From 63a478841aa11a7593bd8970d917e8b92d16c9f4 Mon Sep 17 00:00:00 2001
From: fallenoak <git@fallenoak.me>
Date: Sun, 29 Nov 2020 13:32:25 -0600
Subject: [PATCH] feat(matrix): add C44Matrix::Determinant

---
 tempest/matrix/C44Matrix.cpp |  4 ++++
 tempest/matrix/C44Matrix.hpp |  1 +
 test/Matrix.cpp              | 20 ++++++++++++++++++++
 3 files changed, 25 insertions(+)

diff --git a/tempest/matrix/C44Matrix.cpp b/tempest/matrix/C44Matrix.cpp
index c744422..96aeff9 100644
--- a/tempest/matrix/C44Matrix.cpp
+++ b/tempest/matrix/C44Matrix.cpp
@@ -23,3 +23,7 @@ C44Matrix C44Matrix::Adjoint() const {
 
     return C44Matrix(a0, a1, a2, a3, b0, b1, b2, b3, c0, c1, c2, c3, d0, d1, d2, d3);
 }
+
+float C44Matrix::Determinant() {
+    return (this->b1 * this->c2 * this->d3 + this->c3 * this->b2 * this->d1 + this->b3 * this->c1 * this->d2 - this->c2 * this->b3 * this->d1 - this->d3 * (this->c1 * this->b2) - this->b1 * this->c3 * this->d2) * this->a0 - (this->c2 * this->b0 * this->d3 + this->c3 * this->b2 * this->d0 + this->b3 * this->c0 * this->d2 - this->b3 * this->c2 * this->d0 - this->d3 * (this->c0 * this->b2) - this->b0 * this->c3 * this->d2) * this->a1 + (this->c1 * this->b0 * this->d3 + this->c3 * this->b1 * this->d0 + this->b3 * this->c0 * this->d1 - this->b3 * this->c1 * this->d0 - this->d3 * (this->c0 * this->b1) - this->b0 * this->c3 * this->d1) * this->a2 - (this->c1 * this->b0 * this->d2 + this->c2 * this->b1 * this->d0 + this->b2 * this->c0 * this->d1 - this->b2 * this->c1 * this->d0 - this->d2 * (this->c0 * this->b1) - this->b0 * this->c2 * this->d1) * this->a3;
+}
diff --git a/tempest/matrix/C44Matrix.hpp b/tempest/matrix/C44Matrix.hpp
index 268061b..fb7276b 100644
--- a/tempest/matrix/C44Matrix.hpp
+++ b/tempest/matrix/C44Matrix.hpp
@@ -41,6 +41,7 @@ class C44Matrix {
         , d2(d2)
         , d3(d3) {};
     C44Matrix Adjoint() const;
+    float Determinant();
 };
 
 #endif
diff --git a/test/Matrix.cpp b/test/Matrix.cpp
index b229bad..1b61f0b 100644
--- a/test/Matrix.cpp
+++ b/test/Matrix.cpp
@@ -107,3 +107,23 @@ TEST_CASE("C44Matrix::Adjoint", "[matrix]") {
         CHECK(matrix.d3 == 16.0f);
     }
 }
+
+TEST_CASE("C44Matrix::Determinant", "[matrix]") {
+    SECTION("returns determinant of identity matrix") {
+        C44Matrix matrix;
+        auto determinant = matrix.Determinant();
+        REQUIRE(determinant == 1.0f);
+    }
+
+    SECTION("returns determinant of zero matrix") {
+        auto matrix = C44Matrix(0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f);
+        auto determinant = matrix.Determinant();
+        REQUIRE(determinant == 0.0f);
+    }
+
+    SECTION("returns determinant of non-identity matrix") {
+        auto matrix = C44Matrix(-1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.0f, 8.0f, 9.0f, 10.0f, -11.0f, 12.0f, 13.0f, 14.0f, 15.0f, 16.0f);
+        auto determinant = matrix.Determinant();
+        REQUIRE(determinant == -704.0f);
+    }
+}