/**
 * [Gitea #144] Word-level diff for the "(edited)" hover. Plain text only; the
 * caller decides what to do with formatted bodies. LCS over word tokens, with
 * whitespace preserved on the token that precedes it so the rendered result
 * reads like the original message.
 */

export type DiffOp = 'same' | 'add' | 'del';
export type DiffToken = { op: DiffOp; text: string };

/** Above this many words (before + after) we give up: O(n·m) memory and an unreadable tooltip. */
export const WORD_DIFF_LIMIT = 400;

const tokenize = (text: string): string[] => text.match(/\S+\s*|\s+/g) ?? [];
const word = (token: string) => token.trim();

/**
 * Diff `before` → `after` into runs. Returns `undefined` when the texts are
 * identical or too long to diff meaningfully (caller should fall back to the
 * full history viewer).
 */
export function wordDiff(before: string, after: string): DiffToken[] | undefined {
  if (before === after) return undefined;
  const a = tokenize(before);
  const b = tokenize(after);
  if (a.length + b.length > WORD_DIFF_LIMIT) return undefined;

  // LCS table on trimmed words so whitespace-only changes don't count as edits.
  const n = a.length;
  const m = b.length;
  const lcs: Uint16Array[] = Array.from({ length: n + 1 }, () => new Uint16Array(m + 1));
  for (let i = n - 1; i >= 0; i -= 1) {
    for (let j = m - 1; j >= 0; j -= 1) {
      lcs[i][j] =
        word(a[i]) === word(b[j]) ? lcs[i + 1][j + 1] + 1 : Math.max(lcs[i + 1][j], lcs[i][j + 1]);
    }
  }

  const out: DiffToken[] = [];
  const push = (op: DiffOp, text: string) => {
    const last = out[out.length - 1];
    if (last && last.op === op) last.text += text;
    else out.push({ op, text });
  };
  let i = 0;
  let j = 0;
  while (i < n && j < m) {
    if (word(a[i]) === word(b[j])) {
      push('same', b[j]);
      i += 1;
      j += 1;
    } else if (lcs[i + 1][j] >= lcs[i][j + 1]) {
      push('del', a[i]);
      i += 1;
    } else {
      push('add', b[j]);
      j += 1;
    }
  }
  while (i < n) push('del', a[i++]);
  while (j < m) push('add', b[j++]);

  // Only whitespace moved around: nothing worth showing.
  if (out.every((t) => t.op === 'same' || t.text.trim() === '')) return undefined;
  return out;
}

/** Number of changed words in a diff (for a compact summary). */
export const countChanges = (tokens: DiffToken[]): { added: number; removed: number } => {
  let added = 0;
  let removed = 0;
  tokens.forEach((t) => {
    const words = t.text.trim() === '' ? 0 : t.text.trim().split(/\s+/).length;
    if (t.op === 'add') added += words;
    if (t.op === 'del') removed += words;
  });
  return { added, removed };
};