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# Copyright 2020 The Pigweed Authors
#
# 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
#
# https://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.
"""Provides functionality for encoding tokenized messages."""
import struct
from typing import Union
_INT32_MAX = 2**31 - 1
_UINT32_MAX = 2**32 - 1
def _zig_zag_encode(value: int) -> int:
"""Encodes signed integers to give a compact varint encoding."""
return value << 1 if value >= 0 else (value << 1) ^ (~0)
def _little_endian_base128_encode(integer: int) -> bytearray:
data = bytearray()
while True:
# Grab 7 bits; the eighth bit is set to 1 to indicate more data coming.
data.append((integer & 0x7f) | 0x80)
integer >>= 7
if not integer:
break
data[-1] &= 0x7f # clear the top bit of the last byte
return data
def _encode_int32(arg: int) -> bytearray:
# Convert large unsigned numbers into their corresponding signed values.
if arg > _INT32_MAX:
arg -= 2**32
return _little_endian_base128_encode(_zig_zag_encode(arg))
def _encode_string(arg: bytes) -> bytes:
size_byte = len(arg) if len(arg) < 128 else 0xff
return struct.pack('B', size_byte) + arg[:127]
def encode_token_and_args(token: int, *args: Union[int, float, bytes,
str]) -> bytes:
"""Encodes a tokenized message given its token and arguments.
This function assumes that the token represents a format string with
conversion specifiers that correspond with the provided argument types.
Currently, only 32-bit integers are supported.
"""
if token < 0 or token > _UINT32_MAX:
raise ValueError(
f'The token ({token}) must be an unsigned 32-bit integer')
data = bytearray(struct.pack('<I', token))
for arg in args:
if isinstance(arg, int):
if arg.bit_length() > 32:
raise ValueError(
f'Cannot encode {arg}: only 32-bit integers may be encoded'
)
data += _encode_int32(arg)
elif isinstance(arg, float):
data += struct.pack('<f', arg)
elif isinstance(arg, str):
data += _encode_string(arg.encode())
elif isinstance(arg, bytes):
data += _encode_string(arg)
else:
raise ValueError(
f'{arg} has type {type(arg)}, which is not supported')
return bytes(data)
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