test: add property-based tests for drip contract invariants

contracts/tests/property_based.rs

@@ -0,0 +1,64 @@
+use grant_stream::DripContract; // adjust to your actual contract module
+use proptest::prelude::*;
+
+// Helper enum for randomized actions
+#[derive(Debug, Clone)]
+enum Action {
+    Pause,
+    Resume,
+    Withdraw(u128),
+    ChangeRate(u128),
+}
+
+// Property-based test
+proptest! {
+    #[test]
+    fn drip_invariants(actions in prop::collection::vec(
+        prop_oneof![
+            Just(Action::Pause),
+            Just(Action::Resume),
+            (1u128..1000u128).prop_map(Action::Withdraw),
+            (1u128..100u128).prop_map(Action::ChangeRate),
+        ],
+        1..50 // number of actions in the sequence
+    )) {
+        // Initialize the contract with a random deposit
+        let initial_deposit = 10000u128;
+        let mut drip = DripContract::new(initial_deposit, 10); // 10 tokens per block rate, example
+        let mut total_withdrawn = 0u128;
+        let mut accrued_while_paused = 0u128;
+        let mut is_paused = false;
+
+        for action in actions {
+            match action {
+                Action::Pause => {
+                    drip.pause();
+                    is_paused = true;
+                }
+                Action::Resume => {
+                    drip.resume();
+                    is_paused = false;
+                }
+                Action::Withdraw(amount) => {
+                    let withdrawn = drip.withdraw(amount);
+                    total_withdrawn += withdrawn;
+                }
+                Action::ChangeRate(new_rate) => {
+                    drip.set_rate(new_rate);
+                }
+            }
+
+            // Assert that during paused state, accrued doesn't increase
+            if is_paused {
+                let accrued = drip.accrued_balance();
+                prop_assert_eq!(accrued, accrued_while_paused);
+            } else {
+                accrued_while_paused = drip.accrued_balance();
+            }
+
+            // Assert invariant: total withdrawn + remaining balance == initial deposit
+            let remaining = drip.remaining_balance();
+            prop_assert_eq!(total_withdrawn + remaining, initial_deposit);
+        }
+    }
+}