implement submatrix for matrix3x3

rtchallenge/src/matrices.rs

@@ -10,6 +10,7 @@ pub struct Matrix4x4 {
     m: [[f32; 4]; 4],
 }
 
+#[derive(Debug, PartialEq)]
 pub struct Matrix2x2 {
     m: [[f32; 2]; 2],
 }
@@ -42,6 +43,7 @@ impl Index<(usize, usize)> for Matrix2x2 {
     }
 }
 
+#[derive(Debug, PartialEq)]
 pub struct Matrix3x3 {
     m: [[f32; 3]; 3],
 }
@@ -50,6 +52,35 @@ impl Matrix3x3 {
     pub fn new(r0: [f32; 3], r1: [f32; 3], r2: [f32; 3]) -> Matrix3x3 {
         Matrix3x3 { m: [r0, r1, r2] }
     }
+    /// submatrix extracts a 2x2 matrix ignoring the 0-based `row` and `col` given.
+    ///
+    /// # Examples
+    /// ```
+    /// use rtchallenge::matrices::{Matrix2x2, Matrix3x3};
+    ///
+    /// assert_eq!(
+    ///     Matrix3x3::new([1., 5., 0.], [-3., 2., 7.], [0., 6., -3.],).submatrix(0, 2),
+    ///     Matrix2x2::new([-3., 2.], [0., 6.])
+    /// );
+    /// ```
+    pub fn submatrix(&self, row: usize, col: usize) -> Matrix2x2 {
+        assert!(0 <= row && row < 3);
+        assert!(0 <= col && col < 3);
+        let mut rows = vec![];
+        for r in 0..3 {
+            if r != row {
+                let mut v = vec![];
+                for c in 0..3 {
+                    if c != col {
+                        v.push(self[(r, c)]);
+                    }
+                }
+                rows.push(v);
+            }
+        }
+        let m = [[rows[0][0], rows[0][1]], [rows[1][0], rows[1][1]]];
+        Matrix2x2 { m }
+    }
 }
 impl Index<(usize, usize)> for Matrix3x3 {
     type Output = f32;