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evals/adaptive-change/fixture/SPEC.md

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+# Ledger invariants
+
+`Ledger.apply(operation_id, account, delta)` applies an operation at most once globally.
+
+- A successful operation permanently consumes its operation ID.
+- A rejected overdraft must not consume its operation ID.
+- A caller may fund the account and retry the same rejected operation.
+- Balances may never become negative.
+- Duplicate successful operations return the current balance without applying the delta again.
+
+The current implementation violates the rejected-operation rule.

evals/adaptive-change/fixture/ledger.py

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+class Ledger:
+    def __init__(self) -> None:
+        self._balances: dict[str, int] = {}
+        self._applied: set[str] = set()
+
+    def balance(self, account: str) -> int:
+        return self._balances.get(account, 0)
+
+    def apply(self, operation_id: str, account: str, delta: int) -> int:
+        if operation_id in self._applied:
+            return self.balance(account)
+        self._applied.add(operation_id)
+        updated = self.balance(account) + delta
+        if updated < 0:
+            raise ValueError("insufficient funds")
+        self._balances[account] = updated
+        return updated

evals/adaptive-change/fixture/test_ledger.py

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+import unittest
+
+from ledger import Ledger
+
+
+class LedgerTests(unittest.TestCase):
+    def test_success_is_idempotent(self):
+        ledger = Ledger()
+        self.assertEqual(ledger.apply("deposit-1", "a", 10), 10)
+        self.assertEqual(ledger.apply("deposit-1", "a", 10), 10)
+
+    def test_overdraft_does_not_change_balance(self):
+        ledger = Ledger()
+        with self.assertRaises(ValueError):
+            ledger.apply("withdraw-1", "a", -4)
+        self.assertEqual(ledger.balance("a"), 0)
+
+    def test_rejected_operation_can_be_retried(self):
+        ledger = Ledger()
+        with self.assertRaises(ValueError):
+            ledger.apply("withdraw-1", "a", -4)
+        ledger.apply("deposit-1", "a", 10)
+        self.assertEqual(ledger.apply("withdraw-1", "a", -4), 6)
+
+
+if __name__ == "__main__":
+    unittest.main()