/home/ntakagi/work/tcl8.5.1/libtommath/bn_mp_mod_2d.c Source File

00001 #include <tommath.h>
00002 #ifdef BN_MP_MOD_2D_C
00003 /* LibTomMath, multiple-precision integer library -- Tom St Denis
00004  *
00005  * LibTomMath is a library that provides multiple-precision
00006  * integer arithmetic as well as number theoretic functionality.
00007  *
00008  * The library was designed directly after the MPI library by
00009  * Michael Fromberger but has been written from scratch with
00010  * additional optimizations in place.
00011  *
00012  * The library is free for all purposes without any express
00013  * guarantee it works.
00014  *
00015  * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
00016  */
00017 
00018 /* calc a value mod 2**b */
00019 int
00020 mp_mod_2d (mp_int * a, int b, mp_int * c)
00021 {
00022   int     x, res;
00023 
00024   /* if b is <= 0 then zero the int */
00025   if (b <= 0) {
00026     mp_zero (c);
00027     return MP_OKAY;
00028   }
00029 
00030   /* if the modulus is larger than the value than return */
00031   if (b >= (int) (a->used * DIGIT_BIT)) {
00032     res = mp_copy (a, c);
00033     return res;
00034   }
00035 
00036   /* copy */
00037   if ((res = mp_copy (a, c)) != MP_OKAY) {
00038     return res;
00039   }
00040 
00041   /* zero digits above the last digit of the modulus */
00042   for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
00043     c->dp[x] = 0;
00044   }
00045   /* clear the digit that is not completely outside/inside the modulus */
00046   c->dp[b / DIGIT_BIT] &=
00047     (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
00048   mp_clamp (c);
00049   return MP_OKAY;
00050 }
00051 #endif
00052 
00053 /* $Source: /cvsroot/tcl/libtommath/bn_mp_mod_2d.c,v $ */
00054 /* $Revision: 1.1.1.3 $ */
00055 /* $Date: 2006/12/01 00:08:11 $ */