Merge pull request #313 from remedcu/master

Created BlockhashRNGFallbackRange.sol

Author: clesaege

contracts/standard/rng/BlockhashRNGFallbackRange.sol

@@ -0,0 +1,45 @@
+ /**
+ *  @authors: [@remedcu]
+ *  @reviewers: [@clesaege]
+ *  @auditors: []
+ *  @bounties: []
+ *  @deployments: []
+ */
+
+pragma solidity ^0.4.15;
+
+import "./BlockhashRNG.sol";
+
+/**
+ *  @title Random Number Generator using blockhash with fallback from the last 256 blockhash mined.
+ *  @author Shebin John - <admin@remedcu.com>
+ *
+ *  Random Number Generator returning the blockhash with a backup behaviour.
+ *  This contract implements the RNG standard and gives parties incentives to save the blockhash to avoid it to become unreachable after 256 blocks.
+ *  In case no one called it within the 256 blocks, it returns the blockhash from any one of the last mined 256 blocks.
+ *  Thus allowing the contract to still return a value even after 256 blocks.
+ *  Allows saving the random number for use in the future.
+ *  The first party to call the save function gets the reward.
+ *  This contract must be used when returning 0 is a worse failure mode than returning another blockhash.
+ *  Note that if someone calls it within the timeframe, this contracts acts exactly as BlockHashRNG.
+ */
+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.
+            // Returns the blockhash of _block if accessible (one of the last 256 blocks).
+            // If the blockhash hasn't been saved in time, return a blockhash of a block in range as a fallback.
+            randomNumber[_block] = uint(blockhash((block.number-1) - (block.number-1-_block)%256));
+        }
+
+        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.
+        }
+    }
+
+}