contract DepositContract is IDepositContract, ERC165 {
    uint constant GWEI = 1e9;

    uint constant DEPOSIT_CONTRACT_TREE_DEPTH = 32;
    // NOTE: this also ensures `deposit_count` will fit into 64-bits
    uint constant MAX_DEPOSIT_COUNT = 2**DEPOSIT_CONTRACT_TREE_DEPTH - 1;

    bytes32[DEPOSIT_CONTRACT_TREE_DEPTH] branch;
    uint256 deposit_count;

    bytes32[DEPOSIT_CONTRACT_TREE_DEPTH] zero_hashes;

    constructor() public {
        // Compute hashes in empty sparse Merkle tree
        for (uint height = 0; height < DEPOSIT_CONTRACT_TREE_DEPTH - 1; height++)
            zero_hashes[height + 1] = sha256(abi.encodePacked(zero_hashes[height], zero_hashes[height]));
    }

    /// @notice Query the current deposit root hash.
    /// @return The deposit root hash.
    function get_deposit_root() override external view returns (bytes32) {
        bytes32 node;
        uint size = deposit_count;
        for (uint height = 0; height < DEPOSIT_CONTRACT_TREE_DEPTH; height++) {
            if ((size & 1) == 1)
                node = sha256(abi.encodePacked(branch[height], node));
            else
                node = sha256(abi.encodePacked(node, zero_hashes[height]));
            size /= 2;
        }
        return sha256(abi.encodePacked(
                node,
                to_little_endian_64(uint64(deposit_count)),
                bytes24(0)
            ));
    }

    /// @notice Query the current deposit count.
    /// @return The deposit count encoded as a little endian 64-bit number.
    function get_deposit_count() override external view returns (bytes memory) {
        return to_little_endian_64(uint64(deposit_count));
    }

    /// @notice Submit a Phase 0 DepositData object.
    /// @param pubkey A BLS12-381 public key.
    /// @param withdrawal_credentials Commitment to a public key for withdrawals.
    /// @param signature A BLS12-381 signature.
    /// @param deposit_data_root The SHA-256 hash of the SSZ-encoded DepositData object.
    /// Used as a protection against malformed input.
    function deposit(
        bytes calldata pubkey,
        bytes calldata withdrawal_credentials,
        bytes calldata signature,
        bytes32 deposit_data_root
    ) override external payable {
        // Extended ABI length checks since dynamic types are used.
        require(pubkey.length == 48, "DepositContract: invalid pubkey length");
        require(withdrawal_credentials.length == 32, "DepositContract: invalid withdrawal_credentials length");
        require(signature.length == 96, "DepositContract: invalid signature length");

        // Check deposit amount
        require(msg.value >= 1 ether, "DepositContract: deposit value too low");
        require(msg.value % GWEI == 0, "DepositContract: deposit value not multiple of gwei");
        uint deposit_amount = msg.value / GWEI;
        require(deposit_amount <= 2**64, "DepositContract: deposit value too high");

        // Emit `DepositEvent` log
        bytes memory amount = to_little_endian_64(uint64(deposit_amount));
        emit DepositEvent(
            pubkey,
            withdrawal_credentials,
            amount,
            signature,
            to_little_endian_64(uint64(deposit_count))
        );


        // Compute deposit data root (`DepositData` hash tree root)
        bytes32 pubkey_root = sha256(abi.encodePacked(pubkey, bytes16(0)));
        bytes32 signature_root = sha256(abi.encodePacked(
            sha256(abi.encodePacked(signature[:64])),
            sha256(abi.encodePacked(signature[64:], bytes32(0)))
        ));
        bytes32 node = sha256(abi.encodePacked(
                sha256(abi.encodePacked(pubkey_root, withdrawal_credentials)),
                sha256(abi.encodePacked(amount, bytes24(0), signature_root))
            ));

        // Verify computed and expected deposit data roots match
        require(node == deposit_data_root, "DepositContract: reconstructed DepositData does not match supplied deposit_data_root");

        // Avoid overflowing the Merkle tree (and prevent edge case in computing `branch`)
        require(deposit_count < MAX_DEPOSIT_COUNT, "DepositContract: merkle tree full");

        // Add deposit data root to Merkle tree (update a single `branch` node)
        deposit_count += 1;
        uint size = deposit_count;
        for (uint height = 0; height < DEPOSIT_CONTRACT_TREE_DEPTH; height++) {
            if ((size & 1) == 1) {
                branch[height] = node;
                return;
            }
            node = sha256(abi.encodePacked(branch[height], node));
            size /= 2;
        }
        // As the loop should always end prematurely with the `return` statement,
        // this code should be unreachable. We assert `false` just to be safe.
        assert(false);
    }

    function supportsInterface(bytes4 interfaceId) override external pure returns (bool) {
        return interfaceId == type(ERC165).interfaceId || interfaceId == type(IDepositContract).interfaceId;
    }

    function to_little_endian_64(uint64 value) internal pure returns (bytes memory ret) {
        ret = new bytes(8);
        bytes8 bytesValue = bytes8(value);
        // Byteswapping during copying to bytes.
        ret[0] = bytesValue[7];
        ret[1] = bytesValue[6];
        ret[2] = bytesValue[5];
        ret[3] = bytesValue[4];
        ret[4] = bytesValue[3];
        ret[5] = bytesValue[2];
        ret[6] = bytesValue[1];
        ret[7] = bytesValue[0];
    }
}
/// @notice This is the Ethereum 2.0 deposit contract interface.
/// For more information see the Phase 0 specification under https://github.com/ethereum/eth2.0-specs
function get_deposit_count() override external view returns (bytes memory) {
        return to_little_endian_64(uint64(deposit_count));
    }
/// @notice Query the current deposit count.
/// @return The deposit count encoded as a little endian 64-bit number.