// SPDX-License-Identifier: GPL-2.0-or-later
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/* mpihelp-add_2.c - MPI helper functions
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* Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.
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*
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* This file is part of GnuPG.
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*
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* Note: This code is heavily based on the GNU MP Library.
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* Actually it's the same code with only minor changes in the
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* way the data is stored; this is to support the abstraction
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* of an optional secure memory allocation which may be used
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* to avoid revealing of sensitive data due to paging etc.
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* The GNU MP Library itself is published under the LGPL;
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* however I decided to publish this code under the plain GPL.
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*/
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#include "mpi-internal.h"
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#include "longlong.h"
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mpi_limb_t
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mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,
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mpi_ptr_t s2_ptr, mpi_size_t size)
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{
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mpi_limb_t x, y, cy;
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mpi_size_t j;
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/* The loop counter and index J goes from -SIZE to -1. This way
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the loop becomes faster. */
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j = -size;
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/* Offset the base pointers to compensate for the negative indices. */
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s1_ptr -= j;
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s2_ptr -= j;
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res_ptr -= j;
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cy = 0;
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do {
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y = s2_ptr[j];
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x = s1_ptr[j];
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y += cy; /* add previous carry to subtrahend */
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cy = y < cy; /* get out carry from that addition */
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y = x - y; /* main subtract */
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cy += y > x; /* get out carry from the subtract, combine */
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res_ptr[j] = y;
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} while (++j);
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return cy;
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}
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