// SPDX-License-Identifier: MIT OR Apache-2.0

pragma solidity ^0.8.0;

contract ProofOfWorkFreelance {
    /// @dev Structure describing order for finding the PoW hash
    /// @param data Set of bytes to find the PoW salt for
    /// @param targetHash Number that the hashed data must be equal to or less than
    /// @param reward Amount of ether that will be sent to the address that find the PoW salt
    /// @param deadline The time until which the order will be active
    struct Order {
        bytes data;
        bytes32 targetHash;
        uint256 reward;
        uint256 deadline;
    }

    /// @notice Mapping from the order id to the Order structure
    mapping (uint256 => Order) orders;

    /// @notice Serial number of the last added order to the `orders` mapping
    uint256 currentOrder;

    /// @notice The address with permission to create orders
    address owner;
    
    constructor() {
        owner = msg.sender;
    }

    /// @notice Create a new order for PoW hash finding
    function makeOrder(Order memory _order) external payable {
        require(msg.sender == owner);
        require(_order.reward == msg.value);
        require(_order.deadline >= block.timestamp);

        orders[currentOrder] = _order;
        currentOrder++;
    }

    /// @notice Cancel the order if it has not been executed before the deadline
    function cancelOrder(uint256 _index) external {
        require(msg.sender == owner);
        
        Order memory order = orders[_index];
        require(order.deadline < block.timestamp, "Order is still active");

        delete orders[_index];

        (bool success, ) = msg.sender.call{value: order.reward}("");
        require(success);
    }

    /// @notice Provide the PoW salt and get an award
    function fulfillOrder(uint256 _index, bytes memory _salt) external {
        Order memory order = orders[_index];
        require(order.deadline >= block.timestamp);

        bytes32 hash = keccak256(abi.encodePacked(order.data, _salt));
        require(hash <= order.targetHash, "PoW hash doesn't reach the target value");

        delete orders[_index];

        (bool success, ) = msg.sender.call{value: order.reward}("");
        require(success);
    }
}