pragma solidity ^0.8.13;

contract BlockhashReceiptAnchor {
    struct Receipt {
        uint64 blockNumber;
        address signer;
        bytes32 parentHash;
    }

    mapping(bytes32 => Receipt) private receipts;

    event Anchored(
        bytes32 indexed receiptId,
        address indexed signer,
        bytes32 payload,
        bytes32 parentHash,
        uint256 blockNumber
    );

    function pin(bytes32 payload) external {
        bytes32 parentHash = blockhash(block.number - 1);
        require(parentHash != bytes32(0));
        bytes32 receiptId = keccak256(abi.encodePacked(block.chainid, msg.sender, payload, parentHash));
        require(receipts[receiptId].parentHash == bytes32(0));
        receipts[receiptId] = Receipt(uint64(block.number), msg.sender, parentHash);
        emit Anchored(receiptId, msg.sender, payload, parentHash, block.number);
    }
}