Add a reverse round-trip test for ExUnits translation

Author: neilmayhew

libs/cardano-ledger-core/testlib/Test/Cardano/Ledger/Core/Arbitrary.hs

@@ -914,6 +914,15 @@ instance Arbitrary PV1.Data where
               ]
         | otherwise = oneof [PV1.I <$> arbitrary, PV1.B <$> arbitrary]
 
+instance Arbitrary PV1.ExBudget where
+  arbitrary = PV1.ExBudget <$> arbitrary <*> arbitrary
+
+instance Arbitrary PV1.ExCPU where
+  arbitrary = fromInteger . getNonNegative <$> arbitrary
+
+instance Arbitrary PV1.ExMemory where
+  arbitrary = fromInteger . getNonNegative <$> arbitrary
+
 genValidCostModel :: Language -> Gen CostModel
 genValidCostModel lang = do
   newParamValues <- vectorOf (costModelInitParamCount lang) arbitrary

libs/cardano-ledger-core/testlib/Test/Cardano/Ledger/Plutus/ExUnits.hs

@@ -9,13 +9,28 @@ import Test.Cardano.Ledger.Core.Arbitrary ()
 
 spec :: Spec
 spec = do
-  prop "Round-trip to ExBudget" exUnitsTranslationRoundTrip
+  prop "Round-trip to ExBudget" exUnitsToExBudgetRoundTrip
+  prop "Round-trip from ExBudget" exBudgetToExUnitsRoundTrip
 
--- ExUnits should remain intact when translating to and from the plutus type
-exUnitsTranslationRoundTrip :: Gen Property
-exUnitsTranslationRoundTrip = do
+-- ExUnits should remain intact when translating to and from the Plutus ExBudget type
+exUnitsToExBudgetRoundTrip :: Gen Property
+exUnitsToExBudgetRoundTrip = do
   e <- arbitrary
-  let result = exBudgetToExUnits (transExUnits e)
+  let result = exBudgetToExUnits $ transExUnits e
+  pure
+    $ counterexample
+      ( "Before: "
+          <> show e
+          <> "\n After: "
+          <> show result
+      )
+    $ result == Just e
+
+-- Plutus ExBudget should remain intact when translating to and from the ExUnits type
+exBudgetToExUnitsRoundTrip :: Gen Property
+exBudgetToExUnitsRoundTrip = do
+  e <- arbitrary
+  let result = transExUnits <$> exBudgetToExUnits e
   pure
     $ counterexample
       ( "Before: "