/** Process-local read reuse. Every access checks the live database revision, including cache hits. */
export class RevisionReadCache<T> {
  private readonly entries = new Map<string, { value: Promise<T>; bytes: number }>();
  private revision: number | undefined;

  constructor(private readonly maxEntries = 8, private readonly maxBytes = 32 * 1024 * 1024) {}

  async read(key: string, currentRevision: () => Promise<number>, load: () => Promise<T>): Promise<T> {
    for (let attempt = 0; attempt < 3; attempt += 1) {
      const before = await currentRevision();
      if (!Number.isSafeInteger(before) || before < 0) throw new Error('Cannot verify the database revision.');
      if (this.revision !== before) {
        this.entries.clear();
        this.revision = before;
      }
      let entry = this.entries.get(key);
      if (!entry) {
        entry = { value: Promise.resolve().then(load), bytes: 0 };
        this.entries.set(key, entry);
        while (this.entries.size > this.maxEntries) this.entries.delete(this.entries.keys().next().value!);
      }
      // Refresh insertion order for bounded least-recently-used retention.
      if (this.entries.get(key) === entry) {
        this.entries.delete(key);
        this.entries.set(key, entry);
      }
      try {
        const value = await entry.value;
        if (await currentRevision() !== before) {
          if (this.entries.get(key) === entry) this.entries.delete(key);
          continue;
        }
        if (this.entries.get(key) === entry && entry.bytes === 0) {
          entry.bytes = Buffer.byteLength(JSON.stringify(value), 'utf8');
          let retainedBytes = [...this.entries.values()].reduce((sum, item) => sum + item.bytes, 0);
          while (retainedBytes > this.maxBytes && this.entries.size) {
            const oldest = this.entries.keys().next().value!;
            retainedBytes -= this.entries.get(oldest)!.bytes;
            this.entries.delete(oldest);
          }
        }
        return value;
      } catch (error) {
        if (this.entries.get(key) === entry) this.entries.delete(key);
        throw error;
      }
    }
    throw new Error('ASSESSOR data changed repeatedly while results were read. Please retry.');
  }
}