Differential Testing with --ffi flag

.github/workflows/test.yml

@@ -17,6 +17,16 @@ jobs:
         with:
           submodules: recursive
 
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.8'
+
+      - name: Install eth-abi
+        run: |
+          python3 -m pip install --upgrade pip
+          python3 -m pip install eth-abi
+
       - name: Install Foundry
         uses: foundry-rs/foundry-toolchain@v1
         with:
@@ -30,7 +40,7 @@ jobs:
 
       - name: Run Forge tests
         run: |
-          forge test -vvv
+          forge test -vvv --ffi
         id: test
 
       - name: Check formatting

py/compute_premium.py

@@ -0,0 +1,32 @@
+import sys
+from eth_abi import encode
+
+PRECISION = 10 ** 18
+SECONDS_PER_DAY = 86400
+PER_PERIOD_DECAY_PERCENT = 50
+PER_PERIOD_DECAY_PERCENT_WAD = int(PER_PERIOD_DECAY_PERCENT * PRECISION / 100)
+
+class DecayedPriceCalculator:
+    @staticmethod
+    def decayed_premium(start_premium, elapsed_seconds):
+        ratio = elapsed_seconds / SECONDS_PER_DAY
+        percent_wad_remaining_per_period = (PRECISION - PER_PERIOD_DECAY_PERCENT_WAD) / PRECISION
+        multiplier = (percent_wad_remaining_per_period ** ratio)
+        price = (start_premium * multiplier)
+        return int(price) 
+
+    @classmethod
+    def calculate_from_cli(cls):
+        if len(sys.argv) != 3:
+            print("Usage: python3 price.py <start_premium> <elapsed_seconds>")
+            sys.exit(1)
+
+        start_premium = int(sys.argv[1])
+        elapsed_seconds = int(sys.argv[2])
+
+        result = cls.decayed_premium(start_premium, elapsed_seconds)
+        enc = encode(['uint256'], [result])
+        print("0x" + enc.hex())
+
+if __name__ == "__main__":
+    DecayedPriceCalculator.calculate_from_cli()

test/ExponentialPremiumPriceOracle/ExponentialPremiumFuzzTest.t.sol

@@ -34,4 +34,21 @@ contract ExponentialPremiumFuzzTest is ExponentialPremiumOracleBase {
 
         assert(premium1 >= premium2);
     }
+
+    function test_decayedPremium_accuracy(uint256 elapsed) public {
+        uint256 bound = 400 * 1 days;
+        vm.assume(elapsed <= bound);
+        string[] memory input = new string[](4);
+        input[0] = "python3";
+        input[1] = "py/compute_premium.py";
+        input[2] = vm.toString(startPremium);
+        input[3] = vm.toString(elapsed);
+        uint256 result = oracle.decayedPremium(elapsed);
+        bytes memory res = vm.ffi(input);
+        uint256 expected = abi.decode(res, (uint256));
+        uint256 leftBound = (expected * (999)) / 1000;
+        uint256 rightBound = (expected * (1001)) / 1000;
+        bool withinBounds = (leftBound <= result && result <= rightBound); // Checking accuracy within 0.1 percent of the expected result
+        assertTrue(withinBounds);
+    }
 }