rsa/machine_size.py

# -*- coding: utf-8 -*-
#
#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
#
#  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.

"""Detection of 32-bit and 64-bit machines and byte alignment."""

import sys

MAX_INT = sys.maxsize
MAX_INT64 = (1 << 63) - 1
MAX_INT32 = (1 << 31) - 1
MAX_INT16 = (1 << 15) - 1

# Determine the word size of the processor.
if MAX_INT == MAX_INT64:
    # 64-bit processor.
    MACHINE_WORD_SIZE = 64
elif MAX_INT == MAX_INT32:
    # 32-bit processor.
    MACHINE_WORD_SIZE = 32
else:
    # Else we just assume 64-bit processor keeping up with modern times.
    MACHINE_WORD_SIZE = 64


def get_word_alignment(num, force_arch=64,
                       _machine_word_size=MACHINE_WORD_SIZE):
    """
    Returns alignment details for the given number based on the platform
    Python is running on.

    :param num:
        Unsigned integral number.
    :param force_arch:
        If you don't want to use 64-bit unsigned chunks, set this to
        anything other than 64. 32-bit chunks will be preferred then.
        Default 64 will be used when on a 64-bit machine.
    :param _machine_word_size:
        (Internal) The machine word size used for alignment.
    :returns:
        4-tuple::

            (word_bits, word_bytes,
             max_uint, packing_format_type)
    """
    max_uint64 = 0xffffffffffffffff
    max_uint32 = 0xffffffff
    max_uint16 = 0xffff
    max_uint8 = 0xff

    if force_arch == 64 and _machine_word_size >= 64 and num > max_uint32:
        # 64-bit unsigned integer.
        return 64, 8, max_uint64, "Q"
    elif num > max_uint16:
        # 32-bit unsigned integer
        return 32, 4, max_uint32, "L"
    elif num > max_uint8:
        # 16-bit unsigned integer.
        return 16, 2, max_uint16, "H"
    else:
        # 8-bit unsigned integer.
        return 8, 1, max_uint8, "B"