feat: fix expansion rounding issue (#548)

mento-protocol · Nov 25, 2024 · ea5d779 · ea5d779
1 parent 6295656
commit ea5d779
Show file tree

Hide file tree

Showing 2 changed files with 24 additions and 22 deletions.
diff --git a/contracts/goodDollar/GoodDollarExchangeProvider.sol b/contracts/goodDollar/GoodDollarExchangeProvider.sol
@@ -144,7 +144,12 @@ contract GoodDollarExchangeProvider is IGoodDollarExchangeProvider, BancorExchan
     PoolExchange memory exchange = getPoolExchange(exchangeId);
 
     UD60x18 scaledRatio = wrap(uint256(exchange.reserveRatio) * 1e10);
-    UD60x18 newRatio = scaledRatio.mul(wrap(reserveRatioScalar));
+
+    // The division and multiplication by 1e10 here ensures that the new ratio used for calculating the amount to mint
+    // is the same as the one set in the exchange but only scaled to 18 decimals.
+    // Ignored, because the division and multiplication by 1e10 is needed see comment above.
+    // slither-disable-next-line divide-before-multiply
+    UD60x18 newRatio = wrap((unwrap(scaledRatio.mul(wrap(reserveRatioScalar))) / 1e10) * 1e10);
 
     uint32 newRatioUint = uint32(unwrap(newRatio) / 1e10);
     require(newRatioUint > 0, "New ratio must be greater than 0");

diff --git a/test/unit/goodDollar/GoodDollarExchangeProvider.t.sol b/test/unit/goodDollar/GoodDollarExchangeProvider.t.sol
@@ -349,11 +349,12 @@ contract GoodDollarExchangeProviderTest_mintFromExpansion is GoodDollarExchangeP
   function test_mintFromExpansion_whenValidReserveRatioScalar_shouldReturnCorrectAmountAndEmit() public {
     // reserveRatioScalar is (1-0.000288617289022312) based of 10% yearly expansion rate
     // Formula: amountToMint = (tokenSupply * reserveRatio - tokenSupply * newRatio) / newRatio
-    // newRatio = reserveRatio * reserveRatioScalar = 0.28571428 * (1-0.000288617289022312) = 0.285631817919071438
-    // amountToMint = (7_000_000_000 * 0.28571428 - 7_000_000_000 * 0.285631817919071438) / 0.285631817919071438
-    // ≈ 2_020_904,291074052815139287
+    // newRatio = reserveRatio * reserveRatioScalar = 0.28571428 * (1-0.000288617289022312)
+    // newRatio = 0.28563181 (only 8 decimals)
+    // amountToMint = (7_000_000_000 * 0.28571428 - 7_000_000_000 * 0.28563181) / 0.28563181
+    // ≈ 2_021_098,420375517698816528
     uint32 expectedReserveRatio = 28563181;
-    uint256 expectedAmountToMint = 2020904291074052815139287;
+    uint256 expectedAmountToMint = 2021098420375517698816528;
     uint256 priceBefore = exchangeProvider.currentPrice(exchangeId);
 
     vm.expectEmit(true, true, true, true);
@@ -371,18 +372,17 @@ contract GoodDollarExchangeProviderTest_mintFromExpansion is GoodDollarExchangeP
       "Token supply should increase by minted amount"
     );
     assertEq(poolExchangeAfter.reserveRatio, expectedReserveRatio, "Reserve ratio should be updated correctly");
-    // 0.01% relative error tolerance because of precision loss when new reserve ratio is calculated
-    assertApproxEqRel(priceBefore, priceAfter, 1e18 * 0.0001, "Price should remain within 0.01% of initial price");
+    assertEq(priceBefore, priceAfter, "Price should remain unchanged");
   }
 
   function test_mintFromExpansion_withSmallReserveRatioScalar_shouldReturnCorrectAmount() public {
     uint256 smallReserveRatioScalar = 1e18 * 0.00001; // 0.001%
     // Formula: amountToMint = (tokenSupply * reserveRatio - tokenSupply * newRatio) / newRatio
-    // newRatio = reserveRatio * reserveRatioScalar = 0.28571428 * 1e13/1e18 = 0.0000028571428
-    // amountToMint = (7_000_000_000 * 0.28571428 - 7_000_000_000 * 0.0000028571428) /0.0000028571428
-    // amountToMint ≈ 699993000000000
+    // newRatio = reserveRatio * reserveRatioScalar = 0.28571428 * 1e13/1e18 = 0.00000285 (only 8 decimals)
+    // amountToMint = (7_000_000_000 * 0.28571428 - 7_000_000_000 * 0.00000285) /0.00000285
+    // amountToMint ≈ 701.747.371.929.824,561403508771929824
     uint32 expectedReserveRatio = 285;
-    uint256 expectedAmountToMint = 699993000000000 * 1e18;
+    uint256 expectedAmountToMint = 701747371929824561403508771929824;
     uint256 priceBefore = exchangeProvider.currentPrice(exchangeId);
 
