fix: Add arbitrary for coinbase

Function `NeptuneCoins::arbitrary_coinbase` returns an arbitrary
`Option<NeptuneCoins>` with a small, non-negative amount (if set).

This fixes failing test cases in `time_lock.rs`.

Co-authored-by: Alan Szepieniec <[email protected]>

src/models/blockchain/transaction/validity/tasm/coinbase_amount.rs

@@ -9,6 +9,8 @@ pub(crate) struct CoinbaseAmount;
 
 const ILLEGAL_COINBASE_AMOUNT_ERROR: i128 = 1_000_200;
 
+/// Map a pointer to a coinbase object to its amount (if some) or (if none)
+/// zero.
 impl BasicSnippet for CoinbaseAmount {
     fn inputs(&self) -> Vec<(DataType, String)> {
         vec![(DataType::VoidPointer, "*coinbase".to_owned())]
@@ -197,7 +199,10 @@ mod test {
             let new_seed: [u8; 32] = rng.gen();
 
             let mut u = Unstructured::new(&new_seed);
-            let coinbase: Option<NeptuneCoins> = u.arbitrary().unwrap();
+            let coinbase: Option<NeptuneCoins> = u
+                .arbitrary::<Option<NeptuneCoins>>()
+                .unwrap()
+                .map(|c| c.abs());
             let coinbase_ptr: BFieldElement = rng.gen();
 
             let mut memory = HashMap::default();

src/models/blockchain/type_scripts/neptune_coins.rs

@@ -499,6 +499,8 @@ impl Display for NeptuneCoins {
 }
 
 impl<'a> Arbitrary<'a> for NeptuneCoins {
+    /// Generate an arbitrary amount of NeptuneCoins that is small in absolute
+    /// value but can be negative.
     fn arbitrary(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
         let nau: u128 = u.arbitrary()?;
         Ok(NeptuneCoins((nau as i128) >> 10))
@@ -511,6 +513,14 @@ impl NeptuneCoins {
             .prop_map(|uint| NeptuneCoins((uint >> 10) as i128))
             .boxed()
     }
+
+    /// Generate a strategy for an Option of NeptuneCoins, which if set will be
+    /// a small non-negative amount.
+    pub(crate) fn arbitrary_coinbase() -> BoxedStrategy<Option<Self>> {
+        arb::<Option<NeptuneCoins>>()
+            .prop_map(|coinbase| coinbase.map(|c| c.abs()))
+            .boxed()
+    }
 }
 
 #[cfg(test)]

src/models/blockchain/type_scripts/time_lock.rs

@@ -779,7 +779,7 @@ impl Arbitrary for TimeLockWitness {
             vec(arb::<Digest>(), num_outputs),
             vec(NeptuneCoins::arbitrary_non_negative(), num_outputs),
             vec(arb::<PublicAnnouncement>(), num_public_announcements),
-            arb::<Option<NeptuneCoins>>(),
+            NeptuneCoins::arbitrary_coinbase(),
             NeptuneCoins::arbitrary_non_negative(),
         )
             .prop_flat_map(