From 3a5dd0490c7c9be0277ecee31b1bb2de58637608 Mon Sep 17 00:00:00 2001
From: Kyle Isom <kyle@imap.cc>
Date: Sun, 4 Aug 2019 23:29:56 -0700
Subject: [PATCH] Add cross product.

---
 include/wrmath/geom/vector.h | 19 +++++++++++++++++++
 test/vector_test.cc          | 27 +++++++++++++++++++++++++--
 2 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/include/wrmath/geom/vector.h b/include/wrmath/geom/vector.h
index 2af42c7..89e39fb 100644
--- a/include/wrmath/geom/vector.h
+++ b/include/wrmath/geom/vector.h
@@ -15,6 +15,7 @@
 namespace wr {
 namespace geom {
 
+
 /**
  * Vector provides a standard interface for dimensionless fixed-size
  * vectors. Once instantiated, they cannot be modified. Note that while
@@ -203,6 +204,24 @@ public:
 	}
 
 
+	/**
+	 * Compute the cross product of two vectors. This is only defined
+	 * over three-dimensional vectors.
+	 * @param other Another 3D vector.
+	 * @return The cross product vector.
+	 */
+	Vector
+	cross(const Vector<T, N> &other)
+	{
+		assert(N == 3);
+		return Vector<T, N> {
+			(this->arr[1] * other.arr[2]) - (other.arr[1] * this->arr[2]),
+			-((this->arr[0] * other.arr[2]) - (other.arr[0] * this->arr[2])),
+			(this->arr[0] * other.arr[1]) - (other.arr[0] * this->arr[1])
+		};
+	}
+
+
 	/**
 	 * Perform vector addition with another vector.
 	 * @param other The vector to be added.
diff --git a/test/vector_test.cc b/test/vector_test.cc
index 15b4932..e880558 100644
--- a/test/vector_test.cc
+++ b/test/vector_test.cc
@@ -172,7 +172,7 @@ TEST(Vector3FloatTests, ParallelOrthogonalVectors)
 }
 
 
-TEST(Vector3FloatTests, Projections)
+TEST(Vector4FloatTests, Projections)
 {
 	geom::Vector4f	a {3.009, -6.172, 3.692, -2.510};
 	geom::Vector4f	b {6.404, -9.144, 2.759, 8.718};
@@ -184,6 +184,17 @@ TEST(Vector3FloatTests, Projections)
 }
 
 
+TEST(Vector3FloatTests, CrossProduct)
+{
+	geom::Vector3f	a {8.462, 7.893, -8.187};
+	geom::Vector3f	b {6.984, -5.975, 4.778};
+	geom::Vector3f	c {-11.2046, -97.6094, -105.685};
+
+	c.setEpsilon(0.001);
+	EXPECT_EQ(c, a.cross(b));
+}
+
+
 TEST(Vector3DoubleTests, Magnitude)
 {
 	geom::Vector3d	v3d{1.0, -2.0, 3.0};
@@ -302,7 +313,7 @@ TEST(Vector3DoubleTests, ParallelOrthogonalVectors)
 }
 
 
-TEST(Vector3DoubleTests, Projections)
+TEST(Vector4DoubleTests, Projections)
 {
 	geom::Vector4d	a {3.009, -6.172, 3.692, -2.510};
 	geom::Vector4d	b {6.404, -9.144, 2.759, 8.718};
@@ -314,6 +325,18 @@ TEST(Vector3DoubleTests, Projections)
 }
 
 
+TEST(Vector3DoubleTests, CrossProduct)
+{
+	geom::Vector3d	a {8.462, 7.893, -8.187};
+	geom::Vector3d	b {6.984, -5.975, 4.778};
+	geom::Vector3d	c {-11.2046, -97.6094, -105.685};
+
+	c.setEpsilon(0.001); // double trouble
+	EXPECT_EQ(c, a.cross(b));
+}
+
+
+
 int
 main(int argc, char **argv)
 {