mp_mul_2d.c

   1 #include "tommath_private.h"
   2 #ifdef MP_MUL_2D_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* shift left by a certain bit count */
   7 mp_err mp_mul_2d(const mp_int *a, int b, mp_int *c)
   8 {
   9    mp_err err;
  10
  11    if (b < 0) {
  12       return MP_VAL;
  13    }
  14
  15    if ((err = mp_copy(a, c)) != MP_OKAY) {
  16       return err;
  17    }
  18
  19    if ((err = mp_grow(c, c->used + (b / MP_DIGIT_BIT) + 1)) != MP_OKAY) {
  20       return err;
  21    }
  22
  23    /* shift by as many digits in the bit count */
  24    if (b >= MP_DIGIT_BIT) {
  25       if ((err = mp_lshd(c, b / MP_DIGIT_BIT)) != MP_OKAY) {
  26          return err;
  27       }
  28    }
  29
  30    /* shift any bit count < MP_DIGIT_BIT */
  31    b %= MP_DIGIT_BIT;
  32    if (b != 0u) {
  33       mp_digit shift, mask, r;
  34       int x;
  35
  36       /* bitmask for carries */
  37       mask = ((mp_digit)1 << b) - (mp_digit)1;
  38
  39       /* shift for msbs */
  40       shift = (mp_digit)(MP_DIGIT_BIT - b);
  41
  42       /* carry */
  43       r    = 0;
  44       for (x = 0; x < c->used; x++) {
  45          /* get the higher bits of the current word */
  46          mp_digit rr = (c->dp[x] >> shift) & mask;
  47
  48          /* shift the current word and OR in the carry */
  49          c->dp[x] = ((c->dp[x] << b) | r) & MP_MASK;
  50
  51          /* set the carry to the carry bits of the current word */
  52          r = rr;
  53       }
  54
  55       /* set final carry */
  56       if (r != 0u) {
  57          c->dp[(c->used)++] = r;
  58       }
  59    }
  60    mp_clamp(c);
  61    return MP_OKAY;
  62 }
  63 #endif