MCAP: Python parsing library

MCAP is a modular container format and logging library for pub/sub messages with arbitrary message serialization. It is primarily intended for use in robotics applications, and works well under various workloads, resource constraints, and durability requirements.

Time values (log_time, publish_time, create_time) are represented in nanoseconds since a user-understood epoch (i.e. Unix epoch, robot boot time, etc.)

File extension

mcap

KS implementation details

License: Apache-2.0

This page hosts a formal specification of MCAP using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for Python generated by Kaitai Struct depends on the Python runtime library. You have to install it before you can parse data.

The Python runtime library can be installed from PyPI:

python3 -m pip install kaitaistruct

Code

Parse a local file and get structure in memory:

data = Mcap.from_file("path/to/local/file.mcap")

Or parse structure from a bytes:

from kaitaistruct import KaitaiStream, BytesIO

raw = b"\x00\x01\x02..."
data = Mcap(KaitaiStream(BytesIO(raw)))

After that, one can get various attributes from the structure by invoking getter methods like:

data.header_magic # => get header magic

Python source code to parse MCAP

mcap.py

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild
# type: ignore

import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO
from enum import IntEnum


if getattr(kaitaistruct, 'API_VERSION', (0, 9)) < (0, 11):
    raise Exception("Incompatible Kaitai Struct Python API: 0.11 or later is required, but you have %s" % (kaitaistruct.__version__))

class Mcap(KaitaiStruct):
    """MCAP is a modular container format and logging library for pub/sub messages with
    arbitrary message serialization. It is primarily intended for use in robotics
    applications, and works well under various workloads, resource constraints, and
    durability requirements.
    
    Time values (`log_time`, `publish_time`, `create_time`) are represented in
    nanoseconds since a user-understood epoch (i.e. Unix epoch, robot boot time,
    etc.)
    
    .. seealso::
       Source - https://github.com/foxglove/mcap/tree/c1cc51d/docs/specification#readme
    """

    class Opcode(IntEnum):
        header = 1
        footer = 2
        schema = 3
        channel = 4
        message = 5
        chunk = 6
        message_index = 7
        chunk_index = 8
        attachment = 9
        attachment_index = 10
        statistics = 11
        metadata = 12
        metadata_index = 13
        summary_offset = 14
        data_end = 15
    def __init__(self, _io, _parent=None, _root=None):
        super(Mcap, self).__init__(_io)
        self._parent = _parent
        self._root = _root or self
        self._read()

    def _read(self):
        self.header_magic = Mcap.Magic(self._io, self, self._root)
        self.records = []
        i = 0
        while True:
            _ = Mcap.Record(self._io, self, self._root)
            self.records.append(_)
            if _.op == Mcap.Opcode.footer:
                break
            i += 1
        self.footer_magic = Mcap.Magic(self._io, self, self._root)


    def _fetch_instances(self):
        pass
        self.header_magic._fetch_instances()
        for i in range(len(self.records)):
            pass
            self.records[i]._fetch_instances()

        self.footer_magic._fetch_instances()
        _ = self.footer
        if hasattr(self, '_m_footer'):
            pass
            self._m_footer._fetch_instances()


    class Attachment(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Attachment, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.log_time = self._io.read_u8le()
            self.create_time = self._io.read_u8le()
            self.name = Mcap.PrefixedStr(self._io, self, self._root)
            self.content_type = Mcap.PrefixedStr(self._io, self, self._root)
            self.len_data = self._io.read_u8le()
            self.data = self._io.read_bytes(self.len_data)
            if self.crc32_input_end >= 0:
                pass
                self.invoke_crc32_input_end = self._io.read_bytes(0)

            self.crc32 = self._io.read_u4le()


        def _fetch_instances(self):
            pass
            self.name._fetch_instances()
            self.content_type._fetch_instances()
            if self.crc32_input_end >= 0:
                pass

            _ = self.crc32_input
            if hasattr(self, '_m_crc32_input'):
                pass


