Merge pull request #59 from idbrii/fix-angleto

Fix angle_to to produce signed angle instead of incorrect results

modules/vec2.lua

@@ -228,22 +228,31 @@ function vec2.perpendicular(a)
 	return new(-a.y, a.x)
 end
 
---- Angle from one vector to another.
+--- Signed angle from one vector to another.
+-- Rotations from +x to +y are positive.
 -- @tparam vec2 a Vector
 -- @tparam vec2 b Vector
--- @treturn number angle
+-- @treturn number angle in (-pi, pi]
 function vec2.angle_to(a, b)
 	if b then
-		return atan2(a.y - b.y, a.x - b.x)
+		local angle = atan2(b.y, b.x) - atan2(a.y, a.x)
+		-- convert to (-pi, pi]
+		if angle > math.pi       then
+			angle = angle - 2 * math.pi
+		elseif angle <= -math.pi then
+			angle = angle + 2 * math.pi
+		end
+		return angle
 	end
 
 	return atan2(a.y, a.x)
 end
 
---- Angle between two vectors.
+--- Unsigned angle between two vectors.
+-- Directionless and thus commutative.
 -- @tparam vec2 a Vector
 -- @tparam vec2 b Vector
--- @treturn number angle
+-- @treturn number angle in [0, pi]
 function vec2.angle_between(a, b)
 	if b then
 		if vec2.is_vec2(a) then

spec/utils_spec.lua

@@ -45,6 +45,4 @@ project_from(out, a, b)
 mirror_on(out, a, b)
 reflect(out, i, n)
 refract(out, i, n, ior)
-angle_to(a, b)
-angle_between(a, b)
 --]]

spec/vec2_spec.lua

@@ -203,6 +203,62 @@ describe("vec2:", function()
 		assert.is.equal(temp, vec2(-1, -2))
 	end)
 
+	it("finds angle from one 2-vector to another", function()
+		local d = {
+			right = vec2(1,  0),
+			down  = vec2(0,  -1),
+			left  = vec2(-1, 0),
+			up    = vec2(0,  1),
+		}
+		assert.is.equal(math.deg(d.right:angle_to(d.right)), 0.0)
+		assert.is.equal(math.deg(d.right:angle_to(d.down)), -90.0)
+		assert.is.equal(math.deg(d.right:angle_to(d.left)), 180.0)
+		assert.is.equal(math.deg(d.right:angle_to(d.up)), 90.0)
+
+		assert.is.equal(math.deg(d.down:angle_to(d.right)), 90.0)
+		assert.is.equal(math.deg(d.down:angle_to(d.down)), 0.0)
+		assert.is.equal(math.deg(d.down:angle_to(d.left)), -90.0)
+		assert.is.equal(math.deg(d.down:angle_to(d.up)), 180.0)
+
+		assert.is.equal(math.deg(d.left:angle_to(d.right)), 180.0)
+		assert.is.equal(math.deg(d.left:angle_to(d.down)), 90.0)
+		assert.is.equal(math.deg(d.left:angle_to(d.left)), 0.0)
+		assert.is.equal(math.deg(d.left:angle_to(d.up)), -90.0)
+
+		assert.is.equal(math.deg(d.up:angle_to(d.right)), -90.0)
+		assert.is.equal(math.deg(d.up:angle_to(d.down)), 180.0)
+		assert.is.equal(math.deg(d.up:angle_to(d.left)), 90.0)
+		assert.is.equal(math.deg(d.up:angle_to(d.up)), 0.0)
+	end)
+
+	it("finds angle between two 2-vectors", function()
+		local d = {
+			right = vec2(1,  0),
+			down  = vec2(0,  -1),
+			left  = vec2(-1, 0),
+			up    = vec2(0,  1),
+		}
+		assert.is.equal(math.deg(d.right:angle_between(d.right)), 0.0)
+		assert.is.equal(math.deg(d.right:angle_between(d.down)), 90.0)
+		assert.is.equal(math.deg(d.right:angle_between(d.left)), 180.0)
+		assert.is.equal(math.deg(d.right:angle_between(d.up)), 90.0)
+
+		assert.is.equal(math.deg(d.down:angle_between(d.right)), 90.0)
+		assert.is.equal(math.deg(d.down:angle_between(d.down)), 0.0)
+		assert.is.equal(math.deg(d.down:angle_between(d.left)), 90.0)
+		assert.is.equal(math.deg(d.down:angle_between(d.up)), 180.0)
+
+		assert.is.equal(math.deg(d.left:angle_between(d.right)), 180.0)
+		assert.is.equal(math.deg(d.left:angle_between(d.down)), 90.0)
+		assert.is.equal(math.deg(d.left:angle_between(d.left)), 0.0)
+		assert.is.equal(math.deg(d.left:angle_between(d.up)), 90.0)
+
+		assert.is.equal(math.deg(d.up:angle_between(d.right)), 90.0)
+		assert.is.equal(math.deg(d.up:angle_between(d.down)), 180.0)
+		assert.is.equal(math.deg(d.up:angle_between(d.left)), 90.0)
+		assert.is.equal(math.deg(d.up:angle_between(d.up)), 0.0)
+	end)
+
 	-- Do this last, to insulate tests from accidental state contamination
 	-- Do vec3 tests last, to insulate tests from accidental state contamination
 	it("converts a 2-vector to a 3-vector", function()