added matmul example

Author: BartMassey

README.md

@@ -24,5 +24,6 @@ in a controlled repository now.
 * `sum.rs`: generic iterator sum example
 * `addy.rs`: trait def/impl example
 * `bno-salad.rs`: trait salad example from Blandy and Orendorff
+* `matmul.rs`: example of overloading multiplication for matrices
 * `custom_sum.rs`: sum example with custom `Sum` type
 * `phantom.rs`: phantom type example

matmul.rs

@@ -0,0 +1,52 @@
+use std::ops::*;
+
+#[derive(Debug, Clone, Copy)]
+struct Matrix3([[u64;3];3]);
+
+impl Mul for Matrix3 {
+    type Output = Matrix3;
+    
+    fn mul(self, Matrix3(rhs): Self) -> Self {
+        let Matrix3(lhs) = self;
+        let mut result = [[0;3];3];
+        for i in 0..3 {
+            for j in 0..3 {
+                for k in 0..3 {
+                    result[i][k] += lhs[i][j] * rhs[j][k];
+                }
+            }
+        }
+        Matrix3(result)
+    }
+}
+
+impl PartialEq for Matrix3 {
+    fn eq(&self, &Matrix3(ref rhs): &Matrix3) -> bool {
+        self.0 == *rhs
+    }
+}
+
+const TTT: Matrix3 = Matrix3(
+    [[1,2,3],
+     [4,5,6],
+     [7,8,9]]
+);
+                      
+const ID: Matrix3 = Matrix3(
+    [[1,0,0],
+     [0,1,0],
+     [0,0,1]]
+);
+
+const MAGIC: Matrix3 = Matrix3(
+    [[6,1,8],
+     [7,5,3],
+     [2,9,4]]
+);
+
+fn main() {
+    assert_eq!(TTT * ID, TTT);
+    assert_eq!(ID * TTT, TTT);
+    assert!(TTT * MAGIC != MAGIC * TTT);
+    println!("{:?}", TTT * TTT);
+}