xref: /dflybsd-src/contrib/gmp/mpz/scan1.c (revision d365564473a20a528d07c59cad8ee2f4bea5546f)

/* 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;
  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);

  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 ~(mp_bitcnt_t) 0;

	  /* 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;
}