import { openSaveDB } from './db';
import type { SaveEnvelope } from './envelope';

export interface SnapshotEntry {
  id: string;
  schemaVersion: number;
  savedAt: string;
  envelope: SaveEnvelope;
}

/**
 * Last-N pre-migration snapshot retention. CORE-08 mandates exactly 3.
 * If this needs to grow (e.g., Season 4 prestige rollback), update the
 * constant; do NOT make it a parameter — the single value across the app
 * is part of the contract Phase 2's settings UI shows the user.
 */
const RETAIN = 3;

/**
 * Write a pre-migration snapshot. After every write, prune to the
 * `RETAIN` newest entries by savedAt (descending). Works against both
 * IndexedDB and localStorage backends — `db.transaction(...).objectStore(...)`
 * is the common shape exposed by both.
 *
 * The snapshot ID includes a small entropy suffix because two snapshots
 * can fire in the same millisecond in tests (and theoretically in
 * production during a Phase-2 migration burst).
 */
export async function snapshot(envelope: SaveEnvelope): Promise<void> {
  const db = await openSaveDB();
  const tx = db.transaction('save_snapshots', 'readwrite');
  const store = tx.objectStore('save_snapshots');
  const savedAt = new Date().toISOString();
  const id = `${envelope.schemaVersion}-${savedAt}-${Math.random()
    .toString(36)
    .slice(2, 8)}`;
  await store.put({
    id,
    schemaVersion: envelope.schemaVersion,
    savedAt,
    envelope,
  });

  // Prune oldest beyond RETAIN
  const all = await store.getAll();
  const sorted = all.sort((a, b) => b.savedAt.localeCompare(a.savedAt));
  const toDelete = sorted.slice(RETAIN);
  await Promise.all(toDelete.map((e) => store.delete(e.id)));
  await tx.done;
}

/**
 * List snapshots in newest-first order. Returns `[]` on an empty store.
 */
export async function listSnapshots(): Promise<SnapshotEntry[]> {
  const db = await openSaveDB();
  const all = await db.getAll('save_snapshots');
  return all.sort((a, b) => b.savedAt.localeCompare(a.savedAt));
}