     vm.expectEmit(true, true, true, true);
@@ -400,18 +400,17 @@ contract GoodDollarExchangeProviderTest_mintFromExpansion is GoodDollarExchangeP
       "Token supply should increase by minted amount"
     );
     assertEq(poolExchangeAfter.reserveRatio, expectedReserveRatio, "Reserve ratio should be updated correctly");
-    // 1% relative error tolerance because of precision loss when new reserve ratio is calculated
-    assertApproxEqRel(priceBefore, priceAfter, 1e18 * 0.01, "Price should remain within 1% of initial price");
+    assertEq(priceBefore, priceAfter, "Price should remain unchanged");
   }
 
   function test_mintFromExpansion_withLargeReserveRatioScalar_shouldReturnCorrectAmount() public {
     uint256 largeReserveRatioScalar = 1e18 - 1; // Just below 100%
     // Formula: amountToMint = (tokenSupply * reserveRatio - tokenSupply * newRatio) / newRatio
-    // newRatio = reserveRatio * reserveRatioScalar = 0.28571428 * (1e18 -1)/1e18 ≈ 0.285714279999999999
-    // amountToMint = (7_000_000_000 * 0.28571428 - 7_000_000_000 * 0.285714279999999999) /0.285714279999999999
-    // amountToMint ≈ 0.00000002450000049000
+    // newRatio = reserveRatio * reserveRatioScalar = 0.28571428 * (1e18 -1)/1e18 ≈ 0.28571427 (only 8 decimals)
+    // amountToMint = (7_000_000_000 * 0.28571428 - 7_000_000_000 * 0.28571427) /0.28571427
+    // amountToMint ≈ 245.00001347500074112504
     uint32 expectedReserveRatio = 28571427;
-    uint256 expectedAmountToMint = 24500000490;
+    uint256 expectedAmountToMint = 245000013475000741125;
     uint256 priceBefore = exchangeProvider.currentPrice(exchangeId);
 
     vm.expectEmit(true, true, true, true);
@@ -429,8 +428,7 @@ contract GoodDollarExchangeProviderTest_mintFromExpansion is GoodDollarExchangeP
       "Token supply should increase by minted amount"
     );
     assertEq(poolExchangeAfter.reserveRatio, expectedReserveRatio, "Reserve ratio should be updated correctly");
-    // 0.01% relative error tolerance because of precision loss when new reserve ratio is calculated
-    assertApproxEqRel(priceBefore, priceAfter, 1e18 * 0.0001, "Price should remain within 0.01% of initial price");
+    assertEq(priceBefore, priceAfter, "Price should remain unchanged");
   }
 
   function test_mintFromExpansion_withMultipleConsecutiveExpansions_shouldMintCorrectly() public {
@@ -469,7 +467,7 @@ contract GoodDollarExchangeProviderTest_mintFromExpansion is GoodDollarExchangeP
       1e18 * 0.0001, // 0.01% relative error tolerance because of precision loss when new reserve ratio is calculated
       "Reserve ratio should be updated correctly within 0.01% tolerance"
     );
-    assertApproxEqRel(initialPrice, priceAfter, 1e18 * 0.0001, "Price should remain within 0.01% of initial price");
+    assertEq(initialPrice, priceAfter, "Price should remain unchanged");
   }
 
   function testFuzz_mintFromExpansion(uint256 _reserveRatioScalar) public {
@@ -497,8 +495,7 @@ contract GoodDollarExchangeProviderTest_mintFromExpansion is GoodDollarExchangeP
       initialTokenSupply + amountToMint,
       "Token supply should increase by minted amount"
     );
-    // 1% relative error tolerance because of precision loss when new reserve ratio is calculated
-    assertApproxEqRel(priceBefore, priceAfter, 1e18 * 0.01, "Price should remain within 1% of initial price");
+    assertEq(priceBefore, priceAfter, "Price should remain unchanged");
   }
 }