kc3-lang/libtommath/s_mp_mul_digs.c

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• Hash : 7a68f128
  Author :
  Date : 2019-10-19T16:24:39
  

  Execute move.sh - Rename files from bn_* to match the function names. * git blame <renamed-file> is not affected * git log --follow <renamed-file> can be used to show log across renames

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• s_mp_mul_digs.c

• #include "tommath_private.h"
  #ifdef S_MP_MUL_DIGS_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis */
  /* SPDX-License-Identifier: Unlicense */
  
  /* multiplies |a| * |b| and only computes upto digs digits of result
   * HAC pp. 595, Algorithm 14.12  Modified so you can control how
   * many digits of output are created.
   */
  mp_err s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs)
  {
     mp_int  t;
     mp_err  err;
     int     pa, pb, ix, iy;
     mp_digit u;
     mp_word r;
     mp_digit tmpx, *tmpt, *tmpy;
  
     /* can we use the fast multiplier? */
     if ((digs < MP_WARRAY) &&
         (MP_MIN(a->used, b->used) < MP_MAXFAST)) {
        return s_mp_mul_digs_fast(a, b, c, digs);
     }
  
     if ((err = mp_init_size(&t, digs)) != MP_OKAY) {
        return err;
     }
     t.used = digs;
  
     /* compute the digits of the product directly */
     pa = a->used;
     for (ix = 0; ix < pa; ix++) {
        /* set the carry to zero */
        u = 0;
  
        /* limit ourselves to making digs digits of output */
        pb = MP_MIN(b->used, digs - ix);
  
        /* setup some aliases */
        /* copy of the digit from a used within the nested loop */
        tmpx = a->dp[ix];
  
        /* an alias for the destination shifted ix places */
        tmpt = t.dp + ix;
  
        /* an alias for the digits of b */
        tmpy = b->dp;
  
        /* compute the columns of the output and propagate the carry */
        for (iy = 0; iy < pb; iy++) {
           /* compute the column as a mp_word */
           r       = (mp_word)*tmpt +
                     ((mp_word)tmpx * (mp_word)*tmpy++) +
                     (mp_word)u;
  
           /* the new column is the lower part of the result */
           *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK);
  
           /* get the carry word from the result */
           u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
        }
        /* set carry if it is placed below digs */
        if ((ix + iy) < digs) {
           *tmpt = u;
        }
     }
  
     mp_clamp(&t);
     mp_exch(&t, c);
  
     mp_clear(&t);
     return MP_OKAY;
  }
  #endif
  

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