implement determinant on 3x3 and 4x4 matrices

rtchallenge/src/matrices.rs

@@ -1,5 +1,5 @@
 use std::fmt;
-use std::ops::{Add, Div, Index, Mul, Neg, Sub};
+use std::ops::{Index, Mul};
 
 use crate::tuples::Tuple;
 
@@ -86,7 +86,7 @@ impl Matrix3x3 {
     ///
     /// # Examples
     /// ```
-    /// use rtchallenge::matrices::{Matrix2x2, Matrix3x3};
+    /// use rtchallenge::matrices::Matrix3x3;
     ///
     /// let a = Matrix3x3::new([3., 5., 0.], [2., -1., -7.], [6., -1., 5.]);
     /// let b = a.submatrix(1, 0);
@@ -101,7 +101,7 @@ impl Matrix3x3 {
     ///
     /// # Examples
     /// ```
-    /// use rtchallenge::matrices::{Matrix2x2, Matrix3x3};
+    /// use rtchallenge::matrices::Matrix3x3;
     ///
     /// let a = Matrix3x3::new([3., 5., 0.], [2., -1., -7.], [6., -1., 5.]);
     /// assert_eq!(a.minor(0, 0), -12.);
@@ -113,6 +113,22 @@ impl Matrix3x3 {
         let negate = if (row + col) % 2 == 0 { 1. } else { -1. };
         self.submatrix(row, col).determinant() * negate
     }
+
+    /// Compute determinant of a 3x3 matrix.
+    ///
+    /// # Examples
+    /// ```
+    /// use rtchallenge::matrices::Matrix3x3;
+    ///
+    /// let a = Matrix3x3::new([1., 2., 6.], [-5., 8., -4.], [2., 6., 4.]);
+    /// assert_eq!(a.cofactor(0, 0), 56.);
+    /// assert_eq!(a.cofactor(0, 1), 12.);
+    /// assert_eq!(a.cofactor(0, 2), -46.);
+    /// assert_eq!(a.determinant(), -196.);
+    /// ```
+    pub fn determinant(&self) -> f32 {
+        (0..3).map(|i| self.cofactor(0, i) * self[(0, i)]).sum()
+    }
 }
 impl Index<(usize, usize)> for Matrix3x3 {
     type Output = f32;
@@ -333,6 +349,37 @@ impl Matrix4x4 {
         ];
         Matrix3x3 { m }
     }
+
+    /// Compute minor of a 4x4 matrix.
+    pub fn minor(&self, row: usize, col: usize) -> f32 {
+        self.submatrix(row, col).determinant()
+    }
+    /// Compute cofactor of a 4x4 matrix.
+    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
+    }
+    /// Compute determinant of a 4x4 matrix.
+    ///
+    /// # Examples
+    /// ```
+    /// use rtchallenge::matrices::Matrix4x4;
+    ///
+    /// let a = Matrix4x4::new(
+    ///     [-2., -8., 3., 5.],
+    ///     [-3., 1., 7., 3.],
+    ///     [1., 2., -9., 6.],
+    ///     [-6., 7., 7., -9.],
+    /// );
+    /// assert_eq!(a.cofactor(0, 0), 690.);
+    /// assert_eq!(a.cofactor(0, 1), 447.);
+    /// assert_eq!(a.cofactor(0, 2), 210.);
+    /// assert_eq!(a.cofactor(0, 3), 51.);
+    /// assert_eq!(a.determinant(), -4071.);
+    /// ```
+    pub fn determinant(&self) -> f32 {
+        (0..4).map(|i| self.cofactor(0, i) * self[(0, i)]).sum()
+    }
 }
 
 impl fmt::Debug for Matrix4x4 {