add visibility stake

contracts/communication/VisibilityStake.sol

@@ -0,0 +1,50 @@
+pragma solidity >=0.5.0 <0.6.0;
+
+import "../token/ERC20Token.sol";
+import "../common/Controlled.sol";
+
+
+contract VisibilityStake is Controlled {
+
+    struct Stake {
+        uint256 amount;
+        uint256 time;
+    }
+
+    ERC20Token public token;
+    uint256 public lockDelay;
+    mapping(bytes32 => uint256) public visibility;
+    mapping(address => mapping (bytes32 => Stake)) public stake;
+    
+    constructor(ERC20Token _token, uint256 _lockDelay) public {
+        token = _token;
+        lockDelay = _lockDelay;
+    }
+
+    function deposit(bytes32 _publicKeyHash, uint256 _amount) external {
+        require(token.transferFrom(msg.sender, address(this), _amount), "Transfer error");
+        visibility[_publicKeyHash] += _amount;
+        stake[msg.sender][_publicKeyHash].amount += _amount;
+        stake[msg.sender][_publicKeyHash].time = now;
+
+    }
+
+    function withdraw(bytes32 _publicKeyHash) external {
+        uint256 time = stake[msg.sender][_publicKeyHash].time = now;
+        uint256 amount = stake[msg.sender][_publicKeyHash].amount;
+        require(time != 0 && time > now + lockDelay, "Locked stake");
+        delete stake[msg.sender][_publicKeyHash];
+        if(amount == 0) {
+            return;
+        }
+        require(visibility[_publicKeyHash] > amount, "Not available");
+        visibility[_publicKeyHash] -= amount;
+    }
+    
+    function slash(bytes32 _publicKeyHash) external onlyController {
+        uint256 amount = visibility[_publicKeyHash];
+        delete visibility[_publicKeyHash];
+        require(token.transferFrom(address(this), msg.sender, amount), "Transfer error");
+    }
+
+}