implement cofactor of 3x3 matrix

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;