/** Longest slug generated, so that URLs stay readable. */
export const MAX_SLUG_LENGTH = 80;

/** Valid slug: lowercase ASCII words separated by single hyphens. */
export const SLUG_PATTERN = /^[a-z0-9]+(?:-[a-z0-9]+)*$/;

/** Letters that Unicode normalization does not split into a base letter and an accent. */
const SPECIAL_LETTERS: Record<string, string> = { æ: 'ae', œ: 'oe', ø: 'o', ß: 'ss', đ: 'd', ł: 'l', þ: 'th' };

/**
 * Turns a name into a slug: accents removed, lowercase, every run of other characters replaced by one hyphen,
 * cut at a word boundary to {@link MAX_SLUG_LENGTH}. "&" becomes "and" ("&TEAM" -> "and-team", not "team").
 * @returns an empty string when nothing is left (name without any latin letter or digit, e.g. in hangul).
 */
export function slugify(name: string, maxLength = MAX_SLUG_LENGTH): string {
  const slug = name
    .normalize('NFKD')
    .replace(/[̀-ͯ]/g, '')
    .toLowerCase()
    .replace(/[æœøßđłþ]/g, (letter) => SPECIAL_LETTERS[letter])
    .replace(/&/g, ' and ')
    .replace(/[^a-z0-9]+/g, '-')
    .replace(/^-+|-+$/g, '');
  return truncateSlug(slug, maxLength);
}

/** Cuts a slug to `maxLength`, at the last hyphen when there is one, never leaving a trailing hyphen. */
function truncateSlug(slug: string, maxLength: number): string {
  if (slug.length <= maxLength) return slug;
  const cut = slug.slice(0, maxLength);
  const lastHyphen = cut.lastIndexOf('-');
  return (lastHyphen > 0 ? cut.slice(0, lastHyphen) : cut).replace(/-+$/, '');
}

/** Appends `-suffix` to a slug, shortening the slug so that the result fits in {@link MAX_SLUG_LENGTH}. */
export function withSlugSuffix(slug: string, suffix: string | number): string {
  const end = `-${suffix}`;
  return truncateSlug(slug, MAX_SLUG_LENGTH - end.length) + end;
}

/**
 * Picks the first candidate not in `taken`. When they are all taken, numbers the last one (`-2`, `-3`...)
 * until a free slug is found.
 * @param candidates slugs by order of preference, at least one, none empty.
 */
export function pickAvailableSlug(candidates: string[], taken: ReadonlySet<string>): string {
  const free = candidates.find((candidate) => !taken.has(candidate));
  if (free) return free;

  const last = candidates[candidates.length - 1];
  for (let n = 2; ; n++) {
    const numbered = withSlugSuffix(last, n);
    if (!taken.has(numbered)) return numbered;
  }
}