cmov: add CmovEq impl for slices (#954)

Author: codahale

cmov/src/lib.rs

@@ -134,3 +134,25 @@ impl CmovEq for u128 {
         tmp.cmoveq(&1, input, output);
     }
 }
+
+impl<T: CmovEq> CmovEq for [T] {
+    fn cmoveq(&self, rhs: &Self, input: Condition, output: &mut Condition) {
+        let mut tmp = 1u8;
+        self.cmovne(rhs, 0u8, &mut tmp);
+        tmp.cmoveq(&1, input, output);
+    }
+
+    fn cmovne(&self, rhs: &Self, input: Condition, output: &mut Condition) {
+        // Short-circuit the comparison if the slices are of different lengths, and set the output
+        // condition to the input condition.
+        if self.len() != rhs.len() {
+            *output = input;
+            return;
+        }
+
+        // Compare each byte.
+        for (a, b) in self.iter().zip(rhs.iter()) {
+            a.cmovne(b, input, output);
+        }
+    }
+}

cmov/tests/lib.rs

@@ -332,3 +332,41 @@ mod u128 {
         assert_eq!(o, 55u8);
     }
 }
+
+mod slices {
+    use cmov::CmovEq;
+
+    #[test]
+    fn cmoveq_works() {
+        let mut o = 0u8;
+
+        // Same slices.
+        [1u8, 2, 3].cmoveq(&[1, 2, 3], 43, &mut o);
+        assert_eq!(o, 43);
+
+        // Different lengths.
+        [1u8, 2, 3].cmoveq(&[1, 2], 44, &mut o);
+        assert_ne!(o, 44);
+
+        // Different contents.
+        [1u8, 2, 3].cmoveq(&[1, 2, 4], 45, &mut o);
+        assert_ne!(o, 45);
+    }
+
+    #[test]
+    fn cmovne_works() {
+        let mut o = 0u8;
+
+        // Same slices.
+        [1u8, 2, 3].cmovne(&[1, 2, 3], 43, &mut o);
+        assert_ne!(o, 43);
+
+        // Different lengths.
+        [1u8, 2, 3].cmovne(&[1, 2], 44, &mut o);
+        assert_eq!(o, 44);
+
+        // Different contents.
+        [1u8, 2, 3].cmovne(&[1, 2, 4], 45, &mut o);
+        assert_eq!(o, 45);
+    }
+}