/**
 * Default hint alphabet — qwerty home row first, then top, then bottom.
 * Picked for typing speed; all 26 letters are included so single-char labels
 * cover articles with up to 26 links.
 */
export const DEFAULT_HINT_ALPHABET = 'asdfghjklqwertyuiopzxcvbnm';

/**
 * Generate `count` unique hint labels from `alphabet` such that no label is a
 * prefix of any other (so a single keypress can resolve unambiguously
 * whenever possible).
 *
 * Uses a trie-expansion strategy that works for any count, even beyond the
 * 26-or 676-label single/two-char ranges: we keep a pool of candidate labels
 * (starting with every single letter) and repeatedly expand the front-most
 * candidate into its `A` children until the pool is large enough. Expanding
 * the front keeps the shortest labels for the earliest links. Because every
 * label in the pool is always a leaf of the trie, the result is prefix-free
 * by construction at any length.
 */
export function generateHintLabels(count: number, alphabet: string = DEFAULT_HINT_ALPHABET): string[] {
  if (count <= 0) return [];
  const A = alphabet.length;
  if (A === 0) return [];

  const letters = alphabet.split('');

  if (count <= A) {
    return letters.slice(0, count);
  }

  // Pool of prefix-free candidate labels, shortest-first. Each expansion turns
  // one leaf into A leaves, growing the pool by A - 1.
  const pool = [...letters];
  let head = 0;
  while (pool.length - head < count) {
    const prefix = pool[head++] as string;
    for (const letter of letters) {
      pool.push(prefix + letter);
    }
  }

  // The labels consumed as prefixes (indices < head) are no longer leaves, so
  // drop them; the remaining pool is prefix-free. Take the first `count`,
  // which are the shortest available.
  return pool.slice(head, head + count);
}