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

   1 #include <tommath.h>
   2 #ifdef BN_MP_SQRT_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 /* this function is less generic than mp_n_root, simpler and faster */
  19 int mp_sqrt(mp_int *arg, mp_int *ret)
  20 {
  21   int res;
  22   mp_int t1,t2;
  23
  24   /* must be positive */
  25   if (arg->sign == MP_NEG) {
  26     return MP_VAL;
  27   }
  28
  29   /* easy out */
  30   if (mp_iszero(arg) == MP_YES) {
  31     mp_zero(ret);
  32     return MP_OKAY;
  33   }
  34
  35   if ((res = mp_init_copy(&t1, arg)) != MP_OKAY) {
  36     return res;
  37   }
  38
  39   if ((res = mp_init(&t2)) != MP_OKAY) {
  40     goto E2;
  41   }
  42
  43   /* First approx. (not very bad for large arg) */
  44   mp_rshd (&t1,t1.used/2);
  45
  46   /* t1 > 0  */
  47   if ((res = mp_div(arg,&t1,&t2,NULL)) != MP_OKAY) {
  48     goto E1;
  49   }
  50   if ((res = mp_add(&t1,&t2,&t1)) != MP_OKAY) {
  51     goto E1;
  52   }
  53   if ((res = mp_div_2(&t1,&t1)) != MP_OKAY) {
  54     goto E1;
  55   }
  56   /* And now t1 > sqrt(arg) */
  57   do {
  58     if ((res = mp_div(arg,&t1,&t2,NULL)) != MP_OKAY) {
  59       goto E1;
  60     }
  61     if ((res = mp_add(&t1,&t2,&t1)) != MP_OKAY) {
  62       goto E1;
  63     }
  64     if ((res = mp_div_2(&t1,&t1)) != MP_OKAY) {
  65       goto E1;
  66     }
  67     /* t1 >= sqrt(arg) >= t2 at this point */
  68   } while (mp_cmp_mag(&t1,&t2) == MP_GT);
  69
  70   mp_exch(&t1,ret);
  71
  72 E1: mp_clear(&t2);
  73 E2: mp_clear(&t1);
  74   return res;
  75 }
  76
  77 #endif
  78
  79 /* $Source: /cvs/libtom/libtommath/bn_mp_sqrt.c,v $ */
  80 /* $Revision: 1.3 $ */
  81 /* $Date: 2006/03/31 14:18:44 $ */