/* -*- c++ -*- ----------------------------------------------------------
   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
   http://lammps.sandia.gov, Sandia National Laboratories
   Steve Plimpton, sjplimp@sandia.gov

   Copyright (2003) Sandia Corporation.  Under the terms of Contract
   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
   certain rights in this software.  This software is distributed under
   the GNU General Public License.

   See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */

// loop counters for doing a pack/unpack

struct pack_plan_3d {
  int nfast;                 // # of elements in fast index
  int nmid;                  // # of elements in mid index
  int nslow;                 // # of elements in slow index
  int nstride_line;          // stride between successive mid indices
  int nstride_plane;         // stride between successive slow indices
  int nqty;                  // # of values/element
};

#if !defined(PACK_POINTER) && !defined(PACK_MEMCPY)
#define PACK_ARRAY
#endif

#ifndef PACK_DATA
#define PACK_DATA double
#endif

/* ----------------------------------------------------------------------
   Pack and unpack functions:

   pack routines copy strided values from data into contiguous locs in buf
   unpack routines copy contiguous values from buf into strided locs in data
   different versions of unpack depending on permutation
     and # of values/element
   PACK_ARRAY routines work via array indices (default)
   PACK_POINTER routines work via pointers
   PACK_MEMCPY routines work via pointers and memcpy function
------------------------------------------------------------------------- */

/* ----------------------------------------------------------------------
   pack/unpack with array indices
------------------------------------------------------------------------- */

#ifdef PACK_ARRAY

/* ----------------------------------------------------------------------
   pack from data -> buf
------------------------------------------------------------------------- */

static void pack_3d(PACK_DATA *data, PACK_DATA *buf, struct pack_plan_3d *plan)
{
  register int in,out,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  in = 0;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_plane;
    for (mid = 0; mid < nmid; mid++) {
      out = plane + mid*nstride_line;
      for (fast = 0; fast < nfast; fast++)
        buf[in++] = data[out++];
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data
------------------------------------------------------------------------- */

static void unpack_3d(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)
{
  register int in,out,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = 0;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_plane;
    for (mid = 0; mid < nmid; mid++) {
      in = plane + mid*nstride_line;
      for (fast = 0; fast < nfast; fast++)
        data[in++] = buf[out++];
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, 1 value/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_1(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)
{
  register int in,out,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = 0;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      in = plane + mid;
      for (fast = 0; fast < nfast; fast++, in += nstride_plane)
        data[in] = buf[out++];
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, 2 values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_2(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)
{
  register int in,out,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = 0;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      in = plane + 2*mid;
      for (fast = 0; fast < nfast; fast++, in += nstride_plane) {
        data[in] = buf[out++];
        data[in+1] = buf[out++];
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, nqty values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_n(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register int in,out,iqty,instart,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane,nqty;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;
  nqty = plan->nqty;

  out = 0;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      instart = plane + nqty*mid;
      for (fast = 0; fast < nfast; fast++, instart += nstride_plane) {
        in = instart;
        for (iqty = 0; iqty < nqty; iqty++) data[in++] = buf[out++];
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, 1 value/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_1(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register int in,out,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = 0;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      in = slow + mid*nstride_plane;
      for (fast = 0; fast < nfast; fast++, in += nstride_line)
        data[in] = buf[out++];
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, 2 values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_2(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register int in,out,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = 0;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      in = 2*slow + mid*nstride_plane;
      for (fast = 0; fast < nfast; fast++, in += nstride_line) {
        data[in] = buf[out++];
        data[in+1] = buf[out++];
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, nqty values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_n(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register int in,out,iqty,instart,fast,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,nqty;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;
  nqty = plan->nqty;

  out = 0;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      instart = nqty*slow + mid*nstride_plane;
      for (fast = 0; fast < nfast; fast++, instart += nstride_line) {
        in = instart;
        for (iqty = 0; iqty < nqty; iqty++) data[in++] = buf[out++];
      }
    }
  }
}

#endif

/* ----------------------------------------------------------------------
   pack/unpack with pointers
------------------------------------------------------------------------- */

#ifdef PACK_POINTER

/* ----------------------------------------------------------------------
   pack from data -> buf
------------------------------------------------------------------------- */

static void pack_3d(PACK_DATA *data, PACK_DATA *buf, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  in = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_plane;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+mid*nstride_line]);
      end = begin + nfast;
      for (out = begin; out < end; out++)
        *(in++) = *out;
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data
------------------------------------------------------------------------- */

static void unpack_3d(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_plane;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+mid*nstride_line]);
      end = begin + nfast;
      for (in = begin; in < end; in++)
        *in = *(out++);
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, 1 value/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_1(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+mid]);
      end = begin + nfast*nstride_plane;
      for (in = begin; in < end; in += nstride_plane)
        *in = *(out++);
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, 2 values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_2(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+2*mid]);
      end = begin + nfast*nstride_plane;
      for (in = begin; in < end; in += nstride_plane) {
        *in = *(out++);
        *(in+1) = *(out++);
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, nqty values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_n(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*instart,*begin,*end;
  register int iqty,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane,nqty;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;
  nqty = plan->nqty;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+nqty*mid]);
      end = begin + nfast*nstride_plane;
      for (instart = begin; instart < end; instart += nstