/* mpz_hamdist -- calculate hamming distance.

Copyright 1994, 1996, 2001, 2002 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"


mp_bitcnt_t
mpz_hamdist (mpz_srcptr u, mpz_srcptr v)
{
  mp_srcptr      up, vp;
  mp_size_t      usize, vsize;
  mp_bitcnt_t    count;

  usize = SIZ(u);
  vsize = SIZ(v);

  up = PTR(u);
  vp = PTR(v);

  if (usize >= 0)
    {
      if (vsize < 0)
        return ~ (mp_bitcnt_t) 0;

      /* positive/positive */

      if (usize < vsize)
        MPN_SRCPTR_SWAP (up,usize, vp,vsize);

      count = 0;
      if (vsize != 0)
        count = mpn_hamdist (up, vp, vsize);

      usize -= vsize;
      if (usize != 0)
        count += mpn_popcount (up + vsize, usize);

      return count;
    }
  else
    {
      mp_limb_t  ulimb, vlimb;
      mp_size_t  old_vsize, step;

      if (vsize >= 0)
        return ~ (mp_limb_t) 0;

      /* negative/negative */

      usize = -usize;
      vsize = -vsize;

      /* skip common low zeros */
      for (;;)
        {
          ASSERT (usize > 0);
          ASSERT (vsize > 0);

          usize--;
          vsize--;

          ulimb = *up++;
          vlimb = *vp++;

          if (ulimb != 0)
            break;

          if (vlimb != 0)
            {
              MPN_SRCPTR_SWAP (up,usize, vp,vsize);
              ulimb = vlimb;
              vlimb = 0;
              break;
            }
        }

      /* twos complement first non-zero limbs (ulimb is non-zero, but vlimb
         might be zero) */
      ulimb = -ulimb;
      vlimb = -vlimb;
      popc_limb (count, (ulimb ^ vlimb) & GMP_NUMB_MASK);

      if (vlimb == 0)
        {
          mp_bitcnt_t  twoscount;

          /* first non-zero of v */
          old_vsize = vsize;
          do
            {
              ASSERT (vsize > 0);
              vsize--;
              vlimb = *vp++;
            }
          while (vlimb == 0);

          /* part of u corresponding to skipped v zeros */
          step = old_vsize - vsize - 1;
          count += step * GMP_NUMB_BITS;
          step = MIN (step, usize);
          if (step != 0)
            {
              count -= mpn_popcount (up, step);
              usize -= step;
              up += step;
            }

          /* First non-zero vlimb as twos complement, xor with ones
             complement ulimb.  Note -v^(~0^u) == (v-1)^u. */
          vlimb--;
          if (usize != 0)
            {
              usize--;
              vlimb ^= *up++;
            }
          popc_limb (twoscount, vlimb);
          count += twoscount;
        }

      /* Overlapping part of u and v, if any.  Ones complement both, so just
         plain hamdist. */
      step = MIN (usize, vsize);
      if (step != 0)
        {
          count += mpn_hamdist (up, vp, step);
          usize -= step;
          vsize -= step;
          up += step;
          vp += step;
        }

      /* Remaining high part of u or v, if any, ones complement but xor
         against all ones in the other, so plain popcount. */
      if (usize != 0)
        {
        remaining:
          count += mpn_popcount (up, usize);
        }
      else if (vsize != 0)
        {
          up = vp;
          usize = vsize;
          goto remaining;
        }
      return count;
    }
}