Feat: Touch Gestures (Pinch Zoom) for Reader (#29)

src/features/reader/lib/pinchZoom.ts

@@ -0,0 +1,78 @@
+import { clampZoom } from "./zoomHelpers";
+import { ZOOM_MIN, ZOOM_MAX } from "../store/readerState.svelte";
+
+export interface PinchTrackerOptions {
+  getZoom:         () => number;
+  setZoom:         (z: number) => void;
+  getInspectScale: () => number;
+  setInspectScale: (s: number) => void;
+  resetInspectPan: () => void;
+  isLongstrip:     () => boolean;
+}
+
+export interface PinchTracker {
+  onPointerDown: (e: PointerEvent) => void;
+  onPointerMove: (e: PointerEvent) => void;
+  onPointerUp:   (e: PointerEvent) => void;
+  isPinching:    () => boolean;
+}
+
+const INSPECT_ZOOM_MAX = 8;
+
+export function createPinchTracker(opts: PinchTrackerOptions): PinchTracker {
+  const pointers = new Map<number, { x: number; y: number }>();
+  let startDist    = 0;
+  let startZoom    = 0;
+  let startInspect = 0;
+  let pinching     = false;
+
+  function dist(a: { x: number; y: number }, b: { x: number; y: number }): number {
+    return Math.hypot(b.x - a.x, b.y - a.y);
+  }
+
+  function onPointerDown(e: PointerEvent) {
+    pointers.set(e.pointerId, { x: e.clientX, y: e.clientY });
+    if (pointers.size === 2) {
+      const [a, b] = [...pointers.values()];
+      startDist    = dist(a, b);
+      startZoom    = opts.getZoom();
+      startInspect = opts.getInspectScale();
+      pinching     = true;
+    }
+  }
+
+  function onPointerMove(e: PointerEvent) {
+    if (!pinching || !pointers.has(e.pointerId)) return;
+    pointers.set(e.pointerId, { x: e.clientX, y: e.clientY });
+    if (pointers.size < 2) return;
+
+    const [a, b]   = [...pointers.values()];
+    const current  = dist(a, b);
+    if (startDist === 0) return;
+    const ratio    = current / startDist;
+
+    if (opts.isLongstrip()) {
+      opts.setZoom(clampZoom(startZoom * ratio));
+    } else {
+      const next = Math.max(1, Math.min(INSPECT_ZOOM_MAX, startInspect * ratio));
+      if (next !== opts.getInspectScale()) {
+        if (next === 1) opts.resetInspectPan();
+        opts.setInspectScale(next);
+      }
+    }
+  }
+
+  function onPointerUp(e: PointerEvent) {
+    pointers.delete(e.pointerId);
+    if (pointers.size < 2) {
+      pinching     = false;
+      startDist    = 0;
+      startZoom    = 0;
+      startInspect = 0;
+    }
+  }
+
+  function isPinching() { return pinching; }
+
+  return { onPointerDown, onPointerMove, onPointerUp, isPinching };
+}