Payment channel example.

Author: chriseth

docs/solidity-by-example.rst

@@ -620,4 +620,52 @@ Safe Remote Purchase
 Micropayment Channel
 ********************
 
-To be written.
+::
+
+    pragma solidity ^0.4.11;
+    contract Channel {
+        address[2] users;
+        uint deposit;
+        uint timeout;
+        uint closeTimestamp;
+        uint closeSequenceNumber;
+        uint closeBalanceUser0;
+        enum State { Opened, Accepted, Closing, Closed }
+        State state;
+        function Channel(address _other, uint _timeout) payable {
+            users[0] = msg.sender;
+            users[1] = _other;
+            deposit = msg.value;
+            require(deposit < 2**250);
+            require(_timeout < 2 days);
+            timeout = _timeout;
+        }
+        function accept() payable {
+            require(msg.sender == users[1]);
+            require(msg.value == deposit);
+            require(state == State.Opened);
+            state = State.Accepted;
+        }
+        function requestClose(uint _balanceUser0, uint _sequenceNumber, uint8 v, bytes32 r, bytes32 s) {
+            require(state == State.Accepted || state == State.Closing);
+            require(_sequenceNumber > closeSequenceNumber);
+            require(_balanceUser0 <= 2 * deposit);
+            require(closeTimestamp == 0 || now <= closeTimestamp + timeout);
+            require(ecrecover(sha3(_balanceUser0, _sequenceNumber), v, r, s) == users[_sequenceNumber % 2]);
+            closeTimestamp = now;
+            closeSequenceNumber = _sequenceNumber;
+            closeBalanceUser0 = _balanceUser0;
+            state = State.Closing;
+        }
+        function close()
+        {
+            // TODO Can we actually include this second condition?
+            require(now >= closeTimestamp + timeout || msg.sender == users[(closeSequenceNumber + 1) % 2]);
+            require(state == State.Closing || state == State.Closed);
+            state = State.Closed;
+            if (users[0].send(closeBalanceUser0))
+                closeBalanceUser0 = 0;
+            if (users[1].send(2 * deposit - closeBalanceUser0))
+                deposit = 0;
+        }
+    }