From 7aa15ed38fcf1aa2562152eee62fe8a1cb551473 Mon Sep 17 00:00:00 2001
From: "Stephen M. Cameron" <stephenmcameron@gmail.com>
Date: Wed, 5 Aug 2015 18:19:57 -0700
Subject: [PATCH] Add function to compute elastic collision response

Signed-off-by: Stephen M. Cameron <stephenmcameron@gmail.com>
---
 Makefile            |  3 +++
 elastic_collision.c | 61 +++++++++++++++++++++++++++++++++++++++++++++
 elastic_collision.h | 44 ++++++++++++++++++++++++++++++++
 3 files changed, 108 insertions(+)
 create mode 100644 elastic_collision.c
 create mode 100644 elastic_collision.h

diff --git a/Makefile b/Makefile
index 5a87e695..c8138dd7 100644
--- a/Makefile
+++ b/Makefile
@@ -697,6 +697,9 @@ arbitrary_spin.o:	arbitrary_spin.c arbitrary_spin.h Makefile
 mtwist.o:	mtwist.c Makefile
 	$(Q)$(COMPILE)
 
+elastic_collision.o:	elastic_collision.c elastic_collision.h Makefile
+	$(Q)$(COMPILE)
+
 fleet.o:	fleet.c Makefile
 	$(Q)$(COMPILE)
 
diff --git a/elastic_collision.c b/elastic_collision.c
new file mode 100644
index 00000000..5d4097d8
--- /dev/null
+++ b/elastic_collision.c
@@ -0,0 +1,61 @@
+/*
+	Copyright (C) 2015 Stephen M. Cameron
+	Author: Stephen M. Cameron
+
+	This file is part of Spacenerds In Space.
+
+	Spacenerds in Space is free software; you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation; either version 2 of the License, or
+	(at your option) any later version.
+
+	Spacenerds in Space is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with Spacenerds in Space; if not, write to the Free Software
+	Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+#include "elastic_collision.h"
+
+void elastic_collision(float m1, union vec3 *p1, union vec3 *v1i, float r1,
+		float m2, union vec3 *p2, union vec3 *v2i, float r2,
+		float energy_transmission_factor, union vec3 *v1o, union vec3 *v2o)
+{
+	union vec3 to2, to2n, to1n, deltav1, deltav2;
+	float dist;
+
+	vec3_sub(&to2, p2, p1);
+	dist = vec3_dist(p1, p2);
+
+	vec3_normalize(&to2n, &to2);
+	to1n.v.x = -to2n.v.x;
+	to1n.v.y = -to2n.v.y;
+	to1n.v.z = -to2n.v.z;
+
+	/* Separate the particles, if necessary */
+	if (dist < r1 + r2) {
+		/* split the difference, plus a little */
+		float amount = (1.01 * (r1 - r2) - dist) / 2.0;
+		union vec3 d1, d2;
+		vec3_mul(&d1, &to1n, amount);
+		vec3_mul(&d2, &to2n, amount);
+		vec3_add_self(p1, &d1);
+		vec3_add_self(p2, &d2);
+	}
+
+	/* transfer momentum... */
+	float p1momentum = vec3_dot(v1i, &to2n) * m1 / m2;
+	float p2momentum = vec3_dot(v2i, &to1n) * m2 / m1;
+
+	p1momentum *= sqrt(energy_transmission_factor);
+	p2momentum *= sqrt(energy_transmission_factor);
+
+	vec3_mul(&deltav1, &to1n, p2momentum);
+	vec3_mul(&deltav2, &to2n, p1momentum);
+
+	vec3_add(v1o, v1i, &deltav1);
+	vec3_add(v2o, v2i, &deltav2);
+}
diff --git a/elastic_collision.h b/elastic_collision.h
new file mode 100644
index 00000000..0d401d54
--- /dev/null
+++ b/elastic_collision.h
@@ -0,0 +1,44 @@
+#ifndef ELASTIC_COLLISION_H__
+#define ELASTIC_COLLISION_H__
+
+/*
+	Copyright (C) 2015 Stephen M. Cameron
+	Author: Stephen M. Cameron
+
+	This file is part of Spacenerds In Space.
+
+	Spacenerds in Space is free software; you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation; either version 2 of the License, or
+	(at your option) any later version.
+
+	Spacenerds in Space is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with Spacenerds in Space; if not, write to the Free Software
+	Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+#include <math.h>
+
+#include "quat.h"
+
+/*
+ * elastic_collision():
+ *
+ * Inputs:
+ * m1, p1, v1i, and r1 are mass, position, input velocity and radius of particle 1.
+ * m2, p2, v2i, and r2 are mass, position, input velocity and radius of particle 2.
+ *
+ * Outputs:
+ * p1, and p2 are output as adjusted positions of particles (separated by at least r1 + r2 after
+ * collisions)
+ * v1o and v2o are new velocities of particles 1 and 2, respectively.
+ */
+void elastic_collision(float m1, union vec3 *p1, union vec3 *v1i, float r1,
+		float m2, union vec3 *p2, union vec3 *v2i, float r2,
+		float energy_transmission_factor, union vec3 *v1o, union vec3 *v2o);
+
+#endif