        @property
        def crc32_input(self):
            if hasattr(self, '_m_crc32_input'):
                return self._m_crc32_input

            _pos = self._io.pos()
            self._io.seek(0)
            self._m_crc32_input = self._io.read_bytes(self.crc32_input_end)
            self._io.seek(_pos)
            return getattr(self, '_m_crc32_input', None)

        @property
        def crc32_input_end(self):
            if hasattr(self, '_m_crc32_input_end'):
                return self._m_crc32_input_end

            self._m_crc32_input_end = self._io.pos()
            return getattr(self, '_m_crc32_input_end', None)


    class AttachmentIndex(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.AttachmentIndex, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.ofs_attachment = self._io.read_u8le()
            self.len_attachment = self._io.read_u8le()
            self.log_time = self._io.read_u8le()
            self.create_time = self._io.read_u8le()
            self.data_size = self._io.read_u8le()
            self.name = Mcap.PrefixedStr(self._io, self, self._root)
            self.content_type = Mcap.PrefixedStr(self._io, self, self._root)


        def _fetch_instances(self):
            pass
            self.name._fetch_instances()
            self.content_type._fetch_instances()
            _ = self.attachment
            if hasattr(self, '_m_attachment'):
                pass
                self._m_attachment._fetch_instances()


        @property
        def attachment(self):
            if hasattr(self, '_m_attachment'):
                return self._m_attachment

            io = self._root._io
            _pos = io.pos()
            io.seek(self.ofs_attachment)
            self._raw__m_attachment = io.read_bytes(self.len_attachment)
            _io__raw__m_attachment = KaitaiStream(BytesIO(self._raw__m_attachment))
            self._m_attachment = Mcap.Record(_io__raw__m_attachment, self, self._root)
            io.seek(_pos)
            return getattr(self, '_m_attachment', None)


    class Channel(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Channel, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.id = self._io.read_u2le()
            self.schema_id = self._io.read_u2le()
            self.topic = Mcap.PrefixedStr(self._io, self, self._root)
            self.message_encoding = Mcap.PrefixedStr(self._io, self, self._root)
            self.metadata = Mcap.MapStrStr(self._io, self, self._root)


        def _fetch_instances(self):
            pass
            self.topic._fetch_instances()
            self.message_encoding._fetch_instances()
            self.metadata._fetch_instances()


    class Chunk(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Chunk, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.message_start_time = self._io.read_u8le()
            self.message_end_time = self._io.read_u8le()
            self.uncompressed_size = self._io.read_u8le()
            self.uncompressed_crc32 = self._io.read_u4le()
            self.compression = Mcap.PrefixedStr(self._io, self, self._root)
            self.len_records = self._io.read_u8le()
            _on = self.compression.str
            if _on == u"":
                pass
                self._raw_records = self._io.read_bytes(self.len_records)
                _io__raw_records = KaitaiStream(BytesIO(self._raw_records))
                self.records = Mcap.Records(_io__raw_records, self, self._root)
            else:
                pass
                self.records = self._io.read_bytes(self.len_records)


        def _fetch_instances(self):
            pass
            self.compression._fetch_instances()
            _on = self.compression.str
            if _on == u"":
                pass
                self.records._fetch_instances()
            else:
                pass


    class ChunkIndex(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.ChunkIndex, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.message_start_time = self._io.read_u8le()
            self.message_end_time = self._io.read_u8le()
            self.ofs_chunk = self._io.read_u8le()
            self.len_chunk = self._io.read_u8le()
            self.len_message_index_offsets = self._io.read_u4le()
            self._raw_message_index_offsets = self._io.read_bytes(self.len_message_index_offsets)
            _io__raw_message_index_offsets = KaitaiStream(BytesIO(self._raw_message_index_offsets))
            self.message_index_offsets = Mcap.ChunkIndex.MessageIndexOffsets(_io__raw_message_index_offsets, self, self._root)
            self.message_index_length = self._io.read_u8le()
            self.compression = Mcap.PrefixedStr(self._io, self, self._root)
            self.compressed_size = self._io.read_u8le()
            self.uncompressed_size = self._io.read_u8le()


