refactor: update answer computation

Author: MerlinEgalite

src/adapters/WstEthEthExchangeRateChainlinkAdapter.sol

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 pragma solidity 0.8.21;
 
-import {IStEth} from "../interfaces/IStEth.sol";
+import {IWstEth} from "../interfaces/IWstEth.sol";
 import {MinimalAggregatorV3Interface} from "../interfaces/MinimalAggregatorV3Interface.sol";
 
 import {ErrorsLib} from "../libraries/ErrorsLib.sol";
@@ -14,15 +14,18 @@ import {ErrorsLib} from "../libraries/ErrorsLib.sol";
 contract WstEthEthExchangeRateChainlinkAdapter is MinimalAggregatorV3Interface {
     uint8 public constant decimals = 18;
 
-    IStEth public immutable ST_ETH;
+    IWstEth public immutable WST_ETH;
+    uint256 public immutable WSTETH_DECIMALS;
 
-    constructor(address stEth) {
-        require(stEth != address(0), ErrorsLib.ZERO_ADDRESS);
-        ST_ETH = IStEth(stEth);
+    constructor(address wstEth) {
+        require(wstEth != address(0), ErrorsLib.ZERO_ADDRESS);
+
+        WST_ETH = IWstEth(wstEth);
+        WSTETH_DECIMALS = WST_ETH.decimals();
     }
 
     function latestRoundData() external view returns (uint80, int256, uint256, uint256, uint80) {
-        uint256 answer = ST_ETH.getPooledEthByShares(10 ** decimals);
+        uint256 answer = WST_ETH.stEthPerToken() * 10 ** (WSTETH_DECIMALS - decimals);
         return (0, int256(answer), 0, 0, 0);
     }
 }

src/interfaces/IWstEth.sol

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+pragma solidity >=0.5.0;
+
+interface IWstEth {
+    function decimals() external view returns (uint8);
+    function stEthPerToken() external view returns (uint256);
+}

test/WstEthEthExchangeRateChainlinkAdapterTest.sol

@@ -9,14 +9,14 @@ import "../src/ChainlinkOracle.sol";
 import "./helpers/Constants.sol";
 
 contract WstEthEthExchangeRateChainlinkAdapterTest is Test {
-    IStEth internal constant ST_ETH = IStEth(0xae7ab96520DE3A18E5e111B5EaAb095312D7fE84);
+    IWstEth internal constant WST_ETH = IWstEth(0x7f39C581F595B53c5cb19bD0b3f8dA6c935E2Ca0);
 
     WstEthEthExchangeRateChainlinkAdapter internal oracle;
     ChainlinkOracle internal chainlinkOracle;
 
     function setUp() public {
         vm.createSelectFork(vm.envString("ETH_RPC_URL"));
-        oracle = new WstEthEthExchangeRateChainlinkAdapter(address(ST_ETH));
+        oracle = new WstEthEthExchangeRateChainlinkAdapter(address(WST_ETH));
         chainlinkOracle = new ChainlinkOracle(vaultZero, oracle, feedZero, feedZero, feedZero, 1, 18, 18);
     }
 
@@ -33,7 +33,7 @@ contract WstEthEthExchangeRateChainlinkAdapterTest is Test {
         (uint80 roundId, int256 answer, uint256 startedAt, uint256 updatedAt, uint80 answeredInRound) =
             oracle.latestRoundData();
         assertEq(roundId, 0);
-        assertEq(uint256(answer), ST_ETH.getPooledEthByShares(10 ** 18));
+        assertEq(uint256(answer), WST_ETH.stEthPerToken());
         assertEq(startedAt, 0);
         assertEq(updatedAt, 0);
         assertEq(answeredInRound, 0);