From c840bd01a55b7169874a70aae2bfcbf436071c52 Mon Sep 17 00:00:00 2001
From: Max Cahill <max@1bardesign.com>
Date: Wed, 14 Apr 2021 13:32:14 +1000
Subject: [PATCH] [added] more intersection/resolution helpers

---
 intersect.lua | 53 +++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 49 insertions(+), 4 deletions(-)

diff --git a/intersect.lua b/intersect.lua
index 673ba13..5740734 100644
--- a/intersect.lua
+++ b/intersect.lua
@@ -420,13 +420,58 @@ end
 
 --resolution helpers
 
+--resolve a collision between two bodies, given a (minimum) separating vector
+--	from a's frame of reference, like the result of any of the _collide functions
+--requires the two positions of the bodies, the msv, and a balance factor
+--balance should be between 1 and 0;
+--	1 is only a_pos moving to resolve
+--	0 is only b_pos moving to resolve
+--	0.5 is balanced between both (default)
+--note: this wont work as-is for line segments, which have two separate position coordinates
+--		you will need to understand what is going on and move the second coordinate yourself
+function intersect.resolve_msv(a_pos, b_pos, msv, balance)
+	balance = balance or 0.5
+	a_pos:fmai(msv, balance)
+	b_pos:fmai(msv, -(1 - balance))
+end
+
 --bounce a velocity off of a normal (modifying velocity)
-local _bounce_temp = vec2()
-function intersect.bounce_off(velocity, normal)
-	_bounce_temp:vset(velocity)
+--essentially flips the part of the velocity in the direction of the normal
+function intersect.bounce_off(velocity, normal, conservation)
+	--(default)
+	conservation = conservation or 1
+	--take a copy, we need it
+	local old_vel = vec2.pooled_copy(velocity)
+	--reject on the normal (keep velocity tangential to the normal)
 	velocity:vreji(normal)
-	velocity:vsubi(_bounce_temp:vsubi(velocity))
+	--add back the complement of the difference;
+	--basically "flip" the velocity in line with the normal.
+	velocity:fmai(old_vel:vsubi(velocity), -conservation)
+	--clean up
+	old_vel:release()
 	return velocity
 end
 
+--mutual bounce; two similar bodies bounce off each other, transferring energy
+function intersect.mutual_bounce(velocity_a, velocity_b, normal, conservation)
+	--(default)
+	conservation = conservation or 1
+	--take copies, we need them
+	local old_a_vel = vec2.pooled_copy(velocity_a)
+	local old_b_vel = vec2.pooled_copy(velocity_b)
+	--reject on the normal
+	velocity_a:vreji(normal)
+	velocity_b:vreji(normal)
+	--calculate the amount remaining from the old velocity
+	--(transfer ownership)
+	local a_remaining = old_a_vel:vsubi(velocity_a)
+	local b_remaining = old_b_vel:vsubi(velocity_b)
+	--transfer it to the other body
+	velocity_a:fmai(b_remaining, conservation)
+	velocity_b:fmai(a_remaining, conservation)
+	--clean up
+	a_remaining:release()
+	b_remaining:release()
+end
+
 return intersect