        def _fetch_instances(self):
            pass
            self.message_index_offsets._fetch_instances()
            self.compression._fetch_instances()
            _ = self.chunk
            if hasattr(self, '_m_chunk'):
                pass
                self._m_chunk._fetch_instances()


        class MessageIndexOffset(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                super(Mcap.ChunkIndex.MessageIndexOffset, self).__init__(_io)
                self._parent = _parent
                self._root = _root
                self._read()

            def _read(self):
                self.channel_id = self._io.read_u2le()
                self.offset = self._io.read_u8le()


            def _fetch_instances(self):
                pass


        class MessageIndexOffsets(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                super(Mcap.ChunkIndex.MessageIndexOffsets, self).__init__(_io)
                self._parent = _parent
                self._root = _root
                self._read()

            def _read(self):
                self.entries = []
                i = 0
                while not self._io.is_eof():
                    self.entries.append(Mcap.ChunkIndex.MessageIndexOffset(self._io, self, self._root))
                    i += 1



            def _fetch_instances(self):
                pass
                for i in range(len(self.entries)):
                    pass
                    self.entries[i]._fetch_instances()



        @property
        def chunk(self):
            if hasattr(self, '_m_chunk'):
                return self._m_chunk

            io = self._root._io
            _pos = io.pos()
            io.seek(self.ofs_chunk)
            self._raw__m_chunk = io.read_bytes(self.len_chunk)
            _io__raw__m_chunk = KaitaiStream(BytesIO(self._raw__m_chunk))
            self._m_chunk = Mcap.Record(_io__raw__m_chunk, self, self._root)
            io.seek(_pos)
            return getattr(self, '_m_chunk', None)


    class DataEnd(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.DataEnd, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.data_section_crc32 = self._io.read_u4le()


        def _fetch_instances(self):
            pass


    class Footer(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Footer, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.ofs_summary_section = self._io.read_u8le()
            self.ofs_summary_offset_section = self._io.read_u8le()
            self.summary_crc32 = self._io.read_u4le()


        def _fetch_instances(self):
            pass
            _ = self.summary_crc32_input
            if hasattr(self, '_m_summary_crc32_input'):
                pass

            _ = self.summary_offset_section
            if hasattr(self, '_m_summary_offset_section'):
                pass
                self._m_summary_offset_section._fetch_instances()

            _ = self.summary_section
            if hasattr(self, '_m_summary_section'):
                pass
                self._m_summary_section._fetch_instances()


        @property
        def ofs_summary_crc32_input(self):
            if hasattr(self, '_m_ofs_summary_crc32_input'):
                return self._m_ofs_summary_crc32_input

            self._m_ofs_summary_crc32_input = (self.ofs_summary_section if self.ofs_summary_section != 0 else self._root.ofs_footer)
            return getattr(self, '_m_ofs_summary_crc32_input', None)

        @property
        def summary_crc32_input(self):
            if hasattr(self, '_m_summary_crc32_input'):
                return self._m_summary_crc32_input

            io = self._root._io
            _pos = io.pos()
            io.seek(self.ofs_summary_crc32_input)
            self._m_summary_crc32_input = io.read_bytes(((self._root._io.size() - self.ofs_summary_crc32_input) - 8) - 4)
            io.seek(_pos)
            return getattr(self, '_m_summary_crc32_input', None)

        @property
        def summary_offset_section(self):
            if hasattr(self, '_m_summary_offset_section'):
                return self._m_summary_offset_section

            if self.ofs_summary_offset_section != 0:
                pass
                io = self._root._io
                _pos = io.pos()
                io.seek(self.ofs_summary_offset_section)
                self._raw__m_summary_offset_section = io.read_bytes(self._root.ofs_footer - self.ofs_summary_offset_section)
                _io__raw__m_summary_offset_section = KaitaiStream(BytesIO(self._raw__m_summary_offset_section))
                self._m_summary_offset_section = Mcap.Records(_io__raw__m_summary_offset_section, self, self._root)
                io.seek(_pos)

            return getattr(self, '_m_summary_offset_section', None)

