From aeb4669162d97b68b94f9d273cca6ddbbe643e62 Mon Sep 17 00:00:00 2001
From: Godzil <godzil@godzil.net>
Date: Mon, 17 Feb 2020 15:56:08 +0000
Subject: [PATCH] Add vector reflection via a normal vector.

---
 source/include/tuple.h |  2 +-
 source/tuple.cpp       |  5 +++++
 tests/tuple_test.cpp   | 20 ++++++++++++++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/source/include/tuple.h b/source/include/tuple.h
index 67d87ac..88967eb 100644
--- a/source/include/tuple.h
+++ b/source/include/tuple.h
@@ -41,8 +41,8 @@ public:
     double magnitude();
     Tuple normalise();
     double dot(const Tuple &b);
-
     Tuple cross(const Tuple &b) const;
+    Tuple reflect(const Tuple &normal);
 };
 
 class Point: public Tuple
diff --git a/source/tuple.cpp b/source/tuple.cpp
index 19fa169..42a2c27 100644
--- a/source/tuple.cpp
+++ b/source/tuple.cpp
@@ -33,4 +33,9 @@ Tuple Tuple::cross(const Tuple &b) const
                  this->z * b.x - this->x * b.z,
                  this->x * b.y - this->y * b.x,
                  0);
+}
+
+Tuple Tuple::reflect(const Tuple &normal)
+{
+    return *this - normal * 2 * this->dot(normal);
 }
\ No newline at end of file
diff --git a/tests/tuple_test.cpp b/tests/tuple_test.cpp
index cf698f2..933d7b9 100644
--- a/tests/tuple_test.cpp
+++ b/tests/tuple_test.cpp
@@ -181,3 +181,23 @@ TEST(TuplesTests, Cross_product_of_two_vector)
     ASSERT_EQ(a.cross(b), Vector(-1, 2, -1));
     ASSERT_EQ(b.cross(a), Vector(1, -2, 1));
 }
+
+TEST(TuplesTests, Reflecting_a_vector_approaching_at_45)
+{
+    Vector v = Vector(1, -1, 0); /* Vector */
+    Vector n = Vector(0, 1, 0); /* Normal */
+
+    Tuple r = v.reflect(n);
+
+    ASSERT_EQ(r, Vector(1, 1, 0));
+}
+
+TEST(TuplesTests, Reflecting_a_vector_off_a_slanted_surface)
+{
+    Vector v = Vector(0, -1, 0); /* Vector */
+    Vector n = Vector(sqrt(2)/2, sqrt(2)/2, 0); /* Normal */
+
+    Tuple r = v.reflect(n);
+
+    ASSERT_EQ(r, Vector(1, 0, 0));
+}
\ No newline at end of file