pragma solidity ^0.8.13;

contract TimestampedDigestNotary {
    event Attested(bytes32 indexed digest, bytes32 indexed domain, uint256 blockNumber);

    function attest(bytes32 digest, bytes32 domain) external {
        assembly {
            mstore(0, domain)
            mstore(0x20, digest)
            let key := keccak256(0, 0x40)
            if sload(key) { revert(0, 0) }
            sstore(key, number())
        }
        emit Attested(digest, domain, block.number);
    }

    function exists(bytes32 digest, bytes32 domain) external view returns (uint256 value) {
        assembly {
            mstore(0, domain)
            mstore(0x20, digest)
            value := sload(keccak256(0, 0x40))
        }
    }
}