transcoder/output/length_delimited/LengthDelimitedOutputManager.py

#
# Copyright 2023 Google LLC
#
# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements.  See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You under the Apache License, Version 2.0
# (the "License"); you may not use this file except in compliance with
# the License.  You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
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# pylint: disable=broad-exception-raised

import struct
import sys
from transcoder.output import OutputManager


class LengthDelimitedOutputManager(OutputManager):
    """ Output manager for length-prefixed binary files sent to standard output """

    def __init__(self, prefix_length: int=2, endian: int='>'):
        super().__init__()
        self.prefix_length = prefix_length
        self.endian = endian

    @staticmethod
    def output_type_identifier():
        return 'length_delimited'

    def pack_spec(self):
        """ Decide what parameter to pass to struct.pack based on prefix length specified in constructor """
        if self.prefix_length == 2:
            return self.endian + 'H'
        if self.prefix_length == 4:
            return self.endian + 'I'
        if self.prefix_length == 8:
            return self.endian + 'Q'

        raise Exception('Valid values for prefix length are 2, 4 or 8')


    def write_record(self, record_type_name, record):
        byte_len = struct.pack(self.pack_spec(), len(record))
        sys.stdout.buffer.write(byte_len + record)