Contract 0x95AE62E3E2261615970375CC8af8c7E6923627Fa

// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

/**
 * @title BlockhashStore
 * @notice This contract provides a way to access blockhashes older than
 *   the 256 block limit imposed by the BLOCKHASH opcode.
 *   You may assume that any blockhash stored by the contract is correct.
 *   Note that the contract depends on the format of serialized Ethereum
 *   blocks. If a future hardfork of Ethereum changes that format, the
 *   logic in this contract may become incorrect and an updated version
 *   would have to be deployed.
 */
contract BlockhashStore {
    mapping(uint256 => bytes32) internal _sBlockhashes;

    /**
     * @notice stores blockhash of the earliest block still available through BLOCKHASH.
     */
    function storeEarliest() external {
        store(block.number - 256);
    }

    /**
     * @notice gets a blockhash from the store. If no hash is known, this function reverts.
     * @param n the number of the block whose blockhash should be returned
     */
    function getBlockhash(uint256 n) external view returns (bytes32) {
        bytes32 h = _sBlockhashes[n];
        require(h != 0x0, "blockhash not found in store");
        return h;
    }

    /**
     * @notice stores blockhash of a given block, assuming it is available through BLOCKHASH
     * @param n the number of the block whose blockhash should be stored
     */
    function store(uint256 n) public {
        bytes32 h = blockhash(n);
        require(h != 0x0, "blockhash(n) failed");
        _sBlockhashes[n] = h;
    }

    /**
     * @notice stores blockhash after verifying blockheader of child/subsequent block
     * @param n the number of the block whose blockhash should be stored
     * @param header the rlp-encoded blockheader of block n+1. We verify its correctness by checking
     *   that it hashes to a stored blockhash, and then extract parentHash to get the n-th blockhash.
     */
    function storeVerifyHeader(uint256 n, bytes memory header) public {
        require(
            keccak256(header) == _sBlockhashes[n + 1],
            "header has unknown blockhash"
        );

        // At this point, we know that header is the correct blockheader for block n+1.

        // The header is an rlp-encoded list. The head item of that list is the 32-byte blockhash of the parent block.
        // Based on how rlp works, we know that blockheaders always have the following form:
        // 0xf9____a0PARENTHASH...
        //   ^ ^   ^
        //   | |   |
        //   | |   +--- PARENTHASH is 32 bytes. rlpenc(PARENTHASH) is 0xa || PARENTHASH.
        //   | |
        //   | +--- 2 bytes containing the sum of the lengths of the encoded list items
        //   |
        //   +--- 0xf9 because we have a list and (sum of lengths of encoded list items) fits exactly into two bytes.
        //
        // As a consequence, the PARENTHASH is always at offset 4 of the rlp-encoded block header.

        bytes32 parentHash;
        assembly {
            parentHash := mload(add(header, 36)) // 36 = 32 byte offset for length prefix of ABI-encoded array
            //    +  4 byte offset of PARENTHASH (see above)
        }

        _sBlockhashes[n] = parentHash;
    }
}

{
  "remappings": [],
  "optimizer": {
    "enabled": true,
    "runs": 200
  },
  "evmVersion": "istanbul",
  "libraries": {},
  "outputSelection": {
    "*": {
      "*": [
        "evm.bytecode",
        "evm.deployedBytecode",
        "abi"
      ]
    }
  }
}

[{"inputs":[{"internalType":"uint256","name":"n","type":"uint256"}],"name":"getBlockhash","outputs":[{"internalType":"bytes32","name":"","type":"bytes32"}],"stateMutability":"view","type":"function"},{"inputs":[{"internalType":"uint256","name":"n","type":"uint256"}],"name":"store","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[],"name":"storeEarliest","outputs":[],"stateMutability":"nonpayable","type":"function"},{"inputs":[{"internalType":"uint256","name":"n","type":"uint256"},{"internalType":"bytes","name":"header","type":"bytes"}],"name":"storeVerifyHeader","outputs":[],"stateMutability":"nonpayable","type":"function"}]

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