contract Proxy {
    /**
    * @dev Tells the address of the implementation where every call will be delegated.
    * @return address of the implementation to which it will be delegated
    */
    /* solcov ignore next */
    function implementation() public view returns (address);
    /**
    * @dev Fallback function allowing to perform a delegatecall to the given implementation.
    * This function will return whatever the implementation call returns
    */
    function() public payable {
        // solhint-disable-previous-line no-complex-fallback
        address _impl = implementation();
        require(_impl != address(0));
        assembly {
            /*
                0x40 is the "free memory slot", meaning a pointer to next slot of empty memory. mload(0x40)
                loads the data in the free memory slot, so `ptr` is a pointer to the next slot of empty
                memory. It's needed because we're going to write the return data of delegatecall to the
                free memory slot.
            */
            let ptr := mload(0x40)
            /*
                `calldatacopy` is copy calldatasize bytes from calldata
                First argument is the destination to which data is copied(ptr)
                Second argument specifies the start position of the copied data.
                    Since calldata is sort of its own unique location in memory,
                    0 doesn't refer to 0 in memory or 0 in storage - it just refers to the zeroth byte of calldata.
                    That's always going to be the zeroth byte of the function selector.
                Third argument, calldatasize, specifies how much data will be copied.
                    calldata is naturally calldatasize bytes long (same thing as msg.data.length)
            */
            calldatacopy(ptr, 0, calldatasize)
            /*
                delegatecall params explained:
                gas: the amount of gas to provide for the call. `gas` is an Opcode that gives
                    us the amount of gas still available to execution
                _impl: address of the contract to delegate to
                ptr: to pass copied data
                calldatasize: loads the size of `bytes memory data`, same as msg.data.length
                0, 0: These are for the `out` and `outsize` params. Because the output could be dynamic,
                        these are set to 0, 0 so the output data will not be written to memory. The output
                        data will be read using `returndatasize` and `returdatacopy` instead.
                result: This will be 0 if the call fails and 1 if it succeeds
            */
            let result := delegatecall(gas, _impl, ptr, calldatasize, 0, 0)
            /*
            */
            /*
                ptr current points to the value stored at 0x40,
                because we assigned it like ptr := mload(0x40).
                Because we use 0x40 as a free memory pointer,
                we want to make sure that the next time we want to allocate memory,
                we aren't overwriting anything important.
                So, by adding ptr and returndatasize,
                we get a memory location beyond the end of the data we will be copying to ptr.
                We place this in at 0x40, and any reads from 0x40 will now read from free memory
            */
            mstore(0x40, add(ptr, returndatasize))
            /*
                `returndatacopy` is an Opcode that copies the last return data to a slot. `ptr` is the
                    slot it will copy to, 0 means copy from the beginning of the return data, and size is
                    the amount of data to copy.
                `returndatasize` is an Opcode that gives us the size of the last return data. In this case, that is the size of the data returned from delegatecall
            */
            returndatacopy(ptr, 0, returndatasize)
            /*
                if `result` is 0, revert.
                if `result` is 1, return `size` amount of data from `ptr`. This is the data that was
                copied to `ptr` from the delegatecall return data
            */
            switch result
                case 0 {
                    revert(ptr, returndatasize)
                }
                default {
                    return(ptr, returndatasize)
                }
        }
    }
}
/**
 * @title Proxy
 * @dev Gives the possibility to delegate any call to a foreign implementation.
 */
function implementation() public view returns (address);
/* solcov ignore next */