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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 | } |
61 |
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