Add atomic swap contract

contracts/AirDropper.sol

@@ -0,0 +1,68 @@
+pragma solidity 0.4.25;
+
+import "openzeppelin-solidity/contracts/math/SafeMath.sol";
+import "./Broker.sol";
+
+/// @title The AirDropper contract to send ether to users
+/// @author Switcheo Network
+contract AirDropper {
+    using SafeMath for uint256;
+
+    // The Switcheo Broker contract
+    Broker public broker;
+
+    // A record of which hashes have been used before
+    mapping(bytes32 => bool) public usedHashes;
+
+    // Emitted when ether is sent
+    event SendEther(bytes32 indexed id, address indexed receiver, uint256 amount);
+
+    /// @notice Initializes the AirDropper contract
+    /// @dev The broker is initialized to the Switcheo Broker
+    constructor(address brokerAddress)
+        public
+    {
+        broker = Broker(brokerAddress);
+    }
+
+    modifier onlyCoordinator() {
+        require(
+            msg.sender == address(broker.coordinator()),
+            "Invalid sender"
+        );
+        _;
+    }
+
+    /// @notice The payable method to allow this contract to receive ether
+    function depositEther() external payable {}
+
+    /// @notice Sends ether to a receiving address.
+    /// @param _id The unique identifier to prevent double spends
+    /// @param _receiver The address of the receiver
+    /// @param _amount The amount of ether to send
+    function sendEther(
+        bytes32 _id,
+        address _receiver,
+        uint256 _amount
+    )
+        external
+        onlyCoordinator
+    {
+        _validateAndAddHash(_id);
+        _receiver.transfer(_amount);
+        emit SendEther(_id, _receiver, _amount);
+    }
+
+    /// @dev Ensures a hash hasn't been already used.
+    /// This prevents replay attacks.
+    function _validateAndAddHash(bytes32 _hash)
+        private
+    {
+        require(
+            usedHashes[_hash] != true,
+            "hash already used"
+        );
+
+        usedHashes[_hash] = true;
+    }
+}