// SPDX-License-Identifier: MIT

pragma experimental ABIEncoderV2;
pragma solidity 0.6.12;


// 
/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

// 
/**
 * @dev These functions deal with verification of Merkle trees (hash trees),
 */
library MerkleProof {
    /**
     * @dev Returns true if a `leaf` can be proved to be a part of a Merkle tree
     * defined by `root`. For this, a `proof` must be provided, containing
     * sibling hashes on the branch from the leaf to the root of the tree. Each
     * pair of leaves and each pair of pre-images are assumed to be sorted.
     */
    function verify(bytes32[] memory proof, bytes32 root, bytes32 leaf) internal pure returns (bool) {
        bytes32 computedHash = leaf;

        for (uint256 i = 0; i < proof.length; i++) {
            bytes32 proofElement = proof[i];

            if (computedHash <= proofElement) {
                // Hash(current computed hash + current element of the proof)
                computedHash = keccak256(abi.encodePacked(computedHash, proofElement));
            } else {
                // Hash(current element of the proof + current computed hash)
                computedHash = keccak256(abi.encodePacked(proofElement, computedHash));
            }
        }

        // Check if the computed hash (root) is equal to the provided root
        return computedHash == root;
    }
}

// 
/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address payable) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

// 
/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
contract Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor () internal {
        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(_owner == _msgSender(), "Ownable: caller is not the owner");
        _;
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions anymore. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby removing any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        emit OwnershipTransferred(_owner, address(0));
        _owner = address(0);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Can only be called by the current owner.
     */
    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

// 
// Allows anyone to claim a token if they exist in a merkle root.
interface IMerkleDistributor {
  // This event is triggered whenever a call to #claim succeeds.
  event Claimed(uint256 index, address account, uint256 amount);

  // Claim the given amount of the token to the given address. Reverts if the inputs are invalid.
  function claim(
    uint256 index,
    address account,
    uint256 amount,
    bytes32[] calldata merkleProof
  ) external;

  // Returns the address of the token distributed by this contract.
  function token() external view returns (address);

  // Returns the merkle root of the merkle tree containing account balances available to claim.
  function merkleRoot() external view returns (bytes32);

  // Returns true if the index has been marked claimed.
  function isClaimed(uint256 index) external view returns (bool);

  // Returns the block timestamp when claims will end
  function endTime() external view returns (uint256);

  // Returns true if the claim period has not ended.
  function isActive() external view returns (bool);
}

// 
contract MerkleDistributor is IMerkleDistributor, Ownable {
  address public immutable override token;
  bytes32 public immutable override merkleRoot;
  uint256 public immutable override endTime;

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

  constructor(
    address _token,
    bytes32 _merkleRoot,
    uint256 _endTime
  ) public {
    token = _token;
    merkleRoot = _merkleRoot;
    require(block.timestamp < _endTime, "Invalid endTime");
    endTime = _endTime;
  }

  /** @dev Modifier to check that claim period is active.*/
  modifier whenActive() {
    require(isActive(), "Claim period has ended");
    _;
  }

  function claim(
    uint256 _index,
    address _account,
    uint256 _amount,
    bytes32[] calldata merkleProof
  ) external override whenActive {
    require(!isClaimed(_index), "Drop already claimed");

    // Verify the merkle proof.
    bytes32 node = keccak256(abi.encodePacked(_index, _account, _amount));
    require(MerkleProof.verify(merkleProof, merkleRoot, node), "Invalid proof");

    // Mark it claimed and send the token.
    _setClaimed(_index);
    require(IERC20(token).transfer(_account, _amount), "Transfer failed");

    emit Claimed(_index, _account, _amount);
  }

  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 isActive() public view override returns (bool) {
    return block.timestamp < endTime;
  }

  function recoverERC20(address _tokenAddress, uint256 _tokenAmount) public onlyOwner {
    IERC20(_tokenAddress).transfer(owner(), _tokenAmount);
  }

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