shape: Add support for setting mass/density on Shapes (#19)

body_test.go

@@ -0,0 +1,52 @@
+package cp
+
+import (
+	"testing"
+)
+
+func TestBodyMassFromShapes(t *testing.T) {
+	body := NewBody(0, 0)
+	circle := NewCircle(body, 5, Vector{0, 0})
+	circle2 := NewCircle(body, 5, Vector{0, 0})
+
+	mass := 10.0
+	circle.SetMass(mass)
+	body.AddShape(circle)
+
+	if body.Mass() != mass {
+		t.Fail()
+	}
+
+	circle2.SetMass(mass)
+	body.AddShape(circle2)
+
+	if body.Mass() != mass * 2 {
+		t.Fail()
+	}
+}
+
+func TestBodyCoGFromShapes(t *testing.T) {
+	body := NewBody(0, 0)
+	circle := NewCircle(body, 5, Vector{0, 0})
+	circle2 := NewCircle(body, 5, Vector{10, 0})
+
+	mass := 10.0
+
+	circle.SetMass(mass)
+	body.AddShape(circle)
+
+	circle2.SetMass(mass)
+	body.AddShape(circle2)
+
+	cog := body.CenterOfGravity()
+	if cog.X != 5.0 || cog.Y != 0.0 {
+		t.Fail()
+	}
+
+	body.RemoveShape(circle)
+	cog = body.CenterOfGravity()
+	if cog.X != 10.0 || cog.Y != 0.0 {
+		t.Fail()
+	}
+
+}

shape.go

@@ -79,6 +79,37 @@ func (s *Shape) MassInfo() *ShapeMassInfo {
 	return s.massInfo
 }
 
+func (s *Shape) Mass() float64 {
+	return s.massInfo.m
+}
+
+func (s *Shape) SetMass(mass float64) {
+	s.body.Activate()
+
+	s.massInfo.m = mass
+	s.body.AccumulateMassFromShapes()
+}
+
+func (s *Shape) Density() float64 {
+	return s.massInfo.m / s.massInfo.area
+}
+
+func (s *Shape) SetDensity(density float64) {
+	s.SetMass(density * s.massInfo.area)
+}
+
+func (s *Shape) Moment() float64 {
+	return s.massInfo.m * s.massInfo.i
+}
+
+func (s *Shape) Area() float64 {
+	return s.massInfo.area
+}
+
+func (s *Shape) CenterOfGravity() Vector {
+	return s.massInfo.cog
+}
+
 func (s *Shape) HashId() HashValue {
 	return s.hashid
 }

shape_test.go

@@ -0,0 +1,39 @@
+package cp
+
+import (
+	"math"
+	"testing"
+)
+
+func TestShapeMass(t *testing.T) {
+	body := NewBody(0, 0)
+	circle := NewCircle(body, 5, Vector{0, 0})
+
+	mass := 10.0
+	circle.SetMass(mass)
+	body.AddShape(circle)
+
+	if circle.Mass() != mass {
+		t.Fail()
+	}
+}
+
+func TestShapeCircleArea(t *testing.T) {
+	body := NewBody(0, 0)
+	circle := NewCircle(body, 2, Vector{0, 0})
+
+	if circle.Area() != 4 * math.Pi {
+		t.Fail()
+	}
+}
+
+func TestShapeCircleDensity(t *testing.T) {
+	body := NewBody(0, 0)
+	circle := NewCircle(body, 1, Vector{0, 0})
+
+	circle.SetMass(math.Pi)
+
+	if circle.Density() != 1.0 {
+		t.Fail()
+	}
+}