Adding ElasticCollision2D (#6802)

* Adding ElasticCollision2D

* Fixing build issues.

src/main/java/com/thealgorithms/physics/ElasticCollision2D.java

@@ -0,0 +1,73 @@
+package com.thealgorithms.physics;
+
+/**
+ * 2D Elastic collision between two circular bodies
+ * Based on principles of conservation of momentum and kinetic energy.
+ *
+ * @author [Yash Rajput](https://github.com/the-yash-rajput)
+ */
+public final class ElasticCollision2D {
+
+    private ElasticCollision2D() {
+        throw new AssertionError("No instances. Utility class");
+    }
+
+    public static class Body {
+        public double x;
+        public double y;
+        public double vx;
+        public double vy;
+        public double mass;
+        public double radius;
+
+        public Body(double x, double y, double vx, double vy, double mass, double radius) {
+            this.x = x;
+            this.y = y;
+            this.vx = vx;
+            this.vy = vy;
+            this.mass = mass;
+            this.radius = radius;
+        }
+    }
+
+    /**
+     * Resolve instantaneous elastic collision between two circular bodies.
+     *
+     * @param a first body
+     * @param b second body
+     */
+    public static void resolveCollision(Body a, Body b) {
+        double dx = b.x - a.x;
+        double dy = b.y - a.y;
+        double dist = Math.hypot(dx, dy);
+
+        if (dist == 0) {
+            return; // overlapping
+        }
+
+        double nx = dx / dist;
+        double ny = dy / dist;
+
+        // relative velocity along normal
+        double rv = (b.vx - a.vx) * nx + (b.vy - a.vy) * ny;
+
+        if (rv > 0) {
+            return; // moving apart
+        }
+
+        // impulse with masses
+        double m1 = a.mass;
+        double m2 = b.mass;
+
+        double j = -(1 + 1.0) * rv / (1.0 / m1 + 1.0 / m2);
+
+        // impulse vector
+        double impulseX = j * nx;
+        double impulseY = j * ny;
+
+        a.vx -= impulseX / m1;
+        a.vy -= impulseY / m1;
+        b.vx += impulseX / m2;
+        b.vy += impulseY / m2;
+    }
+}

src/test/java/com/thealgorithms/physics/ElasticCollision2DTest.java

@@ -0,0 +1,91 @@
+package com.thealgorithms.physics;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class ElasticCollision2DTest {
+
+    @Test
+    void testEqualMassHeadOnCollision() {
+        ElasticCollision2D.Body a = new ElasticCollision2D.Body(0, 0, 1, 0, 1.0, 0.5);
+        ElasticCollision2D.Body b = new ElasticCollision2D.Body(1, 0, -1, 0, 1.0, 0.5);
+
+        ElasticCollision2D.resolveCollision(a, b);
+
+        assertEquals(-1.0, a.vx, 1e-6);
+        assertEquals(0.0, a.vy, 1e-6);
+        assertEquals(1.0, b.vx, 1e-6);
+        assertEquals(0.0, b.vy, 1e-6);
+    }
+
+    @Test
+    void testUnequalMassHeadOnCollision() {
+        ElasticCollision2D.Body a = new ElasticCollision2D.Body(0, 0, 2, 0, 2.0, 0.5);
+        ElasticCollision2D.Body b = new ElasticCollision2D.Body(1, 0, -1, 0, 1.0, 0.5);
+
+        ElasticCollision2D.resolveCollision(a, b);
+
+        // 1D head-on collision results
+        assertEquals(0.0, a.vx, 1e-6);
+        assertEquals(0.0, a.vy, 1e-6);
+        assertEquals(3.0, b.vx, 1e-6);
+        assertEquals(0.0, b.vy, 1e-6);
+    }
+
+    @Test
+    void testMovingApartNoCollision() {
+        ElasticCollision2D.Body a = new ElasticCollision2D.Body(0, 0, -1, 0, 1.0, 0.5);
+        ElasticCollision2D.Body b = new ElasticCollision2D.Body(1, 0, 1, 0, 1.0, 0.5);
+
+        ElasticCollision2D.resolveCollision(a, b);
+
+        assertEquals(-1.0, a.vx, 1e-6);
+        assertEquals(0.0, a.vy, 1e-6);
+        assertEquals(1.0, b.vx, 1e-6);
+        assertEquals(0.0, b.vy, 1e-6);
+    }
+
+    @Test
+    void testGlancingCollision() {
+        ElasticCollision2D.Body a = new ElasticCollision2D.Body(0, 0, 1, 1, 1.0, 0.5);
+        ElasticCollision2D.Body b = new ElasticCollision2D.Body(1, 1, -1, -0.5, 1.0, 0.5);
+
+        ElasticCollision2D.resolveCollision(a, b);
+
+        // Ensure relative velocity along normal is reversed
+        double nx = (b.x - a.x) / Math.hypot(b.x - a.x, b.y - a.y);
+        double ny = (b.y - a.y) / Math.hypot(b.x - a.x, b.y - a.y);
+        double relVelAfter = (b.vx - a.vx) * nx + (b.vy - a.vy) * ny;
+
+        assertTrue(relVelAfter > 0);
+    }
+
+    @Test
+    void testOverlappingBodies() {
+        ElasticCollision2D.Body a = new ElasticCollision2D.Body(0, 0, 1, 0, 1.0, 0.5);
+        ElasticCollision2D.Body b = new ElasticCollision2D.Body(0, 0, -1, 0, 1.0, 0.5);
+
+        ElasticCollision2D.resolveCollision(a, b);
+
+        // Should not crash, velocities may remain unchanged
+        assertEquals(1.0, a.vx, 1e-6);
+        assertEquals(0.0, a.vy, 1e-6);
+        assertEquals(-1.0, b.vx, 1e-6);
+        assertEquals(0.0, b.vy, 1e-6);
+    }
+
+    @Test
+    void testStationaryBodyHit() {
+        ElasticCollision2D.Body a = new ElasticCollision2D.Body(0, 0, 2, 0, 1.0, 0.5);
+        ElasticCollision2D.Body b = new ElasticCollision2D.Body(1, 0, 0, 0, 1.0, 0.5);
+
+        ElasticCollision2D.resolveCollision(a, b);
+
+        assertEquals(0.0, a.vx, 1e-6);
+        assertEquals(0.0, a.vy, 1e-6);
+        assertEquals(2.0, b.vx, 1e-6);
+        assertEquals(0.0, b.vy, 1e-6);
+    }
+}