feat(libraries): add router quoting and reserve helpers

src/libraries/FluxLibrary.sol

@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.36;
+
+import {IFluxFactory} from "../interfaces/IFluxFactory.sol";
+import {IFluxPair} from "../interfaces/IFluxPair.sol";
+
+/// @title Pricing and reserve helpers shared by the router.
+library FluxLibrary {
+    function sortTokens(address tokenA, address tokenB) internal pure returns (address token0, address token1) {
+        require(tokenA != tokenB, "FluxLibrary: IDENTICAL_ADDRESSES");
+        (token0, token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA);
+        require(token0 != address(0), "FluxLibrary: ZERO_ADDRESS");
+    }
+
+    function pairFor(address factory, address tokenA, address tokenB) internal view returns (address pair) {
+        pair = IFluxFactory(factory).getPair(tokenA, tokenB);
+        require(pair != address(0), "FluxLibrary: PAIR_NOT_FOUND");
+    }
+
+    function getReserves(address factory, address tokenA, address tokenB)
+        internal
+        view
+        returns (uint256 reserveA, uint256 reserveB)
+    {
+        (address token0,) = sortTokens(tokenA, tokenB);
+        (uint112 reserve0, uint112 reserve1,) = IFluxPair(pairFor(factory, tokenA, tokenB)).getReserves();
+        (reserveA, reserveB) =
+            tokenA == token0 ? (uint256(reserve0), uint256(reserve1)) : (uint256(reserve1), uint256(reserve0));
+    }
+
+    /// @notice Equal-value counterpart amount given current reserves.
+    function quote(uint256 amountA, uint256 reserveA, uint256 reserveB) internal pure returns (uint256 amountB) {
+        require(amountA > 0, "FluxLibrary: INSUFFICIENT_AMOUNT");
+        require(reserveA > 0 && reserveB > 0, "FluxLibrary: INSUFFICIENT_LIQUIDITY");
+        amountB = amountA * reserveB / reserveA;
+    }
+
+    /// @notice Output for an exact input, 0.3% fee applied to the input.
+    function getAmountOut(uint256 amountIn, uint256 reserveIn, uint256 reserveOut)
+        internal
+        pure
+        returns (uint256 amountOut)
+    {
+        require(amountIn > 0, "FluxLibrary: INSUFFICIENT_INPUT_AMOUNT");
+        require(reserveIn > 0 && reserveOut > 0, "FluxLibrary: INSUFFICIENT_LIQUIDITY");
+        uint256 amountInWithFee = amountIn * 997;
+        uint256 numerator = amountInWithFee * reserveOut;
+        uint256 denominator = reserveIn * 1000 + amountInWithFee;
+        amountOut = numerator / denominator;
+    }
+
+    /// @notice Input required for an exact output, 0.3% fee applied to the input.
+    function getAmountIn(uint256 amountOut, uint256 reserveIn, uint256 reserveOut)
+        internal
+        pure
+        returns (uint256 amountIn)
+    {
+        require(amountOut > 0, "FluxLibrary: INSUFFICIENT_OUTPUT_AMOUNT");
+        require(reserveIn > 0 && reserveOut > 0, "FluxLibrary: INSUFFICIENT_LIQUIDITY");
+        uint256 numerator = reserveIn * amountOut * 1000;
+        uint256 denominator = (reserveOut - amountOut) * 997;
+        amountIn = numerator / denominator + 1;
+    }
+
+    function getAmountsOut(address factory, uint256 amountIn, address[] memory path)
+        internal
+        view
+        returns (uint256[] memory amounts)
+    {
+        require(path.length >= 2, "FluxLibrary: INVALID_PATH");
+        amounts = new uint256[](path.length);
+        amounts[0] = amountIn;
+        for (uint256 i; i < path.length - 1; i++) {
+            (uint256 reserveIn, uint256 reserveOut) = getReserves(factory, path[i], path[i + 1]);
+            amounts[i + 1] = getAmountOut(amounts[i], reserveIn, reserveOut);
+        }
+    }
+
+    function getAmountsIn(address factory, uint256 amountOut, address[] memory path)
+        internal
+        view
+        returns (uint256[] memory amounts)
+    {
+        require(path.length >= 2, "FluxLibrary: INVALID_PATH");
+        amounts = new uint256[](path.length);
+        amounts[amounts.length - 1] = amountOut;
+        for (uint256 i = path.length - 1; i > 0; i--) {
+            (uint256 reserveIn, uint256 reserveOut) = getReserves(factory, path[i - 1], path[i]);
+            amounts[i - 1] = getAmountIn(amounts[i], reserveIn, reserveOut);
+        }
+    }
+}