From c97bc25323e341563d1d22d69a576745f8bb427e Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Thu, 1 Jul 2021 20:54:00 -0700
Subject: [PATCH] Implement 2x2 determinant

---
 rtchallenge/src/matrices.rs | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/rtchallenge/src/matrices.rs b/rtchallenge/src/matrices.rs
index 677f3b1..d9ea7d3 100644
--- a/rtchallenge/src/matrices.rs
+++ b/rtchallenge/src/matrices.rs
@@ -18,6 +18,22 @@ impl Matrix2x2 {
     pub fn new(r0: [f32; 2], r1: [f32; 2]) -> Matrix2x2 {
         Matrix2x2 { m: [r0, r1] }
     }
+
+    /// Calculate the determinant of a 2x2.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use rtchallenge::matrices::Matrix2x2;
+    ///
+    /// let a = Matrix2x2::new([1., 5.], [-3., 2.]);
+    ///
+    /// assert_eq!(a.determinant(), 17.);
+    /// ```
+    pub fn determinant(&self) -> f32 {
+        let m = self;
+        m[(0, 0)] * m[(1, 1)] - m[(0, 1)] * m[(1, 0)]
+    }
 }
 impl Index<(usize, usize)> for Matrix2x2 {
     type Output = f32;
@@ -131,7 +147,7 @@ impl Matrix4x4 {
     /// use rtchallenge::matrices::Matrix4x4;
     ///
     /// let i = Matrix4x4::identity();
-    /// assert_eq!(i.inverse() * i, i);
+    /// assert_eq!(i.inverse_old() * i, i);
     ///
     /// let m = Matrix4x4::new(
     ///     [2., 0., 0., 0.],
@@ -139,10 +155,10 @@ impl Matrix4x4 {
     ///     [0., 0., 4., 0.],
     ///     [0., 0., 0., 1.],
     /// );
-    /// assert_eq!(m.inverse() * m, i);
-    /// assert_eq!(m * m.inverse(), i);
+    /// assert_eq!(m.inverse_old() * m, i);
+    /// assert_eq!(m * m.inverse_old(), i);
     /// ```
-    pub fn inverse(&self) -> Matrix4x4 {
+    pub fn inverse_old(&self) -> Matrix4x4 {
         // TODO(wathiede): how come the C++ version doesn't need to deal with non-invertable
         // matrix.
         let mut indxc: [usize; 4] = Default::default();