/**
 * Per-peer outbound queue + queue-transport for browser-friendly
 * pull-mode streams.
 *
 * The default `ShadeTransferHttpTransport` POSTs each chunk directly
 * to the receiver's `/v1/transfer/<streamId>/chunk` route. That
 * requires the receiver to host an HTTP server, which a browser tab
 * cannot. `QueueTransferTransport` flips the direction: it queues
 * chunks per peer and lets the receiver pull them via a long-poll
 * endpoint.
 *
 * The companion `OutboundQueue` data structure is plain server-side
 * state — wired up by `@shade/files`'s `transferQueueRoute()` (and any
 * future consumer) to expose the long-poll surface and feed envelopes
 * + chunks into the browser receiver.
 */
import type {
  ChunkAck,
  ChunkSendOptions,
  ITransferTransport,
  TransferResumeState,
} from './transport.js';
import { TransferTransportError } from '../errors.js';

/**
 * Discriminated union of items the queue ships to the browser
 * receiver. Both kinds carry **wire-encoded bytes** of an envelope —
 * the receiver decodes via `decodeEnvelope` (control envelopes) or
 * forwards directly to `engine.receiveChunk` (chunk envelopes).
 */
export type QueuedEvent =
  | {
      id: number;
      timestampMs: number;
      kind: 'envelope';
      /** Wire-encoded `0x02` ratchet envelope (or `0x01` first-message). */
      bytes: Uint8Array;
    }
  | {
      id: number;
      timestampMs: number;
      kind: 'chunk';
      /** Wire-encoded `0x11` stream-chunk envelope. */
      bytes: Uint8Array;
      meta: {
        streamId: string;
        laneId: number;
        seq: number;
      };
    };

/** Caller-supplied shape for {@link OutboundQueue.enqueue} — the queue assigns `id` + `timestampMs`. */
export type QueuedEventInput =
  | { kind: 'envelope'; bytes: Uint8Array }
  | {
      kind: 'chunk';
      bytes: Uint8Array;
      meta: { streamId: string; laneId: number; seq: number };
    };

export interface OutboundQueueOptions {
  /**
   * Maximum events held per peer. When the queue is full, the oldest
   * unacked event is dropped on next enqueue. Default 1000 — at the
   * default chunk size (256 KiB plaintext) this caps a single peer's
   * outbound buffer at ~256 MiB. Tune up for fewer/bigger streams,
   * down for many concurrent small flows.
   */
  maxEventsPerPeer?: number;
  /**
   * After a peer has not polled for this long, the queue's events are
   * dropped and any pending waiters are released. Default 10 minutes.
   * Setting to `0` disables idle-eviction.
   */
  idleEvictionMs?: number;
}

const DEFAULT_MAX_EVENTS = 1000;
const DEFAULT_IDLE_EVICTION_MS = 10 * 60 * 1000;

interface PendingWaiter {
  resolve(events: QueuedEvent[]): void;
  reject(err: Error): void;
  timer: ReturnType<typeof setTimeout>;
  abortHandler?: () => void;
  signal?: AbortSignal;
}

interface PerPeerState {
  nextId: number;
  events: QueuedEvent[];
  waiters: PendingWaiter[];
  lastTouchedMs: number;
}

/**
 * Per-peer monotonic event log with long-poll semantics.
 *
 * `enqueue` appends; `drain` returns all events with `id > since`,
 * blocking up to `blockMs` if there are none. `since`-based pagination
 * is the resume mechanism: a client crashing mid-stream restarts with
 * its last-processed id and the queue replays everything after it
 * (subject to `maxEventsPerPeer` retention).
 */
export class OutboundQueue {
  private peers = new Map<string, PerPeerState>();
  private readonly maxEvents: number;
  private readonly idleEvictionMs: number;
  private evictTimer: ReturnType<typeof setTimeout> | null = null;
  private destroyed = false;

  constructor(opts: OutboundQueueOptions = {}) {
    this.maxEvents = opts.maxEventsPerPeer ?? DEFAULT_MAX_EVENTS;
    this.idleEvictionMs = opts.idleEvictionMs ?? DEFAULT_IDLE_EVICTION_MS;
    if (this.idleEvictionMs > 0) this.scheduleEviction();
  }

  /** Append an event and wake any waiters for that peer. */
  enqueue(peer: string, ev: QueuedEventInput): QueuedEvent {
    if (this.destroyed) throw new Error('OutboundQueue: destroyed');
    const state = this.getOrCreate(peer);
    const event: QueuedEvent =
      ev.kind === 'chunk'
        ? {
            id: state.nextId++,
            timestampMs: Date.now(),
            kind: 'chunk',
            bytes: ev.bytes,
            meta: ev.meta,
          }
        : {
            id: state.nextId++,
            timestampMs: Date.now(),
            kind: 'envelope',
            bytes: ev.bytes,
          };
    state.events.push(event);
    state.lastTouchedMs = Date.now();
    // Cap: drop oldest. Lost events trigger receiver-side resume from
    // last polled id; the @shade/transfer engine handles missing seqs
    // by re-sending on resume.
    while (state.events.length > this.maxEvents) state.events.shift();
    // Wake all waiters with whatever has accumulated.
    const drained = this.collect(state, 0);
    if (drained.length > 0) {
      const waiters = state.waiters.splice(0);
      for (const w of waiters) {
        clearTimeout(w.timer);
        if (w.abortHandler !== undefined && w.signal !== undefined) {
          w.signal.removeEventListener('abort', w.abortHandler);
        }
        w.resolve(drained);
      }
    }
    return event;
  }