        @property
        def summary_section(self):
            if hasattr(self, '_m_summary_section'):
                return self._m_summary_section

            if self.ofs_summary_section != 0:
                pass
                io = self._root._io
                _pos = io.pos()
                io.seek(self.ofs_summary_section)
                self._raw__m_summary_section = io.read_bytes((self.ofs_summary_offset_section if self.ofs_summary_offset_section != 0 else self._root.ofs_footer) - self.ofs_summary_section)
                _io__raw__m_summary_section = KaitaiStream(BytesIO(self._raw__m_summary_section))
                self._m_summary_section = Mcap.Records(_io__raw__m_summary_section, self, self._root)
                io.seek(_pos)

            return getattr(self, '_m_summary_section', None)


    class Header(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Header, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.profile = Mcap.PrefixedStr(self._io, self, self._root)
            self.library = Mcap.PrefixedStr(self._io, self, self._root)


        def _fetch_instances(self):
            pass
            self.profile._fetch_instances()
            self.library._fetch_instances()


    class Magic(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Magic, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.magic = self._io.read_bytes(8)
            if not self.magic == b"\x89\x4D\x43\x41\x50\x30\x0D\x0A":
                raise kaitaistruct.ValidationNotEqualError(b"\x89\x4D\x43\x41\x50\x30\x0D\x0A", self.magic, self._io, u"/types/magic/seq/0")


        def _fetch_instances(self):
            pass


    class MapStrStr(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.MapStrStr, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.len_entries = self._io.read_u4le()
            self._raw_entries = self._io.read_bytes(self.len_entries)
            _io__raw_entries = KaitaiStream(BytesIO(self._raw_entries))
            self.entries = Mcap.MapStrStr.Entries(_io__raw_entries, self, self._root)


        def _fetch_instances(self):
            pass
            self.entries._fetch_instances()

        class Entries(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                super(Mcap.MapStrStr.Entries, self).__init__(_io)
                self._parent = _parent
                self._root = _root
                self._read()

            def _read(self):
                self.entries = []
                i = 0
                while not self._io.is_eof():
                    self.entries.append(Mcap.TupleStrStr(self._io, self, self._root))
                    i += 1



            def _fetch_instances(self):
                pass
                for i in range(len(self.entries)):
                    pass
                    self.entries[i]._fetch_instances()




    class Message(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Message, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.channel_id = self._io.read_u2le()
            self.sequence = self._io.read_u4le()
            self.log_time = self._io.read_u8le()
            self.publish_time = self._io.read_u8le()
            self.data = self._io.read_bytes_full()


        def _fetch_instances(self):
            pass


    class MessageIndex(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.MessageIndex, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.channel_id = self._io.read_u2le()
            self.len_records = self._io.read_u4le()
            self._raw_records = self._io.read_bytes(self.len_records)
            _io__raw_records = KaitaiStream(BytesIO(self._raw_records))
            self.records = Mcap.MessageIndex.MessageIndexEntries(_io__raw_records, self, self._root)


        def _fetch_instances(self):
            pass
            self.records._fetch_instances()

        class MessageIndexEntries(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                super(Mcap.MessageIndex.MessageIndexEntries, self).__init__(_io)
                self._parent = _parent
                self._root = _root
                self._read()

            def _read(self):
                self.entries = []
                i = 0
                while not self._io.is_eof():
                    self.entries.append(Mcap.MessageIndex.MessageIndexEntry(self._io, self, self._root))
                    i += 1



            def _fetch_instances(self):
                pass
                for i in range(len(self.entries)):
                    pass
                    self.entries[i]._fetch_instances()



