add a round_ties_even for Ratio<T>

src/lib.rs

@@ -257,6 +257,46 @@ impl<T: Clone + Integer> Ratio<T> {
         }
     }
 
+    /// Rounds to the nearest integer. Rounds half-way cases away to even.
+    pub fn round_ties_even(&self) -> Ratio<T> {
+        let zero: Ratio<T> = Zero::zero();
+        let one: T = One::one();
+        let two: T = one.clone() + one.clone();
+
+        // Find unsigned fractional part of rational number
+        let mut fractional = self.fract();
+        if fractional < zero {
+            fractional = zero - fractional
+        };
+
+        // Compare the unsigned fractional part with 1/2
+        let half = Ratio::new_raw(one, two);
+        match fractional.cmp(&half) {
+            cmp::Ordering::Greater => {
+                let one: Ratio<T> = One::one();
+                if *self >= Zero::zero() {
+                    self.trunc() + one
+                } else {
+                    self.trunc() - one
+                }
+            }
+            cmp::Ordering::Equal => {
+                let trunc = self.trunc();
+                if trunc.numer().is_even() {
+                    trunc
+                } else {
+                    let one: Ratio<T> = One::one();
+                    if *self >= Zero::zero() {
+                        self.trunc() + one
+                    } else {
+                        self.trunc() - one
+                    }
+                }
+            }
+            cmp::Ordering::Less => self.trunc(),
+        }
+    }
+
     /// Rounds towards zero.
     #[inline]
     pub fn trunc(&self) -> Ratio<T> {
@@ -1742,6 +1782,7 @@ mod test {
     pub const _0: Rational64 = Ratio { numer: 0, denom: 1 };
     pub const _1: Rational64 = Ratio { numer: 1, denom: 1 };
     pub const _2: Rational64 = Ratio { numer: 2, denom: 1 };
+    pub const _3: Rational64 = Ratio { numer: 3, denom: 1 };
     pub const _NEG2: Rational64 = Ratio {
         numer: -2,
         denom: 1,
@@ -2650,38 +2691,51 @@ mod test {
         assert_eq!(_1_3.ceil(), _1);
         assert_eq!(_1_3.floor(), _0);
         assert_eq!(_1_3.round(), _0);
+        assert_eq!(_1_3.round_ties_even(), _0);
         assert_eq!(_1_3.trunc(), _0);
 
         assert_eq!(_NEG1_3.ceil(), _0);
         assert_eq!(_NEG1_3.floor(), -_1);
         assert_eq!(_NEG1_3.round(), _0);
+        assert_eq!(_NEG1_3.round_ties_even(), _0);
         assert_eq!(_NEG1_3.trunc(), _0);
 
         assert_eq!(_2_3.ceil(), _1);
         assert_eq!(_2_3.floor(), _0);
         assert_eq!(_2_3.round(), _1);
+        assert_eq!(_2_3.round_ties_even(), _1);
         assert_eq!(_2_3.trunc(), _0);
 
         assert_eq!(_NEG2_3.ceil(), _0);
         assert_eq!(_NEG2_3.floor(), -_1);
         assert_eq!(_NEG2_3.round(), -_1);
+        assert_eq!(_NEG2_3.round_ties_even(), -_1);
         assert_eq!(_NEG2_3.trunc(), _0);
 
         assert_eq!(_1_2.ceil(), _1);
         assert_eq!(_1_2.floor(), _0);
         assert_eq!(_1_2.round(), _1);
+        assert_eq!(_1_2.round_ties_even(), _0);
         assert_eq!(_1_2.trunc(), _0);
 
         assert_eq!(_NEG1_2.ceil(), _0);
         assert_eq!(_NEG1_2.floor(), -_1);
         assert_eq!(_NEG1_2.round(), -_1);
+        assert_eq!(_NEG1_2.round_ties_even(), _0);
         assert_eq!(_NEG1_2.trunc(), _0);
 
         assert_eq!(_1.ceil(), _1);
         assert_eq!(_1.floor(), _1);
         assert_eq!(_1.round(), _1);
+        assert_eq!(_1.round_ties_even(), _1);
         assert_eq!(_1.trunc(), _1);
 
+        assert_eq!(_5_2.ceil(), _3);
+        assert_eq!(_5_2.floor(), _2);
+        assert_eq!(_5_2.round(), _3);
+        assert_eq!(_5_2.round_ties_even(), _2);
+        assert_eq!(_5_2.trunc(), _2);
+
         // Overflow checks
 
         let _neg1 = Ratio::from_integer(-1);
@@ -2702,6 +2756,14 @@ mod test {
         assert_eq!(_large_rat6.round(), _neg1);
         assert_eq!(_large_rat7.round(), Zero::zero());
         assert_eq!(_large_rat8.round(), Zero::zero());
+        assert_eq!(_large_rat1.round_ties_even(), One::one());
+        assert_eq!(_large_rat2.round_ties_even(), One::one());
+        assert_eq!(_large_rat3.round_ties_even(), One::one());
+        assert_eq!(_large_rat4.round_ties_even(), One::one());
+        assert_eq!(_large_rat5.round_ties_even(), _neg1);
+        assert_eq!(_large_rat6.round_ties_even(), _neg1);
+        assert_eq!(_large_rat7.round_ties_even(), Zero::zero());
+        assert_eq!(_large_rat8.round_ties_even(), Zero::zero());
     }
 
     #[test]