libtommath/bn_mp_lshd.c

#include "tommath_private.h"
#ifdef BN_MP_LSHD_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */

/* shift left a certain amount of digits */
mp_err mp_lshd(mp_int *a, int b)
{
   int x;
   mp_err res;

   /* if its less than zero return */
   if (b <= 0) {
      return MP_OKAY;
   }
   /* no need to shift 0 around */
   if (MP_IS_ZERO(a)) {
      return MP_OKAY;
   }

   /* grow to fit the new digits */
   if (a->alloc < (a->used + b)) {
      if ((res = mp_grow(a, a->used + b)) != MP_OKAY) {
         return res;
      }
   }

   {
      mp_digit *top, *bottom;

      /* increment the used by the shift amount then copy upwards */
      a->used += b;

      /* top */
      top = a->dp + a->used - 1;

      /* base */
      bottom = (a->dp + a->used - 1) - b;

      /* much like mp_rshd this is implemented using a sliding window
       * except the window goes the otherway around.  Copying from
       * the bottom to the top.  see bn_mp_rshd.c for more info.
       */
      for (x = a->used - 1; x >= b; x--) {
         *top-- = *bottom--;
      }

      /* zero the lower digits */
      top = a->dp;
      for (x = 0; x < b; x++) {
         *top++ = 0;
      }
   }
   return MP_OKAY;
}
#endif