e(`v1-${this.source}\t${timestamp}`);
    }
  }

  /**
   * Retrieve the stored history entries for a certain source, sorted by
   * timestamp descending.
   *
   * @returns {Array<object>} a list of objects
   */
  async list() {
    const rkv = await this.#init();
    const entries = [];
    // The "from" and "to" key parameters to nsIKeyValueStore.enumerate()
    // are inclusive and exclusive, respectively, and keys are tuples
    // of source and datetime joined by a tab (\t), which is character code 9;
    // so enumerating ["source", "source\n"), where the line feed (\n)
    // is character code 10, enumerates all pairs with the given source.
    for (const { value } of await rkv.enumerate(
      `v1-${this.source}`,
      `v1-${this.source}\n`
    )) {
      try {
        const stored = JSON.parse(value);
        entries.push({ ...stored, datetime: new Date(stored.timestamp) });
      } catch (e) {
        // Ignore malformed entries.
        console.error(e);
      }
    }
    // Sort entries by `timestamp` descending.
    entries.sort((a, b) => (a.timestamp > b.timestamp ? -1 : 1));
    return entries;
  }

  /**
   * Return the most recent entry.
   */
  async last() {
    // List is sorted from newer to older.
    return (await this.list())[0];
  }

  /**
   * Wipe out the **whole** store.
   */
  async clear() {
    const rkv = await this.#init();
    await rkv.clear();
  }

  /**
   * Initialize the rkv store in the user profile.
   *
   * @returns {object} the underlying `KeyValueService` instance.
   */
  async #init() {
    if (!this.#store) {
      // Get and cache a handle to the kvstore.
      const dir = PathUtils.join(PathUtils.profileDir, "settings");
      await IOUtils.makeDirectory(dir);
      this.#store = await lazy.KeyValueService.getOrCreate(dir, "synchistory");
    }
    return this.#store;
  }
}
PK