contract.sol

pragma solidity >=0.7.0 <0.9.0;

import "github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.3/contracts/cryptography/ECDSA.sol";
import "github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.3/contracts/utils/ReentrancyGuard.sol";

contract UniDirectionalPaymentChannel is ReentrancyGuard {
    using ECDSA for bytes32;

    address payable public sender;
    address payable public receiver;

    constructor(address payable _receiver) payable {
        require(_receiver != address(0), "receiver = zero address");
        sender = msg.sender;
        receiver = _receiver;
    }

    function _getHash(uint _amount) private view returns (bytes32) {
        return keccak256(abi.encodePacked(address(this), _amount));
    }

    function getHash(uint _amount) external view returns (bytes32) {
        return _getHash(_amount);
    }

    function _getEthSignedHash(uint _amount) private view returns (bytes32) {
        return _getHash(_amount).toEthSignedMessageHash();
    }

    function getEthSignedHash(uint _amount) external view returns (bytes32) {
        return _getEthSignedHash(_amount);
    }

    function _verify(uint _amount, bytes memory _sig) private view returns (bool) {
        return _getEthSignedHash(_amount).recover(_sig) == sender;
    }

    function verify(uint _amount, bytes memory _sig) external view returns (bool) {
        return _verify(_amount, _sig);
    }

    function send(uint _amount, bytes memory _sig) external nonReentrant {
        //  verifing signature and sender != receiver
        require(msg.sender != receiver, "sender must be different than receiver");
        require(_verify(_amount, _sig), "invalid signature");
        
        (bool sent, ) = receiver.call{value: _amount}("");
        require(sent, "Failed to send Ether");
        selfdestruct(sender);
    }
}