Add test_borrow_apy (#91)

* Add test_borrow_apy * hevm => vm * Make comment about Compound UI a bit clearer * Add back a test that was falsely removed Co-authored-by: rockyfour <[email protected]>
makerdao · Nov 23, 2022 · 4de0e6c · 4de0e6c
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diff --git a/src/plans/D3MCompoundPlan.sol b/src/plans/D3MCompoundPlan.sol
@@ -23,7 +23,7 @@
 // apy to rate per block off-chain conversion info:
 //         apy = ((1 + (borrowRatePerBlock / WAD) * blocksPerDay) ^ 365 - 1) * 100
 // (0)     borrowRatePerBlock = ((((apy / 100) + 1) ^ (1 / 365) - 1) / blocksPerDay) * WAD
-//         ATTOW the formula matches the borrow apy on (https://compound.finance/markets/DAI) using blocksPerDay = 6570
+//         ATTOW the formula matches the borrow apy on (https://compound.finance/markets/DAI) using blocksPerDay = 7200
 //
 // https://github.com/compound-finance/compound-protocol/blob/a3214f67b73310d547e00fc578e8355911c9d376/contracts/BaseJumpRateModelV2.sol#L96
 //

diff --git a/src/tests/integration/D3MCompound.t.sol b/src/tests/integration/D3MCompound.t.sol
@@ -45,6 +45,8 @@ interface CErc20Like {
     function balanceOfUnderlying(address owner) external returns (uint256);
     function repayBorrow(uint256 repayAmount) external returns (uint256);
     function exchangeRateCurrent() external returns (uint256);
+    function accrueInterest() external returns (uint256);
+    function borrowBalanceCurrent(address) external returns (uint256);
 }
 
 interface CEthLike {
@@ -322,6 +324,30 @@ contract D3MCompoundTest is DSSTest {
         assertEq(vat.dai(address(d3mHub)), 0);
     }
 
+    function test_borrow_apy() public {
+        // target 2% borrow apy, see top of D3MCompoundPlan for the formula explanation
+        uint256 targetBorrowRate = 7535450719; // ((2.00 / 100) + 1) ^ (1 / 365) - 1) / 7200) * 10^18
+
+        d3mCompoundPlan.file("barb", targetBorrowRate);
+        d3mHub.exec(ilk);
+        assertEqInterest(getBorrowRate(), targetBorrowRate);
+
+        cDai.borrow(1 * WAD);
+
+        uint256 borrowBalanceBefore = cDai.borrowBalanceCurrent(address(this));
+
+        // fast forward 1 year while accruing interest each day
+        for (uint256 i = 1; i <= 365; i++) {
+            vm.roll(block.number + 7200);
+            cDai.accrueInterest();
+        }
+
+        uint256 borrowBalanceAfter = cDai.borrowBalanceCurrent(address(this));
+
+        // rates compound so we tolerate a larger difference
+        assertEqApprox(borrowBalanceAfter, borrowBalanceBefore * 102 / 100, INTEREST_RATE_TOLERANCE * 5);
+    }
+
     function test_barb_zero() public {
 
         uint256 targetBorrowRate = _setRelBorrowTarget(7500);