1/* mpihelp-add_2.c - MPI helper functions 2 * Copyright (C) 1994, 1996, 1997, 1998, 2001, 3 * 2002 Free Software Foundation, Inc. 4 * 5 * This file is part of Libgcrypt. 6 * 7 * Libgcrypt is free software; you can redistribute it and/or modify 8 * it under the terms of the GNU Lesser General Public License as 9 * published by the Free Software Foundation; either version 2.1 of 10 * the License, or (at your option) any later version. 11 * 12 * Libgcrypt is distributed in the hope that it will be useful, 13 * but WITHOUT ANY WARRANTY; without even the implied warranty of 14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 15 * GNU Lesser General Public License for more details. 16 * 17 * You should have received a copy of the GNU Lesser General Public 18 * License along with this program; if not, write to the Free Software 19 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA 20 * 21 * Note: This code is heavily based on the GNU MP Library. 22 * Actually it's the same code with only minor changes in the 23 * way the data is stored; this is to support the abstraction 24 * of an optional secure memory allocation which may be used 25 * to avoid revealing of sensitive data due to paging etc. 26 */ 27 28#include <config.h> 29#include <stdio.h> 30#include <stdlib.h> 31#include "mpi-internal.h" 32#include "longlong.h" 33 34mpi_limb_t 35_gcry_mpih_sub_n( mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr, 36 mpi_ptr_t s2_ptr, mpi_size_t size) 37{ 38 mpi_limb_t x, y, cy; 39 mpi_size_t j; 40 41 /* The loop counter and index J goes from -SIZE to -1. This way 42 the loop becomes faster. */ 43 j = -size; 44 45 /* Offset the base pointers to compensate for the negative indices. */ 46 s1_ptr -= j; 47 s2_ptr -= j; 48 res_ptr -= j; 49 50 cy = 0; 51 do 52 { 53 y = s2_ptr[j]; 54 x = s1_ptr[j]; 55 y += cy; /* add previous carry to subtrahend */ 56 cy = y < cy; /* get out carry from that addition */ 57 y = x - y; /* main subtract */ 58 cy += y > x; /* get out carry from the subtract, combine */ 59 res_ptr[j] = y; 60 } 61 while( ++j ); 62 63 return cy; 64} 65 66 67