Merge pull request #22 from maraoz/poex

Add proof of existence example

contracts/examples/ProofOfExistence.sol

+import "../Rejector.sol";
+
+/*
+ * Proof of Existence example contract
+ * see https://medium.com/zeppelin-blog/the-hitchhikers-guide-to-smart-contracts-in-ethereum-848f08001f05
+ */
+contract ProofOfExistence is Rejector {
+
+  mapping (bytes32 => bool) public proofs;
+
+  // store a proof of existence in the contract state
+  function storeProof(bytes32 proof) {
+    proofs[proof] = true;
+  }
+
+  // calculate and store the proof for a document
+  function notarize(string document) {
+    var proof = calculateProof(document);
+    storeProof(proof);
+  }
+
+  // helper function to get a document's sha256
+  function calculateProof(string document) constant returns (bytes32) {
+    return sha256(document);
+  }
+
+  // check if a document has been notarized
+  function checkDocument(string document) constant returns (bool) {
+    var proof = calculateProof(document);
+    return hasProof(proof);
+  }
+
+  // returns true if proof is stored
+  function hasProof(bytes32 proof) constant returns (bool) {
+    return proofs[proof];
+  }
+}

migrations/2_deploy_contracts.js

 module.exports = function(deployer) {
   deployer.deploy(PullPaymentBid);
   deployer.deploy(BadArrayUse);
+  deployer.deploy(ProofOfExistence);
   deployer.deploy(Bounty);
   deployer.deploy(Ownable);
   deployer.deploy(LimitFunds);