/**
 * Copyright (c) Microsoft Corporation. All rights reserved.
 * Licensed under the MIT License.
 */
import { Storage, StoreItem } from './storage';
/**
 * A file-based storage implementation that persists data to the local filesystem.
 *
 * @remarks
 * FileStorage stores all data in a single JSON file named 'state.json' within a specified folder.
 * This implementation is suitable for development scenarios, local testing, and single-instance
 * deployments where shared state across multiple instances is not required.
 *
 * The storage format is a simple key-value JSON object where keys are strings and values
 * can be any JSON-serializable data. All operations are synchronous file I/O operations
 * wrapped in Promise interfaces to match the Storage contract.
 *
 * ### Warning
 * This implementation does not provide:
 * - Thread safety for concurrent access
 * - Optimistic concurrency control (eTag support)
 * - Atomic operations across multiple keys
 * - Scale for large datasets
 *
 * For production scenarios requiring these features, consider using
 * database-backed storage implementations instead.
 *
 * @example
 * ```typescript
 * const storage = new FileStorage('./data');
 *
 * // Write some data
 * await storage.write({
 *   'user123': { name: 'John', lastSeen: new Date().toISOString() },
 *   'conversation456': { turn: 5, context: 'discussing weather' }
 * });
 *
 * // Read specific keys
 * const data = await storage.read(['user123']);
 * console.log(data.user123); // { name: 'John', lastSeen: '...' }
 *
 * // Delete data
 * await storage.delete(['conversation456']);
 * ```
 *
 */
export declare class FileStorage implements Storage {
    private _folder;
    private _stateFile;
    /**
     * Creates a new FileStorage instance that stores data in the specified folder.
     *
     * @param folder The absolute or relative path to the folder where the state.json file will be stored
     * @throws May throw filesystem errors if the folder cannot be created or accessed
     *
     * @remarks
     * The constructor performs the following initialization steps:
     * 1. Creates the target folder if it doesn't exist (including parent directories)
     * 2. Creates an empty state.json file if it doesn't exist
     * 3. Loads existing data from state.json into memory for fast access
     *
     */
    constructor(folder: string);
    /**
     * Reads store items from the filesystem storage.
     *
     * @param keys Array of keys to read from storage
     * @returns Promise resolving to an object containing the requested items (keys that don't exist are omitted)
     * @throws ReferenceError if keys array is empty or undefined
     *
     * @remarks
     * This method reads from the in-memory cache that was loaded during construction,
     * making it very fast but potentially returning stale data if the file was
     * modified by external processes.
     *
     */
    read(keys: string[]): Promise<StoreItem>;
    /**
     * Writes store items to the filesystem storage.
     *
     * @param changes Object containing key-value pairs to write to storage
     * @returns Promise that resolves when the write operation completes
     *
     * @remarks
     * This method updates both the in-memory cache and writes the entire state
     * to the state.json file. The file is written with pretty-printing (2-space indentation)
     * for better readability during development and debugging.
     *
     * > [!NOTE]
     * > This implementation does not support eTag-based optimistic concurrency control.
     * > Any eTag values in the changes object are ignored.
     *
     */
    write(changes: StoreItem): Promise<void>;
    /**
     * Deletes store items from the filesystem storage.
     *
     * @param keys Array of keys to delete from storage
     * @returns Promise that resolves when the delete operation completes
     *
     * @throws ReferenceError if keys array is empty or undefined
     *
     * @remarks
     * This method removes the specified keys from both the in-memory cache
     * and writes the updated state to the state.json file. Keys that don't
     * exist in storage are silently ignored.
     *
     */
    delete(keys: string[]): Promise<void>;
}