release/src/router/dropbear/libtommath/bn_mp_mod_2d.c

   1 #include <tommath.h>
   2 #ifdef BN_MP_MOD_2D_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
   4  *
   5  * LibTomMath is a library that provides multiple-precision
   6  * integer arithmetic as well as number theoretic functionality.
   7  *
   8  * The library was designed directly after the MPI library by
   9  * Michael Fromberger but has been written from scratch with
  10  * additional optimizations in place.
  11  *
  12  * The library is free for all purposes without any express
  13  * guarantee it works.
  14  *
  15  * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
  16  */
  17
  18 /* calc a value mod 2**b */
  19 int
  20 mp_mod_2d (mp_int * a, int b, mp_int * c)
  21 {
  22   int     x, res;
  23
  24   /* if b is <= 0 then zero the int */
  25   if (b <= 0) {
  26     mp_zero (c);
  27     return MP_OKAY;
  28   }
  29
  30   /* if the modulus is larger than the value than return */
  31   if (b >= (int) (a->used * DIGIT_BIT)) {
  32     res = mp_copy (a, c);
  33     return res;
  34   }
  35
  36   /* copy */
  37   if ((res = mp_copy (a, c)) != MP_OKAY) {
  38     return res;
  39   }
  40
  41   /* zero digits above the last digit of the modulus */
  42   for (x = (b / DIGIT_BIT) + ((b % DIGIT_BIT) == 0 ? 0 : 1); x < c->used; x++) {
  43     c->dp[x] = 0;
  44   }
  45   /* clear the digit that is not completely outside/inside the modulus */
  46   c->dp[b / DIGIT_BIT] &=
  47     (mp_digit) ((((mp_digit) 1) << (((mp_digit) b) % DIGIT_BIT)) - ((mp_digit) 1));
  48   mp_clamp (c);
  49   return MP_OKAY;
  50 }
  51 #endif
  52
  53 /* $Source: /cvs/libtom/libtommath/bn_mp_mod_2d.c,v $ */
  54 /* $Revision: 1.3 $ */
  55 /* $Date: 2006/03/31 14:18:44 $ */