From c8de7165d5de1c7891dd93aeafc579c8371faa59 Mon Sep 17 00:00:00 2001
From: Sam Anthony <sam@samanthony.xyz>
Date: Fri, 4 Oct 2024 17:15:37 -0400
Subject: refactor collision

---
 balls.cpp     |  8 ++++++--
 collision.cpp | 47 ++++++++++++++++++++++++++++++-----------------
 2 files changed, 36 insertions(+), 19 deletions(-)

diff --git a/balls.cpp b/balls.cpp
index 5803aef..bdae497 100644
--- a/balls.cpp
+++ b/balls.cpp
@@ -189,12 +189,16 @@ void
 animate(int v) {
 	size_t i, j;
 
+	/* TODO: parallel */
+	for (Ball &ball : balls) {
+		ball.v.y -=G;
+		ball.p = ptAddVec(ball.p, ball.v);
+	}
+
 	/* TODO: parallel */
 	for (i = 0; i < balls.size(); i++) {
 		for (j = i+1; j < balls.size(); j++)
 			collideBall(&balls[i], &balls[j]);
-		balls[i].v.y -= G;
-		balls[i].p = ptAddVec(balls[i].p, balls[i].v);
 		collideWall(&balls[i], bounds);
 	}
 
diff --git a/collision.cpp b/collision.cpp
index 5f0725e..7e69ba7 100644
--- a/collision.cpp
+++ b/collision.cpp
@@ -1,5 +1,7 @@
 #include "balls.h"
 
+static void setPosition(Ball *b1, Ball *b2);
+static Vector reaction(Ball b1, Ball b2);
 static double clamp(double v, double lo, double hi);
 static double min(double a, double b);
 static double max(double a, double b);
@@ -29,30 +31,41 @@ collideWall(Ball *b, Rectangle wall) {
 
 void
 collideBall(Ball *b1, Ball *b2) {
-	Point midpt;
-	Vector n, distv;
-	double mrat, coef;
+	Vector v1, v2;
 
 	if (!isCollision(b1->p, b1->r, b2->p, b2->r))
 		return;
 
-	/* set position of collision */
-	midpt = ptDivS(addPt(b1->p, b2->p), 2);
-	n = unitNorm(VecPt(b1->p, b2->p));
-	b1->p = ptSubVec(midpt, vecMulS(n, b1->r));
-	b2->p = ptAddVec(midpt, vecMulS(n, b2->r));
+	setPosition(b1, b2);
+
+	v1 = reaction(*b1, *b2);
+	v2 = reaction(*b2, *b1);
+	b1->v = v1;
+	b2->v = v2;
+}
 
-	/* reaction velocity */
+/* set the positions of b1 and b2 at the moment of collsion */
+static void
+setPosition(Ball *b1, Ball *b2) {
+	Point mid;
+	Vector n;
 
-	mrat = 2.0 * b2->m / (b1->m + b2->m);
-	distv = VecPt(b2->p, b1->p);
-	coef = vecDot(subVec(b1->v, b2->v), distv) / (vecLen(distv)*vecLen(distv));
-	b1->v = subVec(b1->v, vecMulS(distv, mrat*coef));
+	mid = ptDivS(addPt(b1->p, b2->p), 2);
+	n = unitNorm(VecPt(b1->p, b2->p));
+	b1->p = ptSubVec(mid, vecMulS(n, b1->r));
+	b2->p = ptAddVec(mid, vecMulS(n, b2->r));
+}
+
+/* return the velocity of b1 after colliding with b2 */
+static Vector
+reaction(Ball b1, Ball b2) {
+	double mrat, coef;
+	Vector distv;
 
-	mrat = 2.0 * b1->m / (b1->m + b2->m);
-	distv = VecPt(b1->p, b2->p);
-	coef = vecDot(subVec(b2->v, b1->v), distv) / (vecLen(distv)*vecLen(distv));
-	b2->v = subVec(b2->v, vecMulS(distv, mrat*coef));
+	mrat = 2.0 * b2.m / (b1.m + b2.m);
+	distv = VecPt(b2.p, b1.p);
+	coef = vecDot(subVec(b1.v, b2.v), distv) / (vecLen(distv)*vecLen(distv));
+	return subVec(b1.v, vecMulS(distv, mrat*coef));
 }
 
 static double
-- 
cgit v1.2.3