# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.

from __future__ import absolute_import, division, print_function

import six


def _truncate_digest(digest, order_bits):
    digest_len = len(digest)

    if 8 * digest_len > order_bits:
        digest_len = (order_bits + 7) // 8
        digest = digest[:digest_len]

    if 8 * digest_len > order_bits:
        rshift = 8 - (order_bits & 0x7)
        assert rshift > 0 and rshift < 8

        mask = 0xFF >> rshift << rshift

        # Set the bottom rshift bits to 0
        digest = digest[:-1] + six.int2byte(six.indexbytes(digest, -1) & mask)

    return digest
