fix: scale factor computation

Author: MathisGD

src/chainlink/Oracle2.sol

@@ -31,9 +31,15 @@ contract Oracle2 is IOracle {
     ) {
         BASE_FEED = baseFeed;
         QUOTE_FEED = quoteFeed;
-        // SCALE_FACTOR = 10 ** (36 + (baseTokenDecimals - baseFeedDecimals) - (quoteTokenDecimals - quoteFeedDecimals))
+        // We note pB the base price, and pQ the quote price (price of 1e(decimals) asset).
+        // We want to return 1e36 * (pB/1e(bDecimals) / (pQ/1e(qDecimals)).
+        // Chainlink returns pB * bFeedPrecision and pQ * qFeedPrecision.
+        // So we have 1e36 * (pB/1e(bDecimals) / (pQ/1e(qDecimals) = pB * 1e(bFeedPrecision) * SCALE_FACTOR / (pQ *
+        // 1e(qFeedPrecision))
+        // So SCALE_FACTOR = 1e36 / 1e(bDecimals) * 1e(qDecimals) * 1e(qFeedPrecision) / 1e(bFeedPrecision)
+        // So SCALE_FACTOR = 1e(36 + qDecimals + qFeedPrecision - bDecimals - bFeedPrecision)
         SCALE_FACTOR =
-            10 ** (36 + baseTokenDecimals + quoteFeed.getDecimals() - baseFeed.getDecimals() - quoteTokenDecimals);
+            10 ** (36 + quoteTokenDecimals + quoteFeed.getDecimals() - baseFeed.getDecimals() - baseTokenDecimals);
     }
 
     /* PRICE */

src/chainlink/Oracle4.sol

@@ -41,12 +41,10 @@ contract Oracle4 is IOracle {
         SECOND_BASE_FEED = secondBaseFeed;
         FIRST_QUOTE_FEED = firstQuoteFeed;
         SECOND_QUOTE_FEED = secondQuoteFeed;
-        // SCALE_FACTOR = 10 ** (36 + (firstQuoteFeedDecimals + secondQuoteFeedDecimals - quoteTokenDecimals) -
-        // (firstBaseFeedDecimals + secondBaseFeedDecimals - baseTokenDecimals))
         SCALE_FACTOR = 10
             ** (
-                36 + baseTokenDecimals + firstQuoteFeed.getDecimals() + secondQuoteFeed.getDecimals()
-                    - firstBaseFeed.getDecimals() - secondBaseFeed.getDecimals() - quoteTokenDecimals
+                36 + quoteTokenDecimals + firstQuoteFeed.getDecimals() + secondQuoteFeed.getDecimals()
+                    - firstBaseFeed.getDecimals() - secondBaseFeed.getDecimals() - baseTokenDecimals
             );
     }
 

test/chainlink/Oracle2Test.sol

@@ -11,6 +11,8 @@ AggregatorV3Interface constant stEthEthFeed = AggregatorV3Interface(0x86392dC19c
 AggregatorV3Interface constant usdcEthFeed = AggregatorV3Interface(0x986b5E1e1755e3C2440e960477f25201B0a8bbD4);
 // 8 decimals of precision
 AggregatorV3Interface constant ethUsdFeed = AggregatorV3Interface(0x5f4eC3Df9cbd43714FE2740f5E3616155c5b8419);
+// 8 decimals of precision
+AggregatorV3Interface constant usdcUsd = AggregatorV3Interface(0x8fFfFfd4AfB6115b954Bd326cbe7B4BA576818f6);
 
 contract FakeAggregator {
     int256 public answer;
@@ -37,25 +39,32 @@ contract OracleTest is Test {
         Oracle2 oracle = new Oracle2(stEthEthFeed, usdcEthFeed, 18, 6);
         (, int256 baseAnswer,,,) = stEthEthFeed.latestRoundData();
         (, int256 quoteAnswer,,,) = usdcEthFeed.latestRoundData();
-        assertEq(oracle.price(), uint256(baseAnswer) * 10 ** (36 + 18 + 18 - 18 - 6) / uint256(quoteAnswer));
+        assertEq(oracle.price(), uint256(baseAnswer) * 10 ** (36 + 18 + 6 - 18 - 18) / uint256(quoteAnswer));
     }
 
