mp_mul_d.c

   1 #include "tommath_private.h"
   2 #ifdef MP_MUL_D_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* multiply by a digit */
   7 mp_err mp_mul_d(const mp_int *a, mp_digit b, mp_int *c)
   8 {
   9    mp_digit u;
  10    mp_err   err;
  11    int   ix, oldused;
  12
  13    if (b == 1u) {
  14       return mp_copy(a, c);
  15    }
  16
  17    /* power of two ? */
  18    if (MP_HAS(MP_MUL_2) && (b == 2u)) {
  19       return mp_mul_2(a, c);
  20    }
  21    if (MP_HAS(MP_MUL_2D) && MP_IS_2EXPT(b)) {
  22       ix = 1;
  23       while ((ix < MP_DIGIT_BIT) && (b != (((mp_digit)1)<<ix))) {
  24          ix++;
  25       }
  26       return mp_mul_2d(a, ix, c);
  27    }
  28
  29    /* make sure c is big enough to hold a*b */
  30    if ((err = mp_grow(c, a->used + 1)) != MP_OKAY) {
  31       return err;
  32    }
  33
  34    /* get the original destinations used count */
  35    oldused = c->used;
  36
  37    /* set the sign */
  38    c->sign = a->sign;
  39
  40    /* zero carry */
  41    u = 0;
  42
  43    /* compute columns */
  44    for (ix = 0; ix < a->used; ix++) {
  45       /* compute product and carry sum for this term */
  46       mp_word r       = (mp_word)u + ((mp_word)a->dp[ix] * (mp_word)b);
  47
  48       /* mask off higher bits to get a single digit */
  49       c->dp[ix] = (mp_digit)(r & (mp_word)MP_MASK);
  50
  51       /* send carry into next iteration */
  52       u       = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT);
  53    }
  54
  55    /* store final carry [if any] and increment ix offset  */
  56    c->dp[ix] = u;
  57
  58    /* set used count */
  59    c->used = a->used + 1;
  60
  61    /* now zero digits above the top */
  62    s_mp_zero_digs(c->dp + c->used, oldused - c->used);
  63
  64    mp_clamp(c);
  65
  66    return MP_OKAY;
  67 }
  68 #endif