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LibCrypto: Add support for shift_right more than one word

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- Before: UnsignedBigInteger::shift_right( n ) trigger index
  verification error for n>31. An assumption of
  num_bits<UnsignedBigInteger::BITS_IN_WORD was being made
- After: shift_right( n ) works correctly for n>31.

NOTE: "bonus" change; not necessary for fixing BigFraction::to_double
This commit is contained in:
Manuel Zahariev 2025-01-07 13:37:33 -08:00 • committed by Ali Mohammad Pur
parent add380d6e2
commit 05cfbdd6fb
Notes: github-actions[bot] 2025-03-23 18:34:34 +00:00
2 changed files with 74 additions and 2 deletions
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41
Libraries/LibCrypto/BigInt/Algorithms/BitwiseOperations.cpp
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@ -1,6 +1,7 @@
/*
* Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
* Copyright (c) 2020-2021, Dex♪ <dexes.ttp@gmail.com>
* Copyright (c) 2025, Manuel Zahariev <manuel@duck.com>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -221,13 +222,49 @@ FLATTEN ErrorOr<void> UnsignedBigIntegerAlgorithms::try_shift_left_without_alloc
return {};
}
/**
* Complexity : O(N) where N is the number of words in the number
*/
FLATTEN void UnsignedBigIntegerAlgorithms::shift_right_without_allocation(
UnsignedBigInteger const& number,
size_t num_bits,
UnsignedBigInteger& output)
{
output.m_words.resize_and_keep_capacity(number.length() - (num_bits / UnsignedBigInteger::BITS_IN_WORD));
Ops::shift_right(number.words_span(), num_bits, output.words_span());
size_t const bit_shift = num_bits % UnsignedBigInteger::BITS_IN_WORD;
size_t const bit_shift_complement = UnsignedBigInteger::BITS_IN_WORD - bit_shift;
size_t const zero_based_index_of_highest_set_bit_in_hiword = (number.one_based_index_of_highest_set_bit() - 1) % UnsignedBigInteger::BITS_IN_WORD;
// true if the high word will be zeroed as a result of the shift
bool const hiword_zero = (bit_shift > zero_based_index_of_highest_set_bit_in_hiword);
size_t const word_shift = num_bits / UnsignedBigInteger::BITS_IN_WORD + (hiword_zero ? 1 : 0);
if (word_shift >= number.length()) { // all non-zero digits have been shifted right; result is zero
output.set_to_0();
return;
}
shift_right_by_n_words(number, word_shift, output);
if (bit_shift == 0) // shifting right by an exact number of words)
return;
size_t const output_length = output.length();
size_t number_index = number.length() - 1;
UnsignedBigInteger::Word carry = 0;
if (hiword_zero) {
carry = number.words().at(number_index) << bit_shift_complement;
--number_index;
}
for (size_t i = 0; i < output_length; ++i) {
size_t const output_index = output_length - i - 1; // downto index 0
output.m_words[output_index] = ((number.m_words.at(number_index) >> bit_shift)) | carry;
carry = (number.m_words.at(number_index) << bit_shift_complement);
--number_index;
}
}
void UnsignedBigIntegerAlgorithms::shift_left_by_n_words(