From d6ad12e344ed1e5436d3dc41fc8e978bed0c208c Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Mon, 5 Jul 2021 15:07:18 -0700
Subject: [PATCH] implement cofactor of 3x3 matrix

---
 rtchallenge/src/matrices.rs | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/rtchallenge/src/matrices.rs b/rtchallenge/src/matrices.rs
index 475ae54..223fe6b 100644
--- a/rtchallenge/src/matrices.rs
+++ b/rtchallenge/src/matrices.rs
@@ -82,6 +82,7 @@ impl Matrix3x3 {
         Matrix2x2 { m }
     }
 
+    /// Compute minor of a 3x3 matrix.
     ///
     /// # Examples
     /// ```
@@ -95,6 +96,23 @@ impl Matrix3x3 {
     pub fn minor(&self, row: usize, col: usize) -> f32 {
         self.submatrix(row, col).determinant()
     }
+
+    /// Compute cofactor of a 3x3 matrix.
+    ///
+    /// # Examples
+    /// ```
+    /// use rtchallenge::matrices::{Matrix2x2, Matrix3x3};
+    ///
+    /// let a = Matrix3x3::new([3., 5., 0.], [2., -1., -7.], [6., -1., 5.]);
+    /// assert_eq!(a.minor(0, 0), -12.);
+    /// assert_eq!(a.cofactor(0, 0), -12.);
+    /// assert_eq!(a.minor(1, 0), 25.);
+    /// assert_eq!(a.cofactor(1, 0), -25.);
+    /// ```
+    pub fn cofactor(&self, row: usize, col: usize) -> f32 {
+        let negate = if (row + col) % 2 == 0 { 1. } else { -1. };
+        self.submatrix(row, col).determinant() * negate
+    }
 }
 impl Index<(usize, usize)> for Matrix3x3 {
     type Output = f32;