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

contract GapFreeNonceReceipt {
    struct Receipt {
        uint256 nonce;
        bytes32 receiptHash;
    }

    mapping(address => Receipt) public receipts;

    event ReceiptAccepted(
        address indexed agent,
        uint256 indexed nonce,
        bytes32 dataHash
    );

    function submit(uint256 nonce, bytes32 dataHash) external {
        Receipt storage r = receipts[msg.sender];
        require(nonce == r.nonce + 1, "nonce gap");

        r.nonce = nonce;
        r.receiptHash = keccak256(abi.encodePacked(msg.sender, nonce, dataHash));

        emit ReceiptAccepted(msg.sender, nonce, dataHash);
    }
}