     function testOracleEthUsd() public {
         Oracle2 oracle = new Oracle2(ethUsdFeed, AggregatorV3Interface(address(0)), 18, 0);
         (, int256 expectedPrice,,,) = ethUsdFeed.latestRoundData();
-        assertEq(oracle.price(), uint256(expectedPrice) * 10 ** (36 + 18 - 8));
+        assertEq(oracle.price(), uint256(expectedPrice) * 10 ** (36 - 18 - 8));
     }
 
     function testOracleStEthEth() public {
-        Oracle2 oracle = new Oracle2(stEthEthFeed, AggregatorV3Interface(address(0)), 18, 0);
+        Oracle2 oracle = new Oracle2(stEthEthFeed, AggregatorV3Interface(address(0)), 18, 18);
         (, int256 expectedPrice,,,) = stEthEthFeed.latestRoundData();
-        assertEq(oracle.price(), uint256(expectedPrice) * 10 ** (36 + 18 - 18));
+        assertEq(oracle.price(), uint256(expectedPrice) * 10 ** (36 + 18 - 18 - 18));
+        assertApproxEqRel(oracle.price(), 1e36, 0.01 ether);
     }
 
     function testOracleEthStEth() public {
-        Oracle2 oracle = new Oracle2(AggregatorV3Interface(address(0)), stEthEthFeed, 0, 18);
+        Oracle2 oracle = new Oracle2(AggregatorV3Interface(address(0)), stEthEthFeed, 18, 18);
         (, int256 expectedPrice,,,) = stEthEthFeed.latestRoundData();
-        assertEq(oracle.price(), 10 ** (36 + 18 - 18) / uint256(expectedPrice));
+        assertEq(oracle.price(), 10 ** (36 + 18 + 18 - 18) / uint256(expectedPrice));
+        assertApproxEqRel(oracle.price(), 1e36, 0.01 ether);
+    }
+
+    function testOracleUsdcUsd() public {
+        Oracle2 oracle = new Oracle2(usdcUsd, AggregatorV3Interface(address(0)), 6, 0);
+        assertApproxEqRel(oracle.price(), 1e36 / 1e6, 0.01 ether);
     }
 
     function testNegativeAnswer() public {

test/chainlink/Oracle4Test.sol

@@ -26,8 +26,7 @@ contract OracleTest is Test {
         (, int256 quoteAnswer,,,) = usdcUsdFeed.latestRoundData();
         assertEq(
             oracle.price(),
-            (uint256(firstBaseAnswer) * uint256(secondBaseAnswer) * 10 ** (36 + 8 + 8 - 6 - 8 - 8))
-                / uint256(quoteAnswer)
+            (uint256(firstBaseAnswer) * uint256(secondBaseAnswer) * 10 ** (36 + 8 + 6 - 8 - 8 - 8)) / uint256(quoteAnswer)
         );
     }
 
@@ -38,16 +37,15 @@ contract OracleTest is Test {
         (, int256 secondQuoteAnswer,,,) = btcUsdFeed.latestRoundData();
         assertEq(
             oracle.price(),
-            (uint256(baseAnswer) * 10 ** (36 + 8 + 8 + 6 - 8 - 8))
-                / (uint256(firstQuoteAnswer) * uint256(secondQuoteAnswer))
+            (uint256(baseAnswer) * 10 ** (36 + 8 + 8 + 8 - 6 - 8)) / (uint256(firstQuoteAnswer) * uint256(secondQuoteAnswer))
         );
     }
 
     function testOracleWbtcEth() public {
         Oracle4 oracle =
-        new Oracle4(wBtcBtcFeed, btcEthFeed, AggregatorV3Interface(address(0)), AggregatorV3Interface(address(0)), 8, 0);
+        new Oracle4(wBtcBtcFeed, btcEthFeed, AggregatorV3Interface(address(0)), AggregatorV3Interface(address(0)), 8, 18);
         (, int256 firstBaseAnswer,,,) = wBtcBtcFeed.latestRoundData();
         (, int256 secondBaseAnswer,,,) = btcEthFeed.latestRoundData();
-        assertEq(oracle.price(), (uint256(firstBaseAnswer) * uint256(secondBaseAnswer) * 10 ** (36 + 8 - 8 - 18)));
+        assertEq(oracle.price(), (uint256(firstBaseAnswer) * uint256(secondBaseAnswer) * 10 ** (36 + 18 - 8 - 8 - 18)));
     }
 }