mpir/mpz/scan1.c

/* mpz_scan1 -- search for a 1 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 inverted u<0 search since there might not
   be a 0 bit before the end of the data.  mpn_scan1 could be used under u>0
   (except when in the high limb), but usually the search won't go very far
   so it seems reasonable to inline that code.  */

mp_bitcnt_t
mpz_scan1 (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;

  /* Past the end there's no 1 bits for u>=0, or an immediate 1 bit for u<0.
     Notice this test picks up any u==0 too. */
  if (starting_limb >= abs_size)
    return (size >= 0 ? ULONG_MAX : starting_bit);

  limb = *p;

  if (size >= 0)
    {
      /* 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 it's the high limb which is zero after masking, then there's
             no 1 bits after starting_bit.  */
          p++;
          if (p == u_end)
            return ULONG_MAX;

          /* Otherwise 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);
            }
        }
    }
  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;
        }

      if (limb == 0)
        {
          /* Skip zero limbs, to find the start of twos complement.  The
             high limb is non-zero, if nothing else.  This search is
             necessary so the -limb is applied at the right spot. */
          do
            {
              p++;
              ASSERT (p < u_end);
              limb = *p;
            }
          while (limb == 0);

          /* Apply twos complement, and look for a 1 bit in that.  Since
             limb!=0 here, also have (-limb)!=0 so there's certainly a 1
             bit.  */
          limb = -limb;
          goto got_limb;
        }

      /* Adjust so ~limb implied by searching for 0 bit becomes -limb.  */
      limb--;

    inverted:
      /* Now seeking a 0 bit. */

      /* 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 is not all ones.  If the end is reached
         then the zero immediately past the end is the result.  */
      while (limb == GMP_NUMB_MAX)
        {
          p++;
          if (p == u_end)
            return (mp_bitcnt_t)abs_size * GMP_NUMB_BITS;
          limb = *p;
        }

      /* Now seeking low 1 bit. */
      limb = ~limb;
    }

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