denyhosts/clamscan/libclamav/tomsfastmath/divide/fp_div.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 "bignum_fast.h"

/* a/b => cb + d == a */
int fp_div(fp_int *a, fp_int *b, fp_int *c, fp_int *d)
{
  fp_int  q, x, y, t1, t2;
  int     n, t, i, norm, neg;

  /* is divisor zero ? */
  if (fp_iszero (b) == 1) {
    return FP_VAL;
  }

  /* if a < b then q=0, r = a */
  if (fp_cmp_mag (a, b) == FP_LT) {
    if (d != NULL) {
      fp_copy (a, d);
    } 
    if (c != NULL) {
      fp_zero (c);
    }
    return FP_OKAY;
  }

  fp_init(&q);
  q.used = a->used + 2;

  fp_init(&t1);
  fp_init(&t2);
  fp_init_copy(&x, a);
  fp_init_copy(&y, b);

  /* fix the sign */
  neg = (a->sign == b->sign) ? FP_ZPOS : FP_NEG;
  x.sign = y.sign = FP_ZPOS;

  /* normalize both x and y, ensure that y >= b/2, [b == 2**DIGIT_BIT] */
  norm = fp_count_bits(&y) % DIGIT_BIT;
  if (norm < (int)(DIGIT_BIT-1)) {
     norm = (DIGIT_BIT-1) - norm;
     fp_mul_2d (&x, norm, &x);
     fp_mul_2d (&y, norm, &y);
  } else {
     norm = 0;
  }

  /* note hac does 0 based, so if used==5 then its 0,1,2,3,4, e.g. use 4 */
  n = x.used - 1;
  t = y.used - 1;

  /* while (x >= y*b**n-t) do { q[n-t] += 1; x -= y*b**{n-t} } */
  fp_lshd (&y, n - t);                                             /* y = y*b**{n-t} */

  while (fp_cmp (&x, &y) != FP_LT) {
    ++(q.dp[n - t]);
    fp_sub (&x, &y, &x);
  }

  /* reset y by shifting it back down */
  fp_rshd (&y, n - t);

  /* step 3. for i from n down to (t + 1) */
  for (i = n; i >= (t + 1); i--) {
    if (i > x.used) {
      continue;
    }

    /* step 3.1 if xi == yt then set q{i-t-1} to b-1, 
     * otherwise set q{i-t-1} to (xi*b + x{i-1})/yt */
    if (x.dp[i] == y.dp[t]) {
      q.dp[i - t - 1] = ((((fp_word)1) << DIGIT_BIT) - 1);
    } else {
      fp_word tmp;
      tmp = ((fp_word) x.dp[i]) << ((fp_word) DIGIT_BIT);
      tmp |= ((fp_word) x.dp[i - 1]);
      tmp /= ((fp_word) y.dp[t]);
      q.dp[i - t - 1] = (fp_digit) (tmp);
    }

    /* while (q{i-t-1} * (yt * b + y{t-1})) > 
             xi * b**2 + xi-1 * b + xi-2 
     
       do q{i-t-1} -= 1; 
    */
    q.dp[i - t - 1] = (q.dp[i - t - 1] + 1);
    do {
      q.dp[i - t - 1] = (q.dp[i - t - 1] - 1);

      /* find left hand */
      fp_zero (&t1);
      t1.dp[0] = (t - 1 < 0) ? 0 : y.dp[t - 1];
      t1.dp[1] = y.dp[t];
      t1.used = 2;
      fp_mul_d (&t1, q.dp[i - t - 1], &t1);

      /* find right hand */
      t2.dp[0] = (i - 2 < 0) ? 0 : x.dp[i - 2];
      t2.dp[1] = (i - 1 < 0) ? 0 : x.dp[i - 1];
      t2.dp[2] = x.dp[i];
      t2.used = 3;
    } while (fp_cmp_mag(&t1, &t2) == FP_GT);

    /* step 3.3 x = x - q{i-t-1} * y * b**{i-t-1} */
    fp_mul_d (&y, q.dp[i - t - 1], &t1);
    fp_lshd  (&t1, i - t - 1);
    fp_sub   (&x, &t1, &x);

    /* if x < 0 then { x = x + y*b**{i-t-1}; q{i-t-1} -= 1; } */
    if (x.sign == FP_NEG) {
      fp_copy (&y, &t1);
      fp_lshd (&t1, i - t - 1);
      fp_add (&x, &t1, &x);
      q.dp[i - t - 1] = q.dp[i - t - 1] - 1;
    }
  }

  /* now q is the quotient and x is the remainder 
   * [which we have to normalize] 
   */
  
  /* get sign before writing to c */
  x.sign = x.used == 0 ? FP_ZPOS : a->sign;

  if (c != NULL) {
    fp_clamp (&q);
    fp_copy (&q, c);
    c->sign = neg;
  }

  if (d != NULL) {
    fp_div_2d (&x, norm, &x, NULL);

/* the following is a kludge, essentially we were seeing the right remainder but 
   with excess digits that should have been zero
 */
    for (i = b->used; i < x.used; i++) {
        x.dp[i] = 0;
    }
    fp_clamp(&x);
    fp_copy (&x, d);
  }

  return FP_OKAY;
}

/* $Source: /cvs/libtom/tomsfastmath/src/divide/fp_div.c,v $ */
/* $Revision: 1.1 $ */
/* $Date: 2006/12/31 21:25:53 $ */