Line data    Source code

       1             : #include <tommath.h>
       2             : #ifdef BN_MP_REDUCE_2K_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://libtom.org
      16             :  */
      17             : 
      18             : /* reduces a modulo n where n is of the form 2**p - d */
      19           0 : int mp_reduce_2k(mp_int *a, mp_int *n, mp_digit d)
      20             : {
      21             :    mp_int q;
      22             :    int    p, res;
      23             : 
      24           0 :    if ((res = mp_init(&q)) != MP_OKAY) {
      25           0 :       return res;
      26             :    }
      27             : 
      28           0 :    p = mp_count_bits(n);
      29           0 : top:
      30             :    /* q = a/2**p, a = a mod 2**p */
      31           0 :    if ((res = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
      32           0 :       goto ERR;
      33             :    }
      34             : 
      35           0 :    if (d != 1) {
      36             :       /* q = q * d */
      37           0 :       if ((res = mp_mul_d(&q, d, &q)) != MP_OKAY) {
      38           0 :          goto ERR;
      39             :       }
      40             :    }
      41             : 
      42             :    /* a = a + q */
      43           0 :    if ((res = s_mp_add(a, &q, a)) != MP_OKAY) {
      44           0 :       goto ERR;
      45             :    }
      46             : 
      47           0 :    if (mp_cmp_mag(a, n) != MP_LT) {
      48           0 :       s_mp_sub(a, n, a);
      49           0 :       goto top;
      50             :    }
      51             : 
      52           0 : ERR:
      53           0 :    mp_clear(&q);
      54           0 :    return res;
      55             : }
      56             : 
      57             : #endif
      58             : 
      59             : /* $Source: /cvs/libtom/libtommath/bn_mp_reduce_2k.c,v $ */
      60             : /* $Revision: 1.4 $ */
      61             : /* $Date: 2006/12/28 01:25:13 $ */