/* mpz_scan1 -- search for a 1 bit.

Copyright 2000, 2001, 2002, 2004, 2012 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 "gmp.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) __GMP_NOTHROW
{
  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 - 1;
  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 ? ~(mp_bitcnt_t) 0 : starting_bit);

  /* This is an important case, where sign is not relevant! */
  if (starting_bit == 0)
    goto short_cut;

  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.  */
	  if (p == u_end)
	    return ~(mp_bitcnt_t) 0;

	  /* Otherwise search further for a non-zero limb.  The high limb is
	     non-zero, if nothing else.  */
	search_nonzero:
	  do
	    {
	      ASSERT (p != u_end);
	      p++;
	    short_cut:
	      limb = *p;
	    }
	  while (limb == 0);
	}
    }
  else
    {
      /* If there's a non-zero limb before ours then we're in the ones
	 complement region.  */
      if (mpn_zero_p (u_ptr, starting_limb)) {
	if (limb == 0)
	  /* Seeking for the first non-zero bit, it is the same for u and -u. */
	  goto search_nonzero;

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

      /* 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)
	{
	  if (p == u_end)
	    return (mp_bitcnt_t) abs_size * GMP_NUMB_BITS;
	  p++;
	  limb = *p;
	}

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

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