pancakePair.sol

function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data) external lock {
    require(amount0Out > 0 || amount1Out > 0, 'Pancake: INSUFFICIENT_OUTPUT_AMOUNT');
    (uint112 _reserve0, uint112 _reserve1,) = getReserves(); // gas savings
    require(amount0Out < _reserve0 && amount1Out < _reserve1, 'Pancake: INSUFFICIENT_LIQUIDITY');

    uint balance0;
    uint balance1;
    { // scope for _token{0,1}, avoids stack too deep errors
    address _token0 = token0;
    address _token1 = token1;
    require(to != _token0 && to != _token1, 'Pancake: INVALID_TO');
    if (amount0Out > 0) _safeTransfer(_token0, to, amount0Out); // optimistically transfer tokens
    if (amount1Out > 0) _safeTransfer(_token1, to, amount1Out); // optimistically transfer tokens
    if (data.length > 0) IPancakeCallee(to).pancakeCall(msg.sender, amount0Out, amount1Out, data);
    balance0 = IERC20(_token0).balanceOf(address(this));
    balance1 = IERC20(_token1).balanceOf(address(this));
    }
    uint amount0In = balance0 > _reserve0 - amount0Out ? balance0 - (_reserve0 - amount0Out) : 0;
    uint amount1In = balance1 > _reserve1 - amount1Out ? balance1 - (_reserve1 - amount1Out) : 0;
    require(amount0In > 0 || amount1In > 0, 'Pancake: INSUFFICIENT_INPUT_AMOUNT');
    { // scope for reserve{0,1}Adjusted, avoids stack too deep errors
    uint balance0Adjusted = (balance0.mul(10000).sub(amount0In.mul(25)));
    uint balance1Adjusted = (balance1.mul(10000).sub(amount1In.mul(25)));
    require(balance0Adjusted.mul(balance1Adjusted) >= uint(_reserve0).mul(_reserve1).mul(10000**2), 'Pancake: K');
    }

    _update(balance0, balance1, _reserve0, _reserve1);
    emit Swap(msg.sender, amount0In, amount1In, amount0Out, amount1Out, to);
}