contract BlockHashRNGFallback is BlockHashRNG {
    
    /** @dev Save the random number for this blockhash and give the reward to the caller.
     *  @param _block Block the random number is linked to.
     */
    function saveRN(uint _block) public {
        if (_block<block.number && randomNumber[_block]==0) {// If the random number is not already set and can be.
            if (blockhash(_block)!=0x0) // Normal case.
                randomNumber[_block]=uint(blockhash(_block));
            else // The contract was not called in time. Fallback to returning previous blockhash.
                randomNumber[_block]=uint(blockhash(block.number-1));
        }
        if (randomNumber[_block] != 0) { // If the random number is set.
            uint rewardToSend=reward[_block];
            reward[_block]=0;
            msg.sender.send(rewardToSend); // Note that the use of send is on purpose as we don't want to block in case the msg.sender has a fallback issue.
        }
    }
    
}
/** Random Number Generator returning the blockhash with a backup behaviour.
 *  Allows saving the random number for use in the future. 
 *  It allows the contract to still access the blockhash even after 256 blocks.
 *  The first party to call the save function gets the reward.
 *  If no one calls the contract within 256 blocks, the contract fallback in returning the blockhash of the previous block.
 */
function saveRN(uint _block) public {
        if (_block<block.number && randomNumber[_block]==0) {// If the random number is not already set and can be.
            if (blockhash(_block)!=0x0) // Normal case.
                randomNumber[_block]=uint(blockhash(_block));
            else // The contract was not called in time. Fallback to returning previous blockhash.
                randomNumber[_block]=uint(blockhash(block.number-1));
        }
        if (randomNumber[_block] != 0) { // If the random number is set.
            uint rewardToSend=reward[_block];
            reward[_block]=0;
            msg.sender.send(rewardToSend); // Note that the use of send is on purpose as we don't want to block in case the msg.sender has a fallback issue.
        }
    }
/** @dev Save the random number for this blockhash and give the reward to the caller.
 *  @param _block Block the random number is linked to.
 */