balanced pair optimization

src/PartyPool.sol

@@ -8,6 +8,7 @@ import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
 import "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
 import "./LMSRStabilized.sol";
+import "./LMSRStabilizedBalancedPair.sol";
 import "./IPartyPool.sol";
 import "./IPartyFlashCallback.sol";
 
@@ -46,6 +47,7 @@ contract PartyPool is IPartyPool, ERC20, ReentrancyGuard {
     //
 
     LMSRStabilized.State internal lmsr;
+    bool immutable private _stablePair; // if true, the optimized LMSRStabilizedBalancedPair optimization path is enabled
 
     // Cached on-chain balances (uint) and internal 64.64 representation
     // balance / base = internal
@@ -65,6 +67,7 @@ contract PartyPool is IPartyPool, ERC20, ReentrancyGuard {
     /// @param _targetSlippage target slippage in 64.64 fixed-point (as used by LMSR)
     /// @param _swapFeePpm fee in parts-per-million, taken from swap input amounts before LMSR calculations
     /// @param _flashFeePpm fee in parts-per-million, taken for flash loans
+    /// @param _stable if true and assets.length==2, then the optimization for 2-asset stablecoin pools is activated.
     constructor(
         string memory name_,
         string memory symbol_,
@@ -73,7 +76,8 @@ contract PartyPool is IPartyPool, ERC20, ReentrancyGuard {
         int128 _tradeFrac,
         int128 _targetSlippage,
         uint256 _swapFeePpm,
-        uint256 _flashFeePpm
+        uint256 _flashFeePpm,
+        bool _stable
     ) ERC20(name_, symbol_) {
         require(_tokens.length > 1, "Pool: need >1 asset");
         require(_tokens.length == _bases.length, "Pool: lengths mismatch");
@@ -85,6 +89,7 @@ contract PartyPool is IPartyPool, ERC20, ReentrancyGuard {
         swapFeePpm = _swapFeePpm;
         require(_flashFeePpm < 1_000_000, "Pool: flash fee >= ppm");
         flashFeePpm = _flashFeePpm;
+        _stablePair = _stable && _tokens.length == 2;
 
         uint256 n = _tokens.length;
 
@@ -369,7 +374,11 @@ contract PartyPool is IPartyPool, ERC20, ReentrancyGuard {
         require(deltaInternalI > int128(0), "swap: input too small after fee");
 
         // Compute internal amounts using LMSR (exact-input with price limit)
-        (amountInInternalUsed, amountOutInternal) = lmsr.swapAmountsForExactInput(i, j, deltaInternalI, limitPrice);
+        // if _stablePair is true, use the optimized path
+        console2.log('stablepair optimization?', _stablePair);
+        (amountInInternalUsed, amountOutInternal) =
+            _stablePair ? LMSRStabilizedBalancedPair.swapAmountsForExactInput(lmsr, i, j, deltaInternalI, limitPrice)
+                        : lmsr.swapAmountsForExactInput(i, j, deltaInternalI, limitPrice);
 
         // Convert actual used input internal -> uint (ceil)
         amountInUintNoFee = _internalToUintCeil(amountInInternalUsed, bases[i]);