| Dmitry Kasatkin | cdec9cb | 2011-08-31 14:05:16 +0300 | [diff] [blame^] | 1 | /* mpihelp-add_2.c - MPI helper functions |
| 2 | * Copyright (C) 1994, 1996, 1997, 1998, 2001 Free Software Foundation, Inc. |
| 3 | * |
| 4 | * This file is part of GnuPG. |
| 5 | * |
| 6 | * GnuPG is free software; you can redistribute it and/or modify |
| 7 | * it under the terms of the GNU General Public License as published by |
| 8 | * the Free Software Foundation; either version 2 of the License, or |
| 9 | * (at your option) any later version. |
| 10 | * |
| 11 | * GnuPG is distributed in the hope that it will be useful, |
| 12 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 13 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 14 | * GNU General Public License for more details. |
| 15 | * |
| 16 | * You should have received a copy of the GNU General Public License |
| 17 | * along with this program; if not, write to the Free Software |
| 18 | * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA |
| 19 | * |
| 20 | * Note: This code is heavily based on the GNU MP Library. |
| 21 | * Actually it's the same code with only minor changes in the |
| 22 | * way the data is stored; this is to support the abstraction |
| 23 | * of an optional secure memory allocation which may be used |
| 24 | * to avoid revealing of sensitive data due to paging etc. |
| 25 | * The GNU MP Library itself is published under the LGPL; |
| 26 | * however I decided to publish this code under the plain GPL. |
| 27 | */ |
| 28 | |
| 29 | #include "mpi-internal.h" |
| 30 | #include "longlong.h" |
| 31 | |
| 32 | mpi_limb_t |
| 33 | mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr, |
| 34 | mpi_ptr_t s2_ptr, mpi_size_t size) |
| 35 | { |
| 36 | mpi_limb_t x, y, cy; |
| 37 | mpi_size_t j; |
| 38 | |
| 39 | /* The loop counter and index J goes from -SIZE to -1. This way |
| 40 | the loop becomes faster. */ |
| 41 | j = -size; |
| 42 | |
| 43 | /* Offset the base pointers to compensate for the negative indices. */ |
| 44 | s1_ptr -= j; |
| 45 | s2_ptr -= j; |
| 46 | res_ptr -= j; |
| 47 | |
| 48 | cy = 0; |
| 49 | do { |
| 50 | y = s2_ptr[j]; |
| 51 | x = s1_ptr[j]; |
| 52 | y += cy; /* add previous carry to subtrahend */ |
| 53 | cy = y < cy; /* get out carry from that addition */ |
| 54 | y = x - y; /* main subtract */ |
| 55 | cy += y > x; /* get out carry from the subtract, combine */ |
| 56 | res_ptr[j] = y; |
| 57 | } while (++j); |
| 58 | |
| 59 | return cy; |
| 60 | } |