/** Letters without their accents, so a match survives the editor's typing. */
const fold = (text: string): string =>
  text.normalize('NFD').replace(/[̀-ͯ]/g, '');

const normalize = (text: string): string =>
  fold(text)
    .toLowerCase()
    .replace(/[‘’“”]/g, "'")
    .replace(/\s+/g, ' ');

/**
 * The spans of the HTML that are plain text outside every tag, and outside
 * the links, headings and block comments a new link has no business in.
 */
const textSpans = (html: string): { start: number; end: number }[] => {
  const spans: { start: number; end: number }[] = [];
  const skip =
    /<a\b[\s\S]*?<\/a>|<h[1-6]\b[\s\S]*?<\/h[1-6]>|<(script|style)\b[\s\S]*?<\/\2>|<!--[\s\S]*?-->|<[^>]+>/gi;
  let at = 0;
  for (const match of html.matchAll(skip)) {
    if (match.index !== undefined && match.index > at) {
      spans.push({ start: at, end: match.index });
    }
    at = (match.index ?? 0) + match[0].length;
  }
  if (at < html.length) {
    spans.push({ start: at, end: html.length });
  }
  return spans;
};

/**
 * Wrap the first plain-text occurrence of `anchor` in a link to `url`.
 *
 * The match ignores case, accents and how the whitespace fell, and skips
 * text that already sits in a link or a heading, so a suggestion never
 * nests one link in another or rewrites a subheading.
 *
 * @returns The content with the link, or null when the anchor is not in it.
 */
export const linkAnchor = (
  html: string,
  anchor: string,
  url: string
): string | null => {
  const needle = normalize(anchor).trim();
  if (needle === '' || url.trim() === '') {
    return null;
  }
  // The anchor may have been typed with other whitespace; match its words in order.
  const pattern = new RegExp(
    needle
      .split(' ')
      .map(word => word.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'))
      .join('[\\s\\u00a0]+'),
    'i'
  );
  for (const span of textSpans(html)) {
    const text = html.slice(span.start, span.end);
    const match = pattern.exec(fold(text));
    if (!match) {
      continue;
    }
    const start = span.start + match.index;
    const end = start + match[0].length;
    const shown = html.slice(start, end);
    return `${html.slice(0, start)}<a href="${url}">${shown}</a>${html.slice(end)}`;
  }
  return null;
};