Add abs_sub method to Signed trait

1 files changed, 14 insertions, 0 deletions

diff --git a/src/libstd/num/bigint.rs b/src/libstd/num/bigint.rs
index cd347098e25..a5cf929ed93 100644
--- a/src/libstd/num/bigint.rs
+++ b/src/libstd/num/bigint.rs
@@ -832,6 +832,11 @@ impl Signed for BigInt {
     }
 
     #[inline(always)]
+    fn abs_sub(&self, other: &BigInt) -> BigInt {
+        if *self <= *other { Zero::zero() } else { *self - *other }
+    }
+
+    #[inline(always)]
     fn signum(&self) -> BigInt {
         match self.sign {
             Plus  => BigInt::from_biguint(Plus, One::one()),
@@ -1921,6 +1926,15 @@ mod bigint_tests {
     }
 
     #[test]
+    fn test_abs_sub() {
+        assert_eq!((-One::one::<BigInt>()).abs_sub(&One::one()), Zero::zero());
+        assert_eq!(One::one::<BigInt>().abs_sub(&One::one()), Zero::zero());
+        assert_eq!(One::one::<BigInt>().abs_sub(&Zero::zero()), One::one());
+        assert_eq!(One::one::<BigInt>().abs_sub(&-One::one::<BigInt>()),
+                   IntConvertible::from_int(2));
+    }
+
+    #[test]
     fn test_to_str_radix() {
         fn check(n: int, ans: &str) {
             assert!(ans == IntConvertible::from_int::<BigInt>(n).to_str_radix(10));