define sub in terms of neg and add; add test cases

Author: chfanghr

src/v1/value/value_utils.rs

@@ -3,7 +3,7 @@ use num_bigint::BigInt;
 use num_traits::Zero;
 use std::{
     collections::BTreeMap,
-    ops::{Add, Mul, Not, Sub},
+    ops::{Add, Mul, Neg, Not, Sub},
 };
 
 use super::{CurrencySymbol, TokenName, Value};
@@ -133,6 +133,22 @@ impl Add<&Value> for &Value {
     }
 }
 
+impl Neg for Value {
+    type Output = Value;
+
+    fn neg(self) -> Self::Output {
+        (&self).neg()
+    }
+}
+
+impl Neg for &Value {
+    type Output = Value;
+
+    fn neg(self) -> Self::Output {
+        self.map_amount(|_, _, a| a.neg())
+    }
+}
+
 forward_val_val_binop!(impl Sub for Value, sub);
 forward_ref_val_binop!(impl Sub for Value, sub);
 forward_val_ref_binop!(impl Sub for Value, sub);
@@ -141,11 +157,7 @@ impl Sub<&Value> for &Value {
     type Output = Value;
 
     fn sub(self, rhs: &Value) -> Self::Output {
-        Value(union_b_tree_maps_with(
-            |lhs, rhs| union_b_tree_maps_with(|lhs, rhs| lhs - rhs, lhs, rhs),
-            &self.0,
-            &rhs.0,
-        ))
+        self.add(rhs.neg())
     }
 }
 

tests/value.rs

@@ -1,6 +1,8 @@
 #[cfg(test)]
 mod value_tests {
-    mod semiring {
+    mod ring_ish {
+        use std::ops::Neg;
+
         use num_bigint::BigInt;
         use num_traits::{One, Zero};
         use plutus_ledger_api::{
@@ -60,6 +62,16 @@ mod value_tests {
               assert_eq!(&val * BigInt::zero(), BigInt::zero() * &val);
               assert_eq!(Value::zero(), (BigInt::zero() * &val).normalize());
             }
+
+            #[test]
+            fn test_additive_inverse_annihilation(val in arb_value()) {
+              assert_eq!((&val + (&val).neg()).normalize(), Value::zero());
+            }
+
+            #[test]
+            fn test_minus(x in arb_value(), y in arb_value()) {
+              assert_eq!(&x - &y, &x + (&y).neg());
+            }
         }
     }