mirror of
https://github.com/RGBCube/serenity
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46 lines
1.5 KiB
C++
46 lines
1.5 KiB
C++
/*
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* Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
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* Copyright (c) 2020-2021, Dex♪ <dexes.ttp@gmail.com>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include "UnsignedBigIntegerAlgorithms.h"
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namespace Crypto {
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/**
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* Complexity: O(N^2) where N is the number of words in the larger number
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* Multiplication method:
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* An integer is equal to the sum of the powers of two
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* according to the indices of its 'on' bits.
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* So to multiple x*y, we go over each '1' bit in x (say the i'th bit),
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* and add y<<i to the result.
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*/
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FLATTEN void UnsignedBigIntegerAlgorithms::multiply_without_allocation(
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UnsignedBigInteger const& left,
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UnsignedBigInteger const& right,
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UnsignedBigInteger& temp_shift_result,
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UnsignedBigInteger& temp_shift_plus,
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UnsignedBigInteger& temp_shift,
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UnsignedBigInteger& output)
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{
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output.set_to_0();
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// iterate all bits
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for (size_t word_index = 0; word_index < left.length(); ++word_index) {
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for (size_t bit_index = 0; bit_index < UnsignedBigInteger::BITS_IN_WORD; ++bit_index) {
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// If the bit is off - skip over it
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if (!(left.m_words[word_index] & (1 << bit_index)))
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continue;
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size_t shift_amount = word_index * UnsignedBigInteger::BITS_IN_WORD + bit_index;
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// output += (right << shift_amount);
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shift_left_without_allocation(right, shift_amount, temp_shift_result, temp_shift_plus, temp_shift);
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add_into_accumulator_without_allocation(output, temp_shift);
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}
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}
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}
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}
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