gyopiazza · November 8, 2019 20:48 · Nov 8, 2019 · Oct 3, 2017 · Oct 3, 2017 · Oct 3, 2017
diff --git a/util-elastic-collision.js b/util-elastic-collision.js
@@ -1,3 +1,19 @@
+/**
+ * Check if 2 circles collide
+ *
+ * Takes velocities and alters them as if the coordinate system they're on was rotated
+ *
+ * @param  Number | p1x | x coordinate of the first particle
+ * @param  Number | p1y | y coordinate of the first particle
+ * @param  Number | r1 | radius of the first particle
+ * @param  Number | p2x | x coordinate of the second particle
+ * @param  Number | p2y | y coordinate of the second particle
+ * @param  Number | r2 | radius of the second particle
+ * @return Boolean | true if particles are in collision
+ */
+const checkCollision = (p1x, p1y, r1, p2x, p2y, r2) =>
+  ((r1 + r2) ** 2 > (p1x - p2x) ** 2 + (p1y - p2y) ** 2)
+
 /**
  * Rotates coordinate system for velocities
  *

diff --git a/Utility Functions for Elastic Collision → util-elastic-collision.js b/Utility Functions for Elastic Collision → util-elastic-collision.js
diff --git a/Utility Functions for Elastic Collision b/Utility Functions for Elastic Collision
@@ -0,0 +1,65 @@
+/**
+ * Rotates coordinate system for velocities
+ *
+ * Takes velocities and alters them as if the coordinate system they're on was rotated
+ *
+ * @param  Object | velocity | The velocity of an individual particle
+ * @param  Float  | angle    | The angle of collision between two objects in radians
+ * @return Object | The altered x and y velocities after the coordinate system has been rotated
+ */
+
+function rotate(velocity, angle) {
+    const rotatedVelocities = {
+        x: velocity.x * Math.cos(angle) - velocity.y * Math.sin(angle),
+        y: velocity.x * Math.sin(angle) + velocity.y * Math.cos(angle)
+    };
+
+    return rotatedVelocities;
+}
+
+/**
+ * Swaps out two colliding particles' x and y velocities after running through
+ * an elastic collision reaction equation
+ *
+ * @param  Object | particle      | A particle object with x and y coordinates, plus velocity
+ * @param  Object | otherParticle | A particle object with x and y coordinates, plus velocity
+ * @return Null | Does not return a value
+ */
+
+function resolveCollision(particle, otherParticle) {
+    const xVelocityDiff = particle.velocity.x - otherParticle.velocity.x;
+    const yVelocityDiff = particle.velocity.y - otherParticle.velocity.y;
+
+    const xDist = otherParticle.x - particle.x;
+    const yDist = otherParticle.y - particle.y;
+
+    // Prevent accidental overlap of particles
+    if (xVelocityDiff * xDist + yVelocityDiff * yDist >= 0) {
+
+        // Grab angle between the two colliding particles
+        const angle = -Math.atan2(otherParticle.y - particle.y, otherParticle.x - particle.x);
+
+        // Store mass in var for better readability in collision equation
+        const m1 = particle.mass;
+        const m2 = otherParticle.mass;
+
+        // Velocity before equation
+        const u1 = rotate(particle.velocity, angle);
+        const u2 = rotate(otherParticle.velocity, angle);
+
+        // Velocity after 1d collision equation
+        const v1 = { x: u1.x * (m1 - m2) / (m1 + m2) + u2.x * 2 * m2 / (m1 + m2), y: u1.y };
+        const v2 = { x: u2.x * (m1 - m2) / (m1 + m2) + u1.x * 2 * m2 / (m1 + m2), y: u2.y };
+
+        // Final velocity after rotating axis back to original location
+        const vFinal1 = rotate(v1, -angle);
+        const vFinal2 = rotate(v2, -angle);
+
+        // Swap particle velocities for realistic bounce effect
+        particle.velocity.x = vFinal1.x;
+        particle.velocity.y = vFinal1.y;
+
+        otherParticle.velocity.x = vFinal2.x;
+        otherParticle.velocity.y = vFinal2.y;
+    }
+}