import { Brief, LetterBlock, allBlocks } from './brief';

/**
 * The rejection diff as a PURE function over two immutable `Brief` values — the whole
 * teaching payload of WP-27: because state is one value, "what changed since the letter
 * was rejected" is just a fold over two snapshots, no change-tracking bookkeeping.
 *
 * Blocks are matched by `blockId` (stable `local-N`/seed ids):
 *  - in `after` but not `before` → `added`
 *  - in `before` but not `after` → `removed`
 *  - in both, different content   → `changed`
 *  - in both, same content        → `unchanged`
 */

export type BlockDiffKind = 'added' | 'removed' | 'changed' | 'unchanged';

export interface BlockDiff {
  readonly blockId: string;
  readonly kind: BlockDiffKind;
}

/** Content equality by value. Blocks are JSON-shaped immutable trees, so a canonical
    stringify is an honest deep-equal here (no functions, no cycles). */
function contentEqual(a: LetterBlock, b: LetterBlock): boolean {
  return JSON.stringify(a.content) === JSON.stringify(b.content);
}

export function diffBlocks(before: Brief, after: Brief): BlockDiff[] {
  const beforeById = new Map(allBlocks(before).map((b) => [b.blockId, b]));
  const afterById = new Map(allBlocks(after).map((b) => [b.blockId, b]));
  const out: BlockDiff[] = [];
  for (const a of afterById.values()) {
    const b = beforeById.get(a.blockId);
    out.push({
      blockId: a.blockId,
      kind: !b ? 'added' : contentEqual(a, b) ? 'unchanged' : 'changed',
    });
  }
  for (const b of beforeById.values()) {
    if (!afterById.has(b.blockId)) out.push({ blockId: b.blockId, kind: 'removed' });
  }
  return out;
}

/** Lookup of only the blocks that changed since rejection (drops `unchanged`), for
    badging the canvas. Keyed by `blockId`; removed ids are present too (the caller
    surfaces them as a count — a removed block no longer renders inline). */
export function changedBlocks(diffs: readonly BlockDiff[]): ReadonlyMap<string, BlockDiffKind> {
  return new Map(diffs.filter((d) => d.kind !== 'unchanged').map((d) => [d.blockId, d.kind]));
}