mpf_normalize.c

来自「精确小数算法库,可以实现任意长度的精确小数算法,用c语言实现,主要用于密码学中小」· C语言 代码 · 共 54 行

/* LibTomFloat, multiple-precision floating-point library
 *
 * LibTomFloat is a library that provides multiple-precision
 * floating-point artihmetic as well as trigonometric functionality.
 *
 * This library requires the public domain LibTomMath to be installed.
 * 
 * This library is free for all purposes without any express
 * gurantee it works
 *
 * Tom St Denis, [e-mail], http://float.libtomcrypt.org
 */
#include <tomfloat.h>

int  mpf_normalize(mp_float *a)
{
   long     cb, diff;
   int      err;
   mp_digit c;

   /* sanity */
   if (a->radix < 2) {
      return MP_VAL;
   }

   cb = mp_count_bits(&(a->mantissa));
   if (cb > a->radix) {
      diff    = cb - a->radix;
      a->exp += diff;

      /* round it, add 1 after shift if diff-1'th bit is 1 */
      c = a->mantissa.dp[diff/DIGIT_BIT] & (1U<<(diff%DIGIT_BIT));
      if ((err = mp_div_2d(&(a->mantissa), diff, &(a->mantissa), NULL)) != MP_OKAY) {
         return err;
      }

      if (c != 0) {
         return mp_add_d(&(a->mantissa), 1, &(a->mantissa));
      } else {
         return MP_OKAY;
      }
   } else if (cb < a->radix) {
      if (mp_iszero(&(a->mantissa)) == MP_YES) {
         return mpf_const_0(a);
      } else {
         diff    = a->radix - cb;
         a->exp -= diff;
         return mp_mul_2d(&(a->mantissa), diff, &(a->mantissa));
      }
   }
   return MP_OKAY;
}