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

contract ThresholdReceiptWitness {
    event Attested(bytes32 indexed domain, bytes32 indexed receiptHash, address indexed attester, uint256 totalAttestations);

    uint256 public immutable threshold;

    mapping(bytes32 => mapping(address => bool)) private _attested;
    mapping(bytes32 => uint256) private _count;

    constructor(uint256 threshold_) {
        require(threshold_ > 0);
        threshold = threshold_;
    }

    function attest(bytes32 domain, bytes32 receiptHash) external returns (bool) {
        bytes32 id = keccak256(abi.encodePacked(domain, receiptHash));
        if (_attested[id][msg.sender]) {
            return false;
        }
        _attested[id][msg.sender] = true;
        uint256 total = _count[id] + 1;
        _count[id] = total;
        emit Attested(domain, receiptHash, msg.sender, total);
        return true;
    }

    function isValid(bytes32 domain, bytes32 receiptHash) external view returns (bool) {
        bytes32 id = keccak256(abi.encodePacked(domain, receiptHash));
        return _count[id] >= threshold;
    }
}