release/src/router/dropbear/libtommath/bn_mp_div_2d.c

   1 #include <tommath.h>
   2 #ifdef BN_MP_DIV_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 /* shift right by a certain bit count (store quotient in c, optional remainder in d) */
  19 int mp_div_2d (mp_int * a, int b, mp_int * c, mp_int * d)
  20 {
  21   mp_digit D, r, rr;
  22   int     x, res;
  23   mp_int  t;
  24
  25
  26   /* if the shift count is <= 0 then we do no work */
  27   if (b <= 0) {
  28     res = mp_copy (a, c);
  29     if (d != NULL) {
  30       mp_zero (d);
  31     }
  32     return res;
  33   }
  34
  35   if ((res = mp_init (&t)) != MP_OKAY) {
  36     return res;
  37   }
  38
  39   /* get the remainder */
  40   if (d != NULL) {
  41     if ((res = mp_mod_2d (a, b, &t)) != MP_OKAY) {
  42       mp_clear (&t);
  43       return res;
  44     }
  45   }
  46
  47   /* copy */
  48   if ((res = mp_copy (a, c)) != MP_OKAY) {
  49     mp_clear (&t);
  50     return res;
  51   }
  52
  53   /* shift by as many digits in the bit count */
  54   if (b >= (int)DIGIT_BIT) {
  55     mp_rshd (c, b / DIGIT_BIT);
  56   }
  57
  58   /* shift any bit count < DIGIT_BIT */
  59   D = (mp_digit) (b % DIGIT_BIT);
  60   if (D != 0) {
  61     register mp_digit *tmpc, mask, shift;
  62
  63     /* mask */
  64     mask = (((mp_digit)1) << D) - 1;
  65
  66     /* shift for lsb */
  67     shift = DIGIT_BIT - D;
  68
  69     /* alias */
  70     tmpc = c->dp + (c->used - 1);
  71
  72     /* carry */
  73     r = 0;
  74     for (x = c->used - 1; x >= 0; x--) {
  75       /* get the lower  bits of this word in a temp */
  76       rr = *tmpc & mask;
  77
  78       /* shift the current word and mix in the carry bits from the previous word */
  79       *tmpc = (*tmpc >> D) | (r << shift);
  80       --tmpc;
  81
  82       /* set the carry to the carry bits of the current word found above */
  83       r = rr;
  84     }
  85   }
  86   mp_clamp (c);
  87   if (d != NULL) {
  88     mp_exch (&t, d);
  89   }
  90   mp_clear (&t);
  91   return MP_OKAY;
  92 }
  93 #endif
  94
  95 /* $Source: /cvs/libtom/libtommath/bn_mp_div_2d.c,v $ */
  96 /* $Revision: 1.3 $ */
  97 /* $Date: 2006/03/31 14:18:44 $ */