mp_2expt.c

   1 #include "tommath_private.h"
   2 #ifdef MP_2EXPT_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* computes a = 2**b
   7  *
   8  * Simple algorithm which zeroes the int, grows it then just sets one bit
   9  * as required.
  10  */
  11 mp_err mp_2expt(mp_int *a, int b)
  12 {
  13    mp_err    err;
  14
  15    if (b < 0) {
  16       return MP_VAL;
  17    }
  18
  19    /* zero a as per default */
  20    mp_zero(a);
  21
  22    /* grow a to accommodate the single bit */
  23    if ((err = mp_grow(a, (b / MP_DIGIT_BIT) + 1)) != MP_OKAY) {
  24       return err;
  25    }
  26
  27    /* set the used count of where the bit will go */
  28    a->used = (b / MP_DIGIT_BIT) + 1;
  29
  30    /* put the single bit in its place */
  31    a->dp[b / MP_DIGIT_BIT] = (mp_digit)1 << (mp_digit)(b % MP_DIGIT_BIT);
  32
  33    return MP_OKAY;
  34 }
  35 #endif