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1// SPDX-License-Identifier: GPL-2.0-or-later2/* mpihelp-add_2.c - MPI helper functions3 * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc.4 *5 * This file is part of GnuPG.6 *7 * Note: This code is heavily based on the GNU MP Library.8 * Actually it's the same code with only minor changes in the9 * way the data is stored; this is to support the abstraction10 * of an optional secure memory allocation which may be used11 * to avoid revealing of sensitive data due to paging etc.12 * The GNU MP Library itself is published under the LGPL;13 * however I decided to publish this code under the plain GPL.14 */15 16#include "mpi-internal.h"17#include "longlong.h"18 19mpi_limb_t20mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr,21 mpi_ptr_t s2_ptr, mpi_size_t size)22{23 mpi_limb_t x, y, cy;24 mpi_size_t j;25 26 /* The loop counter and index J goes from -SIZE to -1. This way27 the loop becomes faster. */28 j = -size;29 30 /* Offset the base pointers to compensate for the negative indices. */31 s1_ptr -= j;32 s2_ptr -= j;33 res_ptr -= j;34 35 cy = 0;36 do {37 y = s2_ptr[j];38 x = s1_ptr[j];39 y += cy; /* add previous carry to subtrahend */40 cy = y < cy; /* get out carry from that addition */41 y = x - y; /* main subtract */42 cy += y > x; /* get out carry from the subtract, combine */43 res_ptr[j] = y;44 } while (++j);45 46 return cy;47}48