# Copyright 2017 Google Inc. All rights reserved.
#
# Licensed 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.

from . import encode
from . import number_types
from . import packer


def GetSizePrefix(buf, offset):
  """Extract the size prefix from a buffer."""
  return encode.Get(packer.int32, buf, offset)


def GetBufferIdentifier(buf, offset, size_prefixed=False):
  """Extract the file_identifier from a buffer"""
  if size_prefixed:
    # increase offset by size of UOffsetTFlags
    offset += number_types.UOffsetTFlags.bytewidth
  # increase offset by size of root table pointer
  offset += number_types.UOffsetTFlags.bytewidth
  # end of FILE_IDENTIFIER
  end = offset + encode.FILE_IDENTIFIER_LENGTH
  return buf[offset:end]


def BufferHasIdentifier(buf, offset, file_identifier, size_prefixed=False):
  got = GetBufferIdentifier(buf, offset, size_prefixed=size_prefixed)
  return got == file_identifier


def RemoveSizePrefix(buf, offset):
  """Create a slice of a size-prefixed buffer that has

  its position advanced just past the size prefix.
  """
  return buf, offset + number_types.Int32Flags.bytewidth