  /**
   * Drain events with `id > since`. If none are available, block up
   * to `blockMs` until any arrive. `signal` cancels the wait early.
   */
  async drain(
    peer: string,
    since: number,
    blockMs: number,
    signal?: AbortSignal,
  ): Promise<QueuedEvent[]> {
    if (this.destroyed) throw new Error('OutboundQueue: destroyed');
    const state = this.getOrCreate(peer);
    state.lastTouchedMs = Date.now();
    const ready = this.collect(state, since);
    if (ready.length > 0 || blockMs <= 0) return ready;
    if (signal?.aborted) return [];
    return await new Promise<QueuedEvent[]>((resolve, reject) => {
      const timer = setTimeout(() => {
        const idx = state.waiters.indexOf(waiter);
        if (idx >= 0) state.waiters.splice(idx, 1);
        if (waiter.abortHandler !== undefined && waiter.signal !== undefined) {
          waiter.signal.removeEventListener('abort', waiter.abortHandler);
        }
        // Empty drain on timeout — that's the "no new events" signal.
        resolve([]);
      }, blockMs);
      const waiter: PendingWaiter = { resolve, reject, timer };
      if (signal !== undefined) {
        const handler = () => {
          const idx = state.waiters.indexOf(waiter);
          if (idx >= 0) state.waiters.splice(idx, 1);
          clearTimeout(timer);
          resolve([]);
        };
        signal.addEventListener('abort', handler, { once: true });
        waiter.abortHandler = handler;
        waiter.signal = signal;
      }
      state.waiters.push(waiter);
    });
  }

  /** Drop a peer's queue + reject waiters. */
  evict(peer: string): void {
    const state = this.peers.get(peer);
    if (state === undefined) return;
    this.peers.delete(peer);
    for (const w of state.waiters) {
      clearTimeout(w.timer);
      if (w.abortHandler !== undefined && w.signal !== undefined) {
        w.signal.removeEventListener('abort', w.abortHandler);
      }
      w.reject(new Error('OutboundQueue: peer evicted'));
    }
  }

  /** Peer-specific snapshot for diagnostics. */
  stats(peer: string): { eventCount: number; nextId: number; waiters: number } | null {
    const state = this.peers.get(peer);
    if (state === undefined) return null;
    return {
      eventCount: state.events.length,
      nextId: state.nextId,
      waiters: state.waiters.length,
    };
  }

  /** Tear everything down. Pending waiters are rejected. */
  destroy(): void {
    if (this.destroyed) return;
    this.destroyed = true;
    if (this.evictTimer !== null) clearTimeout(this.evictTimer);
    for (const peer of [...this.peers.keys()]) this.evict(peer);
  }

  // ─── internals ──────────────────────────────────────────────

  private getOrCreate(peer: string): PerPeerState {
    let state = this.peers.get(peer);
    if (state === undefined) {
      state = {
        nextId: 1,
        events: [],
        waiters: [],
        lastTouchedMs: Date.now(),
      };
      this.peers.set(peer, state);
    }
    return state;
  }

  private collect(state: PerPeerState, since: number): QueuedEvent[] {
    if (state.events.length === 0) return [];
    return state.events.filter((e) => e.id > since);
  }

  private scheduleEviction(): void {
    const interval = Math.max(60_000, Math.floor(this.idleEvictionMs / 4));
    this.evictTimer = setTimeout(() => {
      if (this.destroyed) return;
      const cutoff = Date.now() - this.idleEvictionMs;
      for (const [peer, state] of this.peers.entries()) {
        if (state.lastTouchedMs < cutoff) this.evict(peer);
      }
      this.scheduleEviction();
    }, interval);
    (this.evictTimer as unknown as { unref?: () => void }).unref?.();
  }
}

/**
 * Chunk transport that enqueues into an {@link OutboundQueue} instead
 * of POSTing.
 *
 * Returns an optimistic `ChunkAck` immediately because the queue *is*
 * the delivery — the receiver polls and dispatches. Browser receivers
 * cannot synchronously confirm receipt before the next chunk; the
 * engine's stream-protocol catches dropped chunks at finish-time
 * integrity check, and chunk-resume restarts the lane from the last
 * polled `since`.
 */
export class QueueTransferTransport implements ITransferTransport {
  constructor(private readonly queue: OutboundQueue) {}

  async probe(_peer: string): Promise<void> {
    // The queue is local. Reachability is "is there a poller?" which
    // is decided by `idleEvictionMs`. We don't synchronously check
    // here; the engine retries via `withRetry` on `sendChunk` errors.
  }

  async sendChunk(
    peer: string,
    streamId: string,
    laneId: number,
    seq: number | bigint,
    bytes: Uint8Array,
    options?: ChunkSendOptions,
  ): Promise<ChunkAck> {
    if (options?.signal?.aborted) {
      throw new TransferTransportError('sendChunk aborted by caller');
    }
    const seqNum = typeof seq === 'bigint' ? Number(seq) : seq;
    this.queue.enqueue(peer, {
      kind: 'chunk',
      bytes,
      meta: { streamId, laneId, seq: seqNum },
    });
    return { lastSeq: seqNum };
  }

  async fetchResumeState(
    _peer: string,
    _streamId: string,
  ): Promise<TransferResumeState | null> {
    // Pull-mode receivers report resume state by re-polling with the
    // `since` cursor they last successfully processed; the queue does
    // not need to query the receiver. Return null so the engine
    // restarts from seq 0 (deterministic), and the queue replays from
    // `since=0` if the client reconnects fresh.
    return null;
  }
}