Added tutorial 18

README.md

@@ -17,4 +17,5 @@ The companion to the Youtube tutorials
 - [Learning Solidity : Tutorial 14 Transferring Ethereum between contracts](https://www.youtube.com/watch?v=ELWSKMcJfI8)
 - [Learning Solidity : Tutorial 15 Public vs External](https://www.youtube.com/watch?v=Ii4g38mPPlg)
 - [Learning Solidity : Tutorial 16 Time Based Events](https://www.youtube.com/watch?v=HGw-yalqdgs)
-- [Learning Solidity : Tutorial 17 Polymorphism](https://www.youtube.com/watch?v=l_E5F5qnbtk)
+- [Learning Solidity : Tutorial 17 Polymorphism](https://www.youtube.com/watch?v=l_E5F5qnbtk)
+- [Learning Solidity : Tutorial 18 Randomness and Gambling](https://www.youtube.com/watch?v=3wY5PRliphE)

tutorial-18/Casino.sol

@@ -0,0 +1,94 @@
+pragma solidity ^0.4.0;
+
+contract Casino {
+
+    uint private start;
+
+    uint private buyPeriod = 1000;
+    uint private verifyPeriod = 100;
+    uint private checkPeriod = 100;
+
+    mapping(address => uint) private _tickets;
+    mapping(address => uint) private _winnings;
+
+    address[] _entries;
+    address[] _verified;
+
+    uint private winnerSeed;
+    bool private hasWinner;
+    address private winner;
+
+    function Casino()
+        public {
+        start = block.timestamp;    
+    }
+
+    /**
+     * This should NOT be part of the contract!!
+     */
+    function unsafeEntry(uint number, uint salt) 
+        public
+        payable
+        returns (bool) {
+        return buyTicket(generateHash(number, salt));
+    }
+
+    function generateHash(uint number, uint salt)
+        public
+        pure
+        returns (uint) {
+        return uint(keccak256(number + salt));
+    }
+
+    function buyTicket(uint hash)
+        public
+        payable
+        returns (bool) {
+        // Within the timeframe
+        require(block.timestamp < start+buyPeriod);
+        // Correct amount
+        require(1 ether == msg.value);
+        // 1 entry per address
+        require(_tickets[msg.sender] == 0);
+        _tickets[msg.sender] = hash;
+        _entries.push(msg.sender);
+        return true;
+    }
+
+    function verifyTicket(uint number, uint salt)
+        public
+        returns (bool) {
+        // Within the timeframe
+        require(block.timestamp >= start+buyPeriod);
+        require(block.timestamp < start+buyPeriod+verifyPeriod);
+        // Has a valid entry
+        require(_tickets[msg.sender] > 0);
+        // Validate hash
+        require(salt > number);
+        require(generateHash(number, salt) == _tickets[msg.sender]);
+        winnerSeed = winnerSeed ^ salt ^ uint(msg.sender);
+        _verified.push(msg.sender);
+    }
+
+    function checkWinner()
+        public
+        returns (bool) {
+        // Within the timeframe
+        require(block.timestamp >= start+buyPeriod+verifyPeriod);
+        require(block.timestamp < start+buyPeriod+verifyPeriod+checkPeriod);
+        if (!hasWinner) {
+            winner = _verified[winnerSeed % _verified.length];
+            _winnings[winner] = _verified.length-10 ether;
+            hasWinner = true;
+        }
+        return msg.sender == winner;
+    }
+
+    function claim()
+        public {
+        // Has winnings to claim
+        require(_winnings[msg.sender] > 0);
+        msg.sender.transfer(_winnings[msg.sender]);
+        _winnings[msg.sender] = 0;
+    }
+}

tutorial-18/Random.sol

@@ -0,0 +1,19 @@
+pragma solidity ^0.4.0;
+
+contract Random {
+
+    function unsafeBlockRandom() 
+        public
+        returns (uint) {
+        return uint(block.blockhash(block.number-1)) % 100;
+    }
+
+    uint private _baseIncrement;
+
+    function unsafeIncrementRandom() 
+        public
+        returns (uint) {
+        return uint(sha3(_baseIncrement++)) % 100;
+    }
+
+}