tomsfastmath/src/divide/fp_div_d.c

/* TomsFastMath, a fast ISO C bignum library.
 * 
 * This project is meant to fill in where LibTomMath
 * falls short.  That is speed ;-)
 *
 * This project is public domain and free for all purposes.
 * 
 * Tom St Denis, tomstdenis@gmail.com
 */
#include <tfm.h>

static int s_is_power_of_two(fp_digit b, int *p)
{
   int x;

   /* fast return if no power of two */
   if ((b==0) || (b & (b-1))) {
      return 0;
   }

   for (x = 0; x < DIGIT_BIT; x++) {
      if (b == (((fp_digit)1)<<x)) {
         *p = x;
         return 1;
      }
   }
   return 0;
}

/* a/b => cb + d == a */
int fp_div_d(fp_int *a, fp_digit b, fp_int *c, fp_digit *d)
{
  fp_int   q;
  fp_word  w;
  fp_digit t;
  int      ix;

  /* cannot divide by zero */
  if (b == 0) {
     return FP_VAL;
  }

  /* quick outs */
  if (b == 1 || fp_iszero(a) == 1) {
     if (d != NULL) {
        *d = 0;
     }
     if (c != NULL) {
        fp_copy(a, c);
     }
     return FP_OKAY;
  }

  /* power of two ? */
  if (s_is_power_of_two(b, &ix) == 1) {
     if (d != NULL) {
        *d = a->dp[0] & ((((fp_digit)1)<<ix) - 1);
     }
     if (c != NULL) {
        fp_div_2d(a, ix, c, NULL);
     }
     return FP_OKAY;
  }

  /* no easy answer [c'est la vie].  Just division */
  fp_init(&q);
  
  q.used = a->used;
  q.sign = a->sign;
  w = 0;
  for (ix = a->used - 1; ix >= 0; ix--) {
     w = (w << ((fp_word)DIGIT_BIT)) | ((fp_word)a->dp[ix]);
     
     if (w >= b) {
        t = (fp_digit)(w / b);
        w -= ((fp_word)t) * ((fp_word)b);
      } else {
        t = 0;
      }
      q.dp[ix] = (fp_digit)t;
  }
  
  if (d != NULL) {
     *d = (fp_digit)w;
  }
  
  if (c != NULL) {
     fp_clamp(&q);
     fp_copy(&q, c);
  }
 
  return FP_OKAY;
}


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