From d55babb7820a24531ff74765ba5e06f9a476efd5 Mon Sep 17 00:00:00 2001
From: the lemons <citrons@mondecitronne.com>
Date: Mon, 24 Apr 2023 19:42:13 -0500
Subject: mostly functional obj:avel_to_accel

---
 obj.lua       | 38 ++++++++++++++++----------------------
 objects/x.lua |  8 ++++----
 2 files changed, 20 insertions(+), 26 deletions(-)

diff --git a/obj.lua b/obj.lua
index 5fa4c0e..42bb2c2 100644
--- a/obj.lua
+++ b/obj.lua
@@ -5,6 +5,7 @@ local pi = math.pi
 local obj = {}
 
 obj.max_size = 20
+obj.mass = 1
 local types = {}
 
 function obj.load_types()
@@ -152,11 +153,12 @@ function obj:tick(...)
 						self.data.pos[1] - math.cos(angle) * dist + dx,
 						self.data.pos[2] - math.sin(angle) * dist + dy)
 
-					local av1, av2 = self.data.avel, o.data.avel
-					-- transfer angular velocity conserving energy
-					local avel = math.sqrt((av1*av1 + av2*av2) / 2)
-					self.data.avel = avel * (av1 > av2 and 1 or -1)
-					o.data.avel = avel * (av1 > av2 and -1 or 1)
+					local av1, av2 =
+						math.abs(self.data.avel), math.abs(o.data.avel)
+					self.data.avel = ((av1 + av2) / 2) *
+						(self.data.avel >= 0 and 1 or -1)
+					o.data.avel = ((av1 + av2) / 2) *
+						(o.data.avel >= 0 and 1 or -1)
 
 					local vx, vy = unpack(self.data.vel)
 					local ovx, ovy = unpack(o.data.vel)
@@ -199,11 +201,7 @@ end
 function obj:energy()
 	local vx, vy = unpack(self.data.vel)
 	local avel = self.data.avel
-	return 1 + avel*avel + vx*vx + vy*vy
-end
-
-function obj:observe_avel(o)
-	return o.data.avel - util.magnitude(self.data.vel)
+	return self.mass + self.mass * (math.abs(avel) + vx^2 + vy^2)
 end
 
 function obj:observe_vel(o)
@@ -219,21 +217,17 @@ function obj:observe_pos(o)
 end
 
 function obj:avel_to_accel(o, ax, ay)
-	local ovx, ovy = self:observe_vel(o)
+	local vx, vy = unpack(self.data.vel)
+	local ovx, ovy = unpack(o.data.vel)
 	local nvx, nvy = ovx + ax, ovy + ay
-	local v = util.magnitude{ovx, ovy}
-	local nv = util.magnitude{nvx, nvy}
-	local ne = v*v - nv*nv
-	local avel = self:observe_avel(o)
-	local ave = avel*avel + ne
+	local energy = (ovx^2 + ovy^2) - (nvx^2 + nvy^2)
+	local avel = o.data.avel
+	local ave = math.abs(avel) + energy
 	if ave < 0 then
-		return false
+		-- ???
 	end
-	local vx, vy = unpack(self.data.vel)
-	o.data.avel =
-		util.magnitude{vx, vy} + (math.sqrt(ave) * (avel >= 0 and 1 or -1))
-	o.data.vel = {vx + nvx, vy + nvy}
-	return true
+	o.data.avel = ave * (avel >= 0 and 1 or -1)
+	o.data.vel[1], o.data.vel[2] = nvx, nvy
 end
 
 function obj:in_hitbox(px, py)
diff --git a/objects/x.lua b/objects/x.lua
index 080887b..5f37a7a 100644
--- a/objects/x.lua
+++ b/objects/x.lua
@@ -5,10 +5,10 @@ local x_obj = {hitbox = 4}
 
 function x_obj:draw()
 	set_color(0.5, 1, 0.8)
-	line(-4, 2, 0, 0, -2, 4, -4, 2)
-	line(0, 0, 2, 4, 4, 2, 0, 0)
-	line(0, 0, 4, -2, 2, -4, 0, 0)
-	line(0, 0, -2, -4, -4, -2, 0, 0)
+	line(-1, 4, -3, 3, -4, 1, 0, 0, -1, 4)
+	line(0, 0, 1, 4, 3, 3, 4, 1, 0, 0)
+	line(0, 0, 4, -1, 3, -3, 1, -4, 0, 0)
+	line(0, 0, -1, -4, -3, -3, -4, -1, 0, 0)
 end
 
 local range = 40
-- 
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