'use client';

import { useEffect, useRef, useState } from 'react';
import type { StateSnapshot } from './game/state';

// ISS-007 (v0.16.1): the sim clock ticks smoothly on the CLIENT between polls instead of
// jumping once per snapshot. The server's authoritative rate is exactly real-time × timeScale
// (tickOnce in lib/sim/engine.ts: advance = min(elapsed, MAX_TICK_ELAPSED_MS) × timeScale), so
// client-side extrapolation mirrors the server's own model. Every new snapshot re-anchors to
// server truth, bounding any drift by network latency only.

export interface SimClockAnchor {
  // Server truth at anchor time — the snapshot's simState.clockMs.
  clockMs: number;
  // Date.now() when that snapshot was received.
  at: number;
  timeScale: number;
  paused: boolean;
}

// Pure core of the extrapolation — mirrors tickOnce: while running, game time advances by
// real-time × timeScale; while paused it is frozen (advance 0). nowMs before `at` clamps to
// the anchor value instead of going backwards. Unit-testable in node.
export function advanceSimClock(anchor: SimClockAnchor, nowMs: number): number {
  if (anchor.paused) return anchor.clockMs;
  return anchor.clockMs + Math.max(0, nowMs - anchor.at) * anchor.timeScale;
}

// ~10 fps display refresh — smooth for a seconds-precision wall clock without busy re-renders.
const DISPLAY_TICK_MS = 100;

export interface SimClockControls {
  // EFFECTIVE controls (optimistic pending scale / current pause), not the snapshot's:
  // UI changes take effect on click/drag without waiting for a confirming snapshot.
  timeScale: number;
  paused: boolean;
}

// Returns the display clockMs to render, or null until the first snapshot arrives.
export function useSimClock(snapshot: StateSnapshot | null, controls: SimClockControls): number | null {
  const anchorRef = useRef<SimClockAnchor | null>(null);
  const [displayMs, setDisplayMs] = useState<number | null>(null);

  // Re-sync on every snapshot AND effective-control change: snap to server truth. The snapshot
  // object is fresh per poll (useGameState), so this runs once per poll cycle as well.
  useEffect(() => {
    if (!snapshot) return;
    anchorRef.current = {
      clockMs: snapshot.simState.clockMs,
      at: Date.now(),
      timeScale: controls.timeScale,
      paused: controls.paused,
    };
    setDisplayMs(advanceSimClock(anchorRef.current, Date.now()));
  }, [snapshot, controls.timeScale, controls.paused]);

  useEffect(() => {
    const id = setInterval(() => {
      const anchor = anchorRef.current;
      if (anchor) setDisplayMs(advanceSimClock(anchor, Date.now()));
    }, DISPLAY_TICK_MS);
    return () => clearInterval(id);
  }, []);

  // First paint after a snapshot arrives happens before the re-sync effect runs — show the
  // snapshot value directly so the clock never blanks for a frame.
  return displayMs ?? (snapshot ? snapshot.simState.clockMs : null);
}