// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract HashCashSlotAuction {
    mapping(uint256 => address) public ownerOfSlot;
    uint256 public immutable targetDifficulty;

    error SlotTaken();
    error PoWNotMet();

    event SlotAllocated(uint256 indexed slotId, address indexed owner, uint256 nonce);

    constructor(uint256 _targetDifficulty) {
        unchecked {
            require(_targetDifficulty - 1 <= 247);
        }
        targetDifficulty = _targetDifficulty;
    }

    function claim(uint256 slotId, uint256 nonce) external {
        if (ownerOfSlot[slotId] != address(0)) revert SlotTaken();
        if (uint256(keccak256(abi.encodePacked(msg.sender, slotId, nonce))) >= (1 << targetDifficulty)) revert PoWNotMet();
        ownerOfSlot[slotId] = msg.sender;
        emit SlotAllocated(slotId, msg.sender, nonce);
    }
}