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