Fix another negative fraction condition

src/rational.rs

@@ -1,7 +1,7 @@
 //! # Rational Numbers (fractions)
 
-use crate::num::*;
 use crate::num::Sign::*;
+use crate::num::*;
 use std::cmp::{Ord, Ordering, PartialOrd};
 use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 
@@ -77,11 +77,7 @@ impl<T: Unsigned> Frac<T> {
         let numer = n.to_unsigned();
         let denom = d.to_unsigned();
 
-        Frac {
-            numer,
-            denom,
-            sign,
-        }
+        Frac { numer, denom, sign }
     }
 
     /// Create a new rational from all the raw parts
@@ -94,12 +90,13 @@ impl<T: Unsigned> Frac<T> {
             numer: n,
             denom: d,
             sign: s,
-        }.reduce()
+        }
+        .reduce()
     }
 
     /// Determine the output sign given the two input signs
-    fn get_sign(a: Self, b: Self, c: FracOp) -> Sign {
-        if c == FracOp::Addition {
+    fn get_sign(a: Self, b: Self, op: FracOp) -> Sign {
+        if op == FracOp::Addition {
             return if a.sign == Positive && b.sign == Positive {
                 Positive
             } else {
@@ -108,7 +105,7 @@ impl<T: Unsigned> Frac<T> {
         }
 
         if a.sign != b.sign {
-            if c == FracOp::Subtraction && b.sign == Negative {
+            if op == FracOp::Subtraction && b.sign == Negative {
                 Positive
             } else {
                 Negative
@@ -288,30 +285,25 @@ where
         if a.sign == Positive && b.sign == Negative {
             return a + -b;
         } else if a.sign == Negative && b.sign == Positive {
-            return b + -a;
+            return -(b + -a);
         }
 
         if a.denom != b.denom {
             let (numer, overflowed) = (a.numer * b.denom).left_overflowing_sub(b.numer * a.denom);
 
             let denom = a.denom * b.denom;
-            let mut sign = Self::get_sign(a, b, FracOp::Subtraction);
-            if overflowed {
-                sign = !sign
-            }
-
-            return Self::raw(numer, denom, sign);
+            let sign = Self::get_sign(a, b, FracOp::Subtraction);
+            let res = Self::raw(numer, denom, sign);
+            return if overflowed { -res } else { res };
         }
 
         let (numer, overflowed) = a.numer.left_overflowing_sub(b.numer);
 
         let denom = a.denom;
-        let mut sign = Self::get_sign(a, b, FracOp::Subtraction);
-        if overflowed {
-            sign = !sign
-        }
+        let sign = Self::get_sign(a, b, FracOp::Subtraction);
+        let res = Self::raw(numer, denom, sign);
 
-        Self::raw(numer, denom, sign)
+        if overflowed { -res } else { res }
     }
 }
 
@@ -374,7 +366,7 @@ mod tests {
     fn negative_fractions() {
         assert_eq!(-frac!(1 / 3), -frac!(2 / 3) + frac!(1 / 3), "-2/3 + 1/3");
         assert_eq!(frac!(1), frac!(1 / 3) - -frac!(2 / 3), "1/3 - -2/3");
-        // assert_eq!(-frac!(1), -frac!(2 / 3) - frac!(1 / 3), "-2/3 - +1/3");
+        assert_eq!(-frac!(1), -frac!(2 / 3) - frac!(1 / 3), "-2/3 - +1/3");
         assert_eq!(-frac!(1), -frac!(2 / 3) + -frac!(1 / 3), "-2/3 + -1/3");
     }