        class MessageIndexEntry(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                super(Mcap.MessageIndex.MessageIndexEntry, self).__init__(_io)
                self._parent = _parent
                self._root = _root
                self._read()

            def _read(self):
                self.log_time = self._io.read_u8le()
                self.offset = self._io.read_u8le()


            def _fetch_instances(self):
                pass



    class Metadata(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Metadata, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.name = Mcap.PrefixedStr(self._io, self, self._root)
            self.metadata = Mcap.MapStrStr(self._io, self, self._root)


        def _fetch_instances(self):
            pass
            self.name._fetch_instances()
            self.metadata._fetch_instances()


    class MetadataIndex(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.MetadataIndex, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.ofs_metadata = self._io.read_u8le()
            self.len_metadata = self._io.read_u8le()
            self.name = Mcap.PrefixedStr(self._io, self, self._root)


        def _fetch_instances(self):
            pass
            self.name._fetch_instances()
            _ = self.metadata
            if hasattr(self, '_m_metadata'):
                pass
                self._m_metadata._fetch_instances()


        @property
        def metadata(self):
            if hasattr(self, '_m_metadata'):
                return self._m_metadata

            io = self._root._io
            _pos = io.pos()
            io.seek(self.ofs_metadata)
            self._raw__m_metadata = io.read_bytes(self.len_metadata)
            _io__raw__m_metadata = KaitaiStream(BytesIO(self._raw__m_metadata))
            self._m_metadata = Mcap.Record(_io__raw__m_metadata, self, self._root)
            io.seek(_pos)
            return getattr(self, '_m_metadata', None)


    class PrefixedStr(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.PrefixedStr, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.len_str = self._io.read_u4le()
            self.str = (self._io.read_bytes(self.len_str)).decode(u"UTF-8")


        def _fetch_instances(self):
            pass


    class Record(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Record, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.op = KaitaiStream.resolve_enum(Mcap.Opcode, self._io.read_u1())
            self.len_body = self._io.read_u8le()
            _on = self.op
            if _on == Mcap.Opcode.attachment:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Attachment(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.attachment_index:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.AttachmentIndex(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.channel:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Channel(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.chunk:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Chunk(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.chunk_index:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.ChunkIndex(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.data_end:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.DataEnd(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.footer:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Footer(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.header:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Header(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.message:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Message(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.message_index:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.MessageIndex(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.metadata:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Metadata(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.metadata_index:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.MetadataIndex(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.schema:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Schema(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.statistics:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.Statistics(_io__raw_body, self, self._root)
            elif _on == Mcap.Opcode.summary_offset:
                pass
                self._raw_body = self._io.read_bytes(self.len_body)
                _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
                self.body = Mcap.SummaryOffset(_io__raw_body, self, self._root)
            else:
                pass
                self.body = self._io.read_bytes(self.len_body)


        def _fetch_instances(self):
            pass
            _on = self.op
            if _on == Mcap.Opcode.attachment:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.attachment_index:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.channel:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.chunk:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.chunk_index:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.data_end:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.footer:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.header:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.message:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.message_index:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.metadata:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.metadata_index:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.schema:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.statistics:
                pass
                self.body._fetch_instances()
            elif _on == Mcap.Opcode.summary_offset:
                pass
                self.body._fetch_instances()
            else:
                pass


    class Records(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Records, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.records = []
            i = 0
            while not self._io.is_eof():
                self.records.append(Mcap.Record(self._io, self, self._root))
                i += 1



        def _fetch_instances(self):
            pass
            for i in range(len(self.records)):
                pass
                self.records[i]._fetch_instances()



    class Schema(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Schema, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.id = self._io.read_u2le()
            self.name = Mcap.PrefixedStr(self._io, self, self._root)
            self.encoding = Mcap.PrefixedStr(self._io, self, self._root)
            self.len_data = self._io.read_u4le()
            self.data = self._io.read_bytes(self.len_data)


