Fix subtract ScalarVar by Scalar (#68)

Signed-off-by: Michele Orrù <[email protected]>
Co-authored-by: Michele Orrù <[email protected]>

Author: nategraf

src/linear_relation/canonical.rs

@@ -371,7 +371,6 @@ impl<G: PrimeGroup> TryFrom<&LinearRelation<G>> for CanonicalLinearRelation<G> {
 
         // Process each constraint using the modular helper method
         for (lhs, rhs) in iter::zip(&relation.image, &relation.linear_map.linear_combinations) {
-
             let lhs_value = relation
                 .linear_map
                 .group_elements
@@ -384,7 +383,6 @@ impl<G: PrimeGroup> TryFrom<&LinearRelation<G>> for CanonicalLinearRelation<G> {
                 .iter()
                 .all(|weighted| matches!(weighted.term.scalar, ScalarTerm::Unit))
             {
-
                 let rhs_value = rhs.0.iter().fold(G::identity(), |acc, weighted| {
                     acc + relation
                         .linear_map

src/linear_relation/ops.rs

@@ -552,13 +552,13 @@ mod sub {
             impl<G, Rhs> Sub<Rhs> for $type
             where
                 Rhs: Neg,
-                <Rhs as Neg>::Output: Add<Self>,
+                Self: Add<<Rhs as Neg>::Output>,
             {
-                type Output = <<Rhs as Neg>::Output as Add<Self>>::Output;
+                type Output = <Self as Add<<Rhs as Neg>::Output>>::Output;
 
                 #[allow(clippy::suspicious_arithmetic_impl)]
                 fn sub(self, rhs: Rhs) -> Self::Output {
-                    rhs.neg() + self
+                    self + rhs.neg()
                 }
             }
             )+
@@ -790,6 +790,18 @@ mod tests {
         assert_eq!(diff.terms()[1].weight, Scalar::ONE);
     }
 
+    #[test]
+    fn test_scalar_var_subtraction_by_scalar() {
+        let x = scalar_var(0);
+
+        let diff = x - Scalar::ONE;
+        assert_eq!(diff.terms().len(), 2);
+        assert_eq!(diff.terms()[0].term, ScalarTerm::Var(x));
+        assert_eq!(diff.terms()[0].weight, Scalar::ONE);
+        assert_eq!(diff.terms()[1].term, ScalarTerm::Unit);
+        assert_eq!(diff.terms()[1].weight, -Scalar::ONE);
+    }
+
     #[test]
     fn test_group_var_subtraction() {
         let g = group_var(0);