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

contract TombstoneAttestation {
    mapping(address => bytes32) private attestations;
    mapping(address => bool) private tombstones;

    event Attested(address indexed who, bytes32 hash);
    event Retracted(address indexed who, bytes32 hash);

    function attest(bytes32 hash) external {
        require(hash != bytes32(0), "zero hash");
        require(!tombstones[msg.sender], "tombstoned");
        require(attestations[msg.sender] == bytes32(0), "existing");
        attestations[msg.sender] = hash;
        emit Attested(msg.sender, hash);
    }

    function retract() external {
        bytes32 hash = attestations[msg.sender];
        require(hash != bytes32(0), "no attestation");
        require(!tombstones[msg.sender], "tombstoned");
        tombstones[msg.sender] = true;
        emit Retracted(msg.sender, hash);
    }

    function currentHash(address who) external view returns (bytes32) {
        return attestations[who];
    }

    function isTombstone(address who) external view returns (bool) {
        return tombstones[who];
    }
}