Add partially-implemented Mat2 that should have been in last commit

src/math/mat2.rs

@@ -0,0 +1,33 @@
+#[derive(PartialEq, Debug, Copy, Clone)]
+pub struct Mat2 {
+    pub elements: [[f64; 2]; 2],
+}
+
+impl Mat2 {
+    pub fn new(m00: f64, m01: f64, m10: f64, m11: f64) -> Mat2 {
+        Mat2 {
+            elements: [[m00, m01], [m10, m11]],
+        }
+    }
+
+    pub fn determinant(&self) -> f64 {
+        self.elements[0][0] * self.elements[1][1] - self.elements[0][1] * self.elements[1][0]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn determinant_returns_expected_value() {
+        let target1 = Mat2::new(1.0, 2.0, 3.0, 4.0);
+        let target2 = Mat2::new(1.0, -2.0, 3.0, 4.0);
+        let target3 = Mat2::new(1.0, 1.0, 1.0, 1.0);
+        let target4 = Mat2::new(21.0, 45.0, -16.0, 0.0);
+        assert!(target1.determinant() == -2.0);
+        assert!(target2.determinant() == 10.0);
+        assert!(target3.determinant() == 0.0);
+        assert!(target4.determinant() == 720.0);
+    }
+}