callback-style funding option

src/IPartyPool.sol

@@ -178,6 +178,7 @@ interface IPartyPool is IERC20Metadata, IOwnable {
     /// @dev This function transfers the exact gross input (including fee) from payer and sends the computed output to receiver.
     ///      Non-standard tokens (fee-on-transfer, rebasers) are rejected via balance checks.
     /// @param payer address of the account that pays for the swap
+    /// @param selector If zero, then regular ERC20 approvals must be given by the payere to the pool to move the required input amount. If this selector is nonzero, then a callback style funding mechanism is used where the given selector is invoked on the payer, passing the arguments of (address inputToken, uint256 inputAmount). The callback function must send the given amount of input coin to the pool in ordr to continue the swap transaction, otherwise "Insufficient funds" is thrown.
     /// @param receiver address that will receive the output tokens
     /// @param inputTokenIndex index of input asset
     /// @param outputTokenIndex index of output asset
@@ -187,6 +188,7 @@ interface IPartyPool is IERC20Metadata, IOwnable {
     /// @return amountIn actual input used (uint256), amountOut actual output sent (uint256), inFee fee taken from the input (uint256)
     function swap(
         address payer,
+        bytes4 selector,
         address receiver,
         uint256 inputTokenIndex,
         uint256 outputTokenIndex,