        def _fetch_instances(self):
            pass
            self.name._fetch_instances()
            self.encoding._fetch_instances()


    class Statistics(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.Statistics, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.message_count = self._io.read_u8le()
            self.schema_count = self._io.read_u2le()
            self.channel_count = self._io.read_u4le()
            self.attachment_count = self._io.read_u4le()
            self.metadata_count = self._io.read_u4le()
            self.chunk_count = self._io.read_u4le()
            self.message_start_time = self._io.read_u8le()
            self.message_end_time = self._io.read_u8le()
            self.len_channel_message_counts = self._io.read_u4le()
            self._raw_channel_message_counts = self._io.read_bytes(self.len_channel_message_counts)
            _io__raw_channel_message_counts = KaitaiStream(BytesIO(self._raw_channel_message_counts))
            self.channel_message_counts = Mcap.Statistics.ChannelMessageCounts(_io__raw_channel_message_counts, self, self._root)


        def _fetch_instances(self):
            pass
            self.channel_message_counts._fetch_instances()

        class ChannelMessageCount(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                super(Mcap.Statistics.ChannelMessageCount, self).__init__(_io)
                self._parent = _parent
                self._root = _root
                self._read()

            def _read(self):
                self.channel_id = self._io.read_u2le()
                self.message_count = self._io.read_u8le()


            def _fetch_instances(self):
                pass


        class ChannelMessageCounts(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                super(Mcap.Statistics.ChannelMessageCounts, self).__init__(_io)
                self._parent = _parent
                self._root = _root
                self._read()

            def _read(self):
                self.entries = []
                i = 0
                while not self._io.is_eof():
                    self.entries.append(Mcap.Statistics.ChannelMessageCount(self._io, self, self._root))
                    i += 1



            def _fetch_instances(self):
                pass
                for i in range(len(self.entries)):
                    pass
                    self.entries[i]._fetch_instances()




    class SummaryOffset(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.SummaryOffset, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.group_opcode = KaitaiStream.resolve_enum(Mcap.Opcode, self._io.read_u1())
            self.ofs_group = self._io.read_u8le()
            self.len_group = self._io.read_u8le()


        def _fetch_instances(self):
            pass
            _ = self.group
            if hasattr(self, '_m_group'):
                pass
                self._m_group._fetch_instances()


        @property
        def group(self):
            if hasattr(self, '_m_group'):
                return self._m_group

            io = self._root._io
            _pos = io.pos()
            io.seek(self.ofs_group)
            self._raw__m_group = io.read_bytes(self.len_group)
            _io__raw__m_group = KaitaiStream(BytesIO(self._raw__m_group))
            self._m_group = Mcap.Records(_io__raw__m_group, self, self._root)
            io.seek(_pos)
            return getattr(self, '_m_group', None)


    class TupleStrStr(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            super(Mcap.TupleStrStr, self).__init__(_io)
            self._parent = _parent
            self._root = _root
            self._read()

        def _read(self):
            self.key = Mcap.PrefixedStr(self._io, self, self._root)
            self.value = Mcap.PrefixedStr(self._io, self, self._root)


        def _fetch_instances(self):
            pass
            self.key._fetch_instances()
            self.value._fetch_instances()


    @property
    def footer(self):
        if hasattr(self, '_m_footer'):
            return self._m_footer

        _pos = self._io.pos()
        self._io.seek(self.ofs_footer)
        self._raw__m_footer = self._io.read_bytes_full()
        _io__raw__m_footer = KaitaiStream(BytesIO(self._raw__m_footer))
        self._m_footer = Mcap.Record(_io__raw__m_footer, self, self._root)
        self._io.seek(_pos)
        return getattr(self, '_m_footer', None)

    @property
    def ofs_footer(self):
        if hasattr(self, '_m_ofs_footer'):
            return self._m_ofs_footer

        self._m_ofs_footer = (((self._io.size() - 1) - 8) - 20) - 8
        return getattr(self, '_m_ofs_footer', None)