pragma solidity ^0.8.13;

contract ReceiptNullifier {
    mapping(bytes32 => bytes32) private commitments;
    bytes32 private constant SPENT = bytes32(uint256(1));

    event Committed(bytes32 indexed taskId, bytes32 commitment);
    event Revealed(bytes32 indexed taskId, bytes32 outputHash, bytes32 secret);

    function _key(bytes32 taskId) private view returns (bytes32) {
        return keccak256(abi.encodePacked(taskId, block.chainid, address(this)));
    }

    function commit(bytes32 taskId, bytes32 outputHash, bytes32 secret) external {
        bytes32 commitment = keccak256(abi.encodePacked(taskId, outputHash, secret));
        bytes32 key = _key(taskId);
        require(commitment != bytes32(0) && commitment != SPENT && commitments[key] == bytes32(0));
        commitments[key] = commitment;
        emit Committed(taskId, commitment);
    }

    function reveal(bytes32 taskId, bytes32 outputHash, bytes32 secret) external {
        bytes32 commitment = keccak256(abi.encodePacked(taskId, outputHash, secret));
        bytes32 key = _key(taskId);
        require(commitments[key] == commitment);
        commitments[key] = SPENT;
        emit Revealed(taskId, outputHash, secret);
    }
}