feat: v0.16.1 — ISS-007 smooth client-side sim clock with per-snapshot re-sync

lib/useSimClock.ts

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+'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);
+}