"""

    Encodes a Merkle Trie node into its RLP form. The RLP will then be

    serialized into a `Bytes` and hashed unless it is less that 32 bytes

    when serialized.

    This function also accepts `None`, representing the absence of a node,

    which is encoded to `b""`.

    Parameters

    ----------

    node : Optional[InternalNode]

        The node to encode.

    Returns

    -------

    encoded : `rlp.RLP`

        The node encoded as RLP.

    """

    unencoded: rlp.Extended

    if node is None:

        unencoded = b""

    elif isinstance(node, LeafNode):

        unencoded = (

            nibble_list_to_compact(node.rest_of_key, True),

            node.value,

        )

    elif isinstance(node, ExtensionNode):

        unencoded = (

            nibble_list_to_compact(node.key_segment, False),

            node.subnode,

        )

    elif isinstance(node, BranchNode):

        unencoded = node.subnodes + [node.value]

    else:

        raise AssertionError(f"Invalid internal node type {type(node)}!")

    encoded = rlp.encode(unencoded)

    if len(encoded) < 32:

        return unencoded

    else:

        return keccak256(encoded)

    """

    Encode a Node for storage in the Merkle Trie.

    Currently mostly an unimplemented stub.

    """

    if isinstance(node, Account):

        assert storage_root is not None

        return encode_account(node, storage_root)

    elif isinstance(node, (LegacyTransaction, Receipt, U256)):

        return rlp.encode(node)

    elif isinstance(node, Bytes):

        return node

    else:

        return previous_trie.encode_node(node, storage_root)

    """

    Create a copy of `trie`. Since only frozen objects may be stored in tries,

    the contents are reused.

    Parameters

    ----------

    trie: `Trie`

        Trie to copy.

    Returns

    -------

    new_trie : `Trie[K, V]`

        A copy of the trie.

    """

    return Trie(trie.secured, trie.default, copy.copy(trie._data))

    """

    Stores an item in a Merkle Trie.

    This method deletes the key if `value == trie.default`, because the Merkle

    Trie represents the default value by omitting it from the trie.

    Parameters

    ----------

    trie: `Trie`

        Trie to store in.

    key : `Bytes`

        Key to lookup.

    value : `V`

        Node to insert at `key`.

    """

    if value == trie.default:

        if key in trie._data:

            del trie._data[key]

    else:

        trie._data[key] = value

    """

    Gets an item from the Merkle Trie.

    This method returns `trie.default` if the key is missing.

    Parameters

    ----------

    trie:

        Trie to lookup in.

    key :

        Key to lookup.

    Returns

    -------

    node : `V`

        Node at `key` in the trie.

    """

    return trie._data.get(key, trie.default)

    """

    Find the longest common prefix of two sequences.

    """

    for i in range(len(a)):

        if i >= len(b) or a[i] != b[i]:

            return i

    return len(a)

    """

    Compresses nibble-list into a standard byte array with a flag.

    A nibble-list is a list of byte values no greater than `15`. The flag is

    encoded in high nibble of the highest byte. The flag nibble can be broken

    down into two two-bit flags.

    Highest nibble::

        +---+---+----------+--------+

        | _ | _ | is_leaf | parity |

        +---+---+----------+--------+

          3   2      1         0

    The lowest bit of the nibble encodes the parity of the length of the

    remaining nibbles -- `0` when even and `1` when odd. The second lowest bit

    is used to distinguish leaf and extension nodes. The other two bits are not

    used.

    Parameters

    ----------

    x :

        Array of nibbles.

    is_leaf :

        True if this is part of a leaf node, or false if it is an extension

        node.

    Returns

    -------

    compressed : `bytearray`

        Compact byte array.

    """

    compact = bytearray()

    if len(x) % 2 == 0:  # ie even length

        compact.append(16 * (2 * is_leaf))

        for i in range(0, len(x), 2):

            compact.append(16 * x[i] + x[i + 1])

    else:

        compact.append(16 * ((2 * is_leaf) + 1) + x[0])

        for i in range(1, len(x), 2):

            compact.append(16 * x[i] + x[i + 1])

    return Bytes(compact)

    """

    Converts a `Bytes` into to a sequence of nibbles (bytes with value < 16).

    Parameters

    ----------

    bytes_:

        The `Bytes` to convert.

    Returns

    -------

    nibble_list : `Bytes`

        The `Bytes` in nibble-list format.

    """

    nibble_list = bytearray(2 * len(bytes_))

    for byte_index, byte in enumerate(bytes_):

        nibble_list[byte_index * 2] = (byte & 0xF0) >> 4

        nibble_list[byte_index * 2 + 1] = byte & 0x0F

    return Bytes(nibble_list)

    """

    Prepares the trie for root calculation. Removes values that are empty,

    hashes the keys (if `secured == True`) and encodes all the nodes.

    Parameters

    ----------

    trie :

        The `Trie` to prepare.

    get_storage_root :

        Function to get the storage root of an account. Needed to encode

        `Account` objects.

    Returns

    -------

    out : `Mapping[ethereum.base_types.Bytes, Node]`

        Object with keys mapped to nibble-byte form.

    """

    mapped: MutableMapping[Bytes, Bytes] = {}

    for preimage, value in trie._data.items():

        if isinstance(value, Account):

            assert get_storage_root is not None

            address = Address(preimage)

            encoded_value = encode_node(value, get_storage_root(address))

        else:

            encoded_value = encode_node(value)

        if encoded_value == b"":

            raise AssertionError

        key: Bytes

        if trie.secured:

            # "secure" tries hash keys once before construction

            key = keccak256(preimage)

        else:

            key = preimage

        mapped[bytes_to_nibble_list(key)] = encoded_value

    return mapped

    """

    Computes the root of a modified merkle patricia trie (MPT).

    Parameters

    ----------

    trie :

        `Trie` to get the root of.

    get_storage_root :

        Function to get the storage root of an account. Needed to encode

        `Account` objects.

    Returns

    -------

    root : `.fork_types.Root`

        MPT root of the underlying key-value pairs.

    """

    obj = _prepare_trie(trie, get_storage_root)

    root_node = encode_internal_node(patricialize(obj, Uint(0)))

    if len(rlp.encode(root_node)) < 32:

        return keccak256(rlp.encode(root_node))

    else:

        assert isinstance(root_node, Bytes)

        return Root(root_node)

    """

    Structural composition function.

    Used to recursively patricialize and merkleize a dictionary. Includes

    memoization of the tree structure and hashes.

    Parameters

    ----------

    obj :

        Underlying trie key-value pairs, with keys in nibble-list format.

    level :

        Current trie level.

    Returns

    -------

    node : `ethereum.base_types.Bytes`

        Root node of `obj`.

    """

    if len(obj) == 0:

        return None

    arbitrary_key = next(iter(obj))

    # if leaf node

    if len(obj) == 1:

        leaf = LeafNode(arbitrary_key[level:], obj[arbitrary_key])

        return leaf

    # prepare for extension node check by finding max j such that all keys in

    # obj have the same key[i:j]

    substring = arbitrary_key[level:]

    prefix_length = len(substring)

    for key in obj:

        prefix_length = min(

            prefix_length, common_prefix_length(substring, key[level:])

        )

        # finished searching, found another key at the current level

        if prefix_length == 0:

            break

    # if extension node

    if prefix_length > 0:

        prefix = arbitrary_key[int(level) : int(level) + prefix_length]

        return ExtensionNode(

            prefix,

            encode_internal_node(

                patricialize(obj, level + Uint(prefix_length))

            ),

        )

    branches: List[MutableMapping[Bytes, Bytes]] = []

    for _ in range(16):

        branches.append({})

    value = b""

    for key in obj:

        if len(key) == level:

            # shouldn't ever have an account or receipt in an internal node

            if isinstance(obj[key], (Account, Receipt, Uint)):

                raise AssertionError

            value = obj[key]

        else:

            branches[key[level]][key] = obj[key]

    return BranchNode(

        [

            encode_internal_node(patricialize(branches[k], level + Uint(1)))

            for k in range(16)

        ],

        value,

    )