import type { PositionEntry } from './protocol';

export interface Coalescer {
  /** Buffer the latest position for `deviceId`. Replaces any earlier in-flight value. */
  push: (p: PositionEntry) => void;
  /** Drop the buffer and cancel any pending flush. Used on event-switch and shutdown. */
  cancel: () => void;
}

/**
 * Per-frame coalescer at the WebSocket boundary.
 *
 * Every incoming `position` message lands in a per-device buffer; the
 * latest wins. One `requestAnimationFrame` tick flushes the snapshot to
 * the consumer (typically `usePositionStore.getState().applyPositions`).
 * That caps the dispatch rate at the browser's frame rate (~60 Hz)
 * regardless of how fast positions arrive.
 *
 * This is the discipline traccar-web lacks: per-message Redux dispatch
 * cascades through selectors and rebuilds full feature collections at
 * every position arrival, which is the most likely cause of its observed
 * lag at high update rates.
 */
export function createCoalescer(onFlush: (snapshot: PositionEntry[]) => void): Coalescer {
  const buffer = new Map<string, PositionEntry>();
  let rafId: number | null = null;

  function flush(): void {
    rafId = null;
    if (buffer.size === 0) return;
    const snapshot = Array.from(buffer.values());
    buffer.clear();
    onFlush(snapshot);
  }

  return {
    push(p) {
      buffer.set(p.deviceId, p);
      if (rafId === null) {
        rafId = requestAnimationFrame(flush);
      }
    },
    cancel() {
      buffer.clear();
      if (rafId !== null) {
        cancelAnimationFrame(rafId);
        rafId = null;
      }
    },
  };
}