Correct Ratio::is_negative and Ratio::is_positive

Zero is not positive or negative.

This was broken in 8be7e7b.

Author: ollie27

src/rational.rs

@@ -485,11 +485,15 @@ impl<T: Clone + Integer + Signed> Signed for Ratio<T> {
     }
 
     #[inline]
-    fn is_positive(&self) -> bool { !self.is_negative() }
+    fn is_positive(&self) -> bool {
+        (self.numer.is_positive() & self.denom.is_positive()) |
+        (self.numer.is_negative() & self.denom.is_negative())
+    }
 
     #[inline]
     fn is_negative(&self) -> bool {
-        self.numer.is_negative() ^ self.denom.is_negative()
+        (self.numer.is_negative() & self.denom.is_positive()) |
+        (self.numer.is_positive() & self.denom.is_negative())
     }
 }
 
@@ -603,6 +607,8 @@ mod test {
     pub const _1_2: Rational = Ratio { numer: 1, denom: 2};
     pub const _3_2: Rational = Ratio { numer: 3, denom: 2};
     pub const _NEG1_2: Rational = Ratio { numer: -1, denom: 2};
+    pub const _1_NEG2: Rational = Ratio { numer: 1, denom: -2};
+    pub const _NEG1_NEG2: Rational = Ratio { numer: -1, denom: -2};
     pub const _1_3: Rational = Ratio { numer: 1, denom: 3};
     pub const _NEG1_3: Rational = Ratio { numer: -1, denom: 3};
     pub const _2_3: Rational = Ratio { numer: 2, denom: 3};
@@ -1000,9 +1006,17 @@ mod test {
         assert_eq!(_1_2.abs_sub(&_3_2), Zero::zero());
         assert_eq!(_1_2.signum(), One::one());
         assert_eq!(_NEG1_2.signum(), - ::one::<Ratio<isize>>());
+        assert_eq!(_0.signum(), Zero::zero());
         assert!(_NEG1_2.is_negative());
+        assert!(_1_NEG2.is_negative());
         assert!(! _NEG1_2.is_positive());
+        assert!(! _1_NEG2.is_positive());
+        assert!(_1_2.is_positive());
+        assert!(_NEG1_NEG2.is_positive());
         assert!(! _1_2.is_negative());
+        assert!(! _NEG1_NEG2.is_negative());
+        assert!(! _0.is_positive());
+        assert!(! _0.is_negative());
     }
 
     #[test]