pragma solidity ^0.8.13;

contract HashChainReceiptAnchor {
    address public immutable agent;
    bytes32 public head;

    event Append(uint256 indexed blockNumber, bytes32 indexed newHead, address indexed agent);

    modifier onlyAgent() {
        require(msg.sender == agent, "not agent");
        _;
    }

    constructor(address _agent) {
        require(_agent != address(0), "zero");
        agent = _agent;
    }

    function append(bytes calldata attestationData) external onlyAgent returns (bytes32 newHead) {
        newHead = keccak256(abi.encodePacked(head, attestationData));
        head = newHead;
        emit Append(block.number, newHead, msg.sender);
    }
}