// Word-level text diff for the Edit History viewer. Pure and dependency-free.

export type DiffSegment = {
  type: 'equal' | 'added' | 'removed';
  text: string;
};

// Split into a sequence of tokens where each token is either a run of
// whitespace or a run of non-whitespace, so word boundaries and the original
// spacing/newlines are both preserved when segments are re-joined.
const tokenize = (text: string): string[] => text.match(/\s+|\S+/g) ?? [];

// Above this many tokens per side the O(n*m) LCS table gets expensive/large, so
// we fall back to a coarse whole-block "replaced" diff. Message bodies this long
// are rare; the exact word diff isn't worth a multi-million-cell allocation.
const MAX_DIFF_TOKENS = 2000;

const coarseDiff = (oldText: string, newText: string): DiffSegment[] => {
  const segments: DiffSegment[] = [];
  if (oldText) segments.push({ type: 'removed', text: oldText });
  if (newText) segments.push({ type: 'added', text: newText });
  return segments;
};

/**
 * Word-level diff of `oldText` → `newText` via a classic LCS. Returns segments
 * in reading order: `equal` runs, `removed` runs (present only in old), and
 * `added` runs (present only in new). Consecutive same-type tokens are merged.
 * Pure — never mutates its inputs. Whitespace is preserved.
 */
export function diffWords(oldText: string, newText: string): DiffSegment[] {
  const a = tokenize(oldText);
  const b = tokenize(newText);
  if (a.length > MAX_DIFF_TOKENS || b.length > MAX_DIFF_TOKENS) {
    return coarseDiff(oldText, newText);
  }
  const n = a.length;
  const m = b.length;

  // lcs[i][j] = length of the longest common subsequence of a[i..] and b[j..].
  const lcs: number[][] = Array.from({ length: n + 1 }, () => new Array<number>(m + 1).fill(0));
  for (let i = n - 1; i >= 0; i -= 1) {
    for (let j = m - 1; j >= 0; j -= 1) {
      lcs[i][j] = a[i] === b[j] ? lcs[i + 1][j + 1] + 1 : Math.max(lcs[i + 1][j], lcs[i][j + 1]);
    }
  }

  const raw: { type: DiffSegment['type']; text: string }[] = [];
  let i = 0;
  let j = 0;
  while (i < n && j < m) {
    if (a[i] === b[j]) {
      raw.push({ type: 'equal', text: a[i] });
      i += 1;
      j += 1;
    } else if (lcs[i + 1][j] >= lcs[i][j + 1]) {
      raw.push({ type: 'removed', text: a[i] });
      i += 1;
    } else {
      raw.push({ type: 'added', text: b[j] });
      j += 1;
    }
  }
  while (i < n) {
    raw.push({ type: 'removed', text: a[i] });
    i += 1;
  }
  while (j < m) {
    raw.push({ type: 'added', text: b[j] });
    j += 1;
  }

  // Merge consecutive same-type tokens into segments.
  const segments: DiffSegment[] = [];
  raw.forEach((tok) => {
    const last = segments[segments.length - 1];
    if (last && last.type === tok.type) last.text += tok.text;
    else segments.push({ type: tok.type, text: tok.text });
  });
  return segments;
}
