MidasNFTMerkleDistributor.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/utils/cryptography/MerkleProof.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "./interfaces/IMerkleDistributor.sol";

contract MerkleDistributor is IMerkleDistributor, IERC721Receiver {
    address public immutable override token;
    bytes32 public immutable override merkleRoot;

    // This is a packed array of booleans.
    mapping(uint256 => uint256) private claimedBitMap;

    constructor(address token_, bytes32 merkleRoot_) public {
        token = token_;
        merkleRoot = merkleRoot_;
    }

    // Implementing `onERC721Received` so this contract can receive the Midas NFT
    // Note that the operator is recorded as the owner of the Midas NFT 
    function onERC721Received(
        address operator, 
        address from, 
        uint tokenId, 
        bytes calldata
    ) 
        external 
        override 
        returns (bytes4) 
    {
        return IERC721Receiver.onERC721Received.selector;
    }

    function isClaimed(uint256 index) public view override returns (bool) {
        uint256 claimedWordIndex = index / 256;
        uint256 claimedBitIndex = index % 256;
        uint256 claimedWord = claimedBitMap[claimedWordIndex];
        uint256 mask = (1 << claimedBitIndex);
        return claimedWord & mask == mask;
    }

    function _setClaimed(uint256 index) private {
        uint256 claimedWordIndex = index / 256;
        uint256 claimedBitIndex = index % 256;
        claimedBitMap[claimedWordIndex] = claimedBitMap[claimedWordIndex] | (1 << claimedBitIndex);
    }

    function _transferERC721FromContract(address account, uint256 tokenId) private returns (bool){
        IERC721(token).transferFrom(address(this), account, tokenId);
        return true;
    }

    // @dev tokenId should be a random number which could be generated by frontend,
    //      or it could be implemented by this contract if necessary.
    function claim(uint256 index, address account, uint256 tokenId, uint256 amount, bytes32[] calldata merkleProof) external override {
        require(!isClaimed(index), 'MerkleDistributor: Drop already claimed.');

        // Verify the merkle proof.
        bytes32 node = keccak256(abi.encodePacked(index, account, amount));
        require(MerkleProof.verify(merkleProof, merkleRoot, node), 'MerkleDistributor: Invalid proof.');

        // Mark it claimed and send the token.
        _setClaimed(index);
        require(_transferERC721FromContract(account, tokenId), 'MerkleDistributor: Transfer failed.');

        emit Claimed(index, account, tokenId);
    }
}