Improve std::num::pow implementation

The implementation has been made more succinct and no longer requires Clone. The coverage of the associated unit test has also been increased to check more combinations of bases, exponents, and expected results.

Author: brendanzab

src/libstd/num/mod.rs

@@ -304,48 +304,29 @@ pub trait Real: Signed
     fn to_radians(&self) -> Self;
 }
 
-/// Raises a value to the power of exp, using
-/// exponentiation by squaring.
+/// Raises a value to the power of exp, using exponentiation by squaring.
 ///
 /// # Example
 ///
 /// ```rust
 /// use std::num;
 ///
-/// let sixteen = num::pow(2, 4u);
-/// assert_eq!(sixteen, 16);
+/// assert_eq!(num::pow(2, 4), 16);
 /// ```
 #[inline]
-pub fn pow<T: Clone+One+Mul<T, T>>(num: T, exp: uint) -> T {
-    let one: uint = One::one();
-    let num_one: T = One::one();
-
-    if exp.is_zero() { return num_one; }
-    if exp == one { return num.clone(); }
-
-    let mut i: uint = exp;
-    let mut v: T;
-    let mut r: T = num_one;
-
-    // This if is to avoid cloning self.
-    if (i & one) == one {
-        r = r * num;
-        i = i - one;
-    }
-
-    i = i >> one;
-    v = num * num;
-
-    while !i.is_zero() {
-        if (i & one) == one {
-            r = r * v;
-            i = i - one;
+pub fn pow<T: One + Mul<T, T>>(mut base: T, mut exp: uint) -> T {
+    if exp == 1 { base }
+    else {
+        let mut acc = one::<T>();
+        while exp > 0 {
+            if (exp & 1) == 1 {
+                acc = acc * base;
+            }
+            base = base * base;
+            exp = exp >> 1;
         }
-        i = i >> one;
-        v = v * v;
+        acc
     }
-
-    r
 }
 
 /// Raise a number to a power.
@@ -1670,17 +1651,24 @@ mod tests {
 
     #[test]
     fn test_pow() {
-        fn assert_pow<T: Eq+Clone+One+Mul<T, T>>(num: T, exp: uint) -> () {
-            assert_eq!(num::pow(num.clone(), exp),
-                       range(1u, exp).fold(num.clone(), |acc, _| acc * num));
+        fn naive_pow<T: One + Mul<T, T>>(base: T, exp: uint) -> T {
+            range(0, exp).fold(one::<T>(), |acc, _| acc * base)
         }
-
-        assert_eq!(num::pow(3, 0), 1);
-        assert_eq!(num::pow(5, 1), 5);
-        assert_pow(-4, 2);
-        assert_pow(8, 3);
-        assert_pow(8, 5);
-        assert_pow(2u64, 50);
+        macro_rules! assert_pow(
+            (($num:expr, $exp:expr) => $expected:expr) => {{
+                let result = pow($num, $exp);
+                assert_eq!(result, $expected);
+                assert_eq!(result, naive_pow($num, $exp));
+            }}
+        )
+        assert_pow!((3,    0 ) => 1);
+        assert_pow!((5,    1 ) => 5);
+        assert_pow!((-4,   2 ) => 16);
+        assert_pow!((0.5,  5 ) => 0.03125);
+        assert_pow!((8,    3 ) => 512);
+        assert_pow!((8.0,  5 ) => 32768.0);
+        assert_pow!((8.5,  5 ) => 44370.53125);
+        assert_pow!((2u64, 50) => 1125899906842624);
     }
 }