time for a snapshot

workshop/steps/06-ray-vs-aabb/ray.ts

@@ -0,0 +1,52 @@
+import { type AABB, type Span, sweepInterval, type Vec } from "./given.ts";
+
+/** A collision: the fraction of the frame at impact, and the surface normal. */
+export type Hit = { time: number; normal: Vec };
+
+/**
+ * A point at `p` moving by `v` over this frame, against a static box.
+ * Returns the entry Hit, or null if there is no collision this frame.
+ *
+ * Reuse sweepInterval twice (x and y), then combine:
+ *   entry = max of the two entries, exit = min of the two exits.
+ */
+export function rayVsAABB(p: Vec, v: Vec, box: AABB): Hit | null {
+	const spanX = sweepInterval(p.x, v.x, box.x, box.x + box.w);
+	if (spanX === null) {
+		return null;
+	}
+	const spanY = sweepInterval(p.y, v.y, box.y, box.y + box.h);
+	if (spanY === null) {
+		return null;
+	}
+
+	const span: Span = {
+		entry: Math.max(spanX.entry, spanY.entry),
+		exit: Math.min(spanX.exit, spanY.exit),
+	};
+
+	if (span.entry > span.exit) {
+		return null;
+	}
+
+	if (span.entry >= 1 || span.exit <= 0) {
+		return null;
+	}
+
+	const time = span.entry;
+	const normal = {
+		x: 0,
+		y: 0,
+	};
+
+	if (spanX.entry > spanY.entry) {
+		normal.x = v.x > 0 ? -1 : 1;
+	} else {
+		normal.y = v.y > 0 ? -1 : 1;
+	}
+
+	return {
+		time,
+		normal,
+	};
+}