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cdec9cb5 DK |
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 | } |