First Draft for Singular One-to-Many UTxO Indexer

lib/aiken-design-patterns/singular-utxo-indexer-one-to-many.ak

@@ -0,0 +1,42 @@
+use aiken/list
+use aiken/transaction.{Input, Output, ScriptContext, Spend, Transaction}
+
+pub fn spend(
+  validation_logic: fn(Output, List<Output>) -> Bool,
+  redeemer: (Int, List<Int>),
+  ctx: ScriptContext,
+) -> Bool {
+  expect ScriptContext { transaction: tx, purpose: Spend(own_ref) } = ctx
+
+  let Transaction { inputs, outputs, .. } = tx
+
+  let (in_ix, out_ixs) = redeemer
+
+  expect Some(Input { output: in_utxo, output_reference: in_ref }) =
+    inputs |> list.at(in_ix)
+
+  let fold_res =
+    list.foldr(
+      out_ixs,
+      (list.length(outputs), []),
+      fn(curr_ix, acc_tuple) {
+        let (prev_ix, acc) = acc_tuple
+        // Folding from right and expecting ascending order from head to tail
+        // (for a more intuitive API) to disallow duplicates.
+        if curr_ix < prev_ix {
+          expect Some(out_utxo) = outputs |> list.at(curr_ix)
+          (curr_ix, acc |> list.push(out_utxo))
+        } else {
+          // Output indices must be ordered.
+          fail
+        }
+      },
+    )
+
+  let (_, out_utxos) = fold_res
+
+  // Indicated input must match the spending one.
+  expect (own_ref == in_ref)?
+
+  validation_logic(in_utxo, out_utxos)
+}