Basic implementation of HTLC

HTLC.sol

pragma solidity ^0.5.0;

contract HTLC {
	address payable public sender;
	address payable public receiver;
	bytes32 public hashlock;
	uint256 public timelock;
	bytes32 public preimage;

	constructor(address payable _receiver, bytes32 _hashlock, uint256 _timelock) public payable {
		sender = msg.sender;
		receiver = _receiver;
		hashlock = _hashlock;
		timelock = _timelock;
	}

	function redeem(bytes32 _preimage) external returns (bool) {
		require(address(this).balance != 0);
		require(msg.sender == receiver);
		// Check hashlock
		require(hashlock == sha256(abi.encodePacked(preimage)),
			"hashlock unlock failed"
		);
		// Redeem
		preimage = _preimage;
		msg.sender.transfer(address(this).balance);
		return true;
	}

	function refund() external returns (bool) {
		require(address(this).balance != 0);
		require(msg.sender == sender);
		// Check timelock
		require(timelock <= now, "timelock unlock failed");
		// Refund
		msg.sender.transfer(address(this).balance);
		return true;
	}
}