/**
 * State cache with TTL-based expiry and SHA-256 ETag support.
 *
 * Provides a lightweight in-memory cache for sidecar state snapshots,
 * session data, and delegations. Each entry stores the cached value,
 * a computed ETag for HTTP conditional requests, and an expiry timestamp.
 *
 * @module sidecar/server/cache
 */
/** A single cache entry with ETag and TTL metadata. */
export interface CacheEntry<T> {
    data: T;
    etag: string;
    expiresAt: number;
}
/** Cache category used to select the default TTL. */
export type CacheCategory = "snapshot" | "session" | "delegations" | "docs";
/**
 * In-memory state cache with per-category TTLs and SHA-256 ETag.
 *
 * All operations are synchronous and benefit from Node's single-threaded
 * event loop (no concurrent access races on the internal Map).
 *
 * @typeParam T - The type of data stored in each entry.
 *
 * @example
 * ```ts
 * const cache = new SidecarStateCache()
 * const etag = cache.set("sess-1", { id: "sess-1" }, "session")
 * const entry = cache.get<{ id: string }>("sess-1", "session")
 * cache.invalidate("all")
 * ```
 */
export declare class SidecarStateCache {
    #private;
    /**
     * Retrieve a cached entry if it has not expired.
     *
     * Expired entries are evicted before returning `undefined`.
     *
     * @param key      - Cache key (typically a URL path or session ID).
     * @param category - Category for TTL selection (unused on read).
     * @returns The cached data, or `undefined` if missing/expired.
     */
    get<T>(key: string, _category?: CacheCategory): T | undefined;
    /**
     * Store a value in the cache.
     *
     * Computes a SHA-256 ETag from `JSON.stringify(data)`.
     *
     * @param key      - Cache key.
     * @param data     - Value to cache.
     * @param category - Category used to select the default TTL.
     * @returns The computed ETag (strong, quoted per HTTP spec).
     * @throws `[Hivemind]` on serialisation failure.
     */
    set<T>(key: string, data: T, category?: CacheCategory): string;
    /**
     * Invalidate all entries matching a category or all entries.
     *
     * When `category` is `"all"` the entire cache is cleared.
     *
     * @param category - Category to evict, or `"all"` for full clear.
     */
    invalidate(category: CacheCategory | "all"): void;
}
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