slugs.has(key)) {
    // Duplicate slug, add a `-1`, `-2`, etc. to keep ID unique.
    key = `${slug}-${n++}`;
  }

  // Mark this slug as used in the environment.
  slugs.set(key, true);
  return key;
}

const isLevelSelectedNumber = selection => level => level <= selection;
const isLevelSelectedArray = selection => level => selection.includes(level);

const anchor = (md, opts) => {
  opts = Object.assign({}, anchor.defaults, opts);

  md.core.ruler.push('anchor', state => {
    const slugs = new Map();
    const tokens = state.tokens;

    const isLevelSelected = Array.isArray(opts.level)
      ? isLevelSelectedArray(opts.level)
      : isLevelSelectedNumber(opts.level);

    tokens.forEach((token, i) => {
      if (token.type !== 'heading_open') {
        return;
      }

      // Before we do anything, we must collect all previously defined ID attributes to ensure we won't generate any duplicates:
      let slug = token.attrGet('id');

      if (slug != null) {
        // mark existing slug/ID as unique, at least.
        // IFF it collides, FAIL!
        slug = uniqueSlug(slug, slugs, true);
      }
    });

    tokens.forEach((token, i) => {
      if (token.type !== 'heading_open') {
        return;
      }

      if (!isLevelSelected(Number(token.tag.substr(1)))) {
        return;
      }

      // Aggregate the next token children text.
      let keyparts = [];
      for (let j = i + 1, iK = tokens.length; j < iK; j++) {
        const token = tokens[j];
        if (token.type === 'heading_close') { break; }
        if (!token.children) { continue; }
        const keypart = token.children
        .filter(token => token.type === 'text' || token.type === 'code_inline')
        .reduce((acc, t) => acc + t.content, '')
        .trim();
        if (keypart.length > 0) {
          keyparts.push(keypart);
        }
      }
      const title = keyparts.join(' ');
      let slug = token.attrGet('id');

      if (slug == null) {
        slug = uniqueSlug(opts.slugify(title), slugs, false, opts.uniqueSlugStartIndex);
        token.attrSet('id', slug);
      }

      if (opts.permalink) {
        opts.renderPermalink(slug, opts, state, i);
      }

      if (opts.callback) {
        opts.callback(token, { slug, title });
      }
    });
  });
};

anchor.defaults = {
  level: 6,                    // **max** level or array of levels
  slugify,
  uniqueSlugStartIndex: 1,
  permalink: false,
  renderPermalink,
  permalinkClass: 'header-anchor',
  permalinkSpace: true,
  permalinkSymbol: '¶',
  permalinkBefore: false,
  permalinkHref,
  permalinkAttrs
};

export default anchor;