mpir/mpz/scan0.c

/* mpz_scan0 -- search for a 0 bit.

Copyright 2000, 2001, 2002, 2004 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 3 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */

#include "mpir.h"
#include "gmp-impl.h"
#include "longlong.h"


/* mpn_scan0 can't be used for the u>0 search since there might not be a 0
   bit before the end of the data.  mpn_scan1 could be used for the inverted
   search under u<0, but usually the search won't go very far so it seems
   reasonable to inline that code.  */

mp_bitcnt_t
mpz_scan0 (mpz_srcptr u, mp_bitcnt_t starting_bit)
{
  mp_srcptr      u_ptr = PTR(u);
  mp_size_t      size = SIZ(u);
  mp_size_t      abs_size = ABS(size);
  mp_srcptr      u_end = u_ptr + abs_size;
  mp_size_t      starting_limb = starting_bit / GMP_NUMB_BITS;
  mp_srcptr      p = u_ptr + starting_limb;
  mp_limb_t      limb;
  int            cnt;

  /* When past end, there's an immediate 0 bit for u>=0, or no 0 bits for
     u<0.  Notice this test picks up all cases of u==0 too. */
  if (starting_limb >= abs_size)
    return (size >= 0 ? starting_bit : ULONG_MAX);

  limb = *p;

  if (size >= 0)
    {
      /* Mask to 1 all bits before starting_bit, thus ignoring them. */
      limb |= (CNST_LIMB(1) << (starting_bit % GMP_NUMB_BITS)) - 1;

      /* Search for a limb which isn't all ones.  If the end is reached then
         the zero bit immediately past the end is returned.  */
      while (limb == GMP_NUMB_MAX)
        {
          p++;
          if (p == u_end)
            return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
          limb = *p;
        }

      /* Now seek low 1 bit. */
      limb = ~limb;
    }
  else
    {
      mp_srcptr  q;

      /* If there's a non-zero limb before ours then we're in the ones
         complement region.  Search from *(p-1) downwards since that might
         give better cache locality, and since a non-zero in the middle of a
         number is perhaps a touch more likely than at the end.  */
      q = p;
      while (q != u_ptr)
        {
          q--;
          if (*q != 0)
            goto inverted;
        }

      /* Adjust so ~limb implied by searching for 1 bit below becomes -limb.
         If limb==0 here then this isn't the beginning of twos complement
         inversion, but that doesn't matter because limb==0 is a zero bit
         immediately (-1 is all ones for below).  */
      limb--;

    inverted:
      /* Now seeking a 1 bit. */

      /* Mask to 0 all bits before starting_bit, thus ignoring them. */
      limb &= (MP_LIMB_T_MAX << (starting_bit % GMP_NUMB_BITS));

      if (limb == 0)
        {
          /* If the high limb is zero after masking, then no 1 bits past
             starting_bit.  */
          p++;
          if (p == u_end)
            return ULONG_MAX;

          /* Search further for a non-zero limb.  The high limb is non-zero,
             if nothing else.  */
          for (;;)
            {
              limb = *p;
              if (limb != 0)
                break;
              p++;
              ASSERT (p < u_end);
            }
        }
    }

  ASSERT (limb != 0);
  count_trailing_zeros (cnt, limb);
  return (mp_bitcnt_t)((p - u_ptr) * GMP_NUMB_BITS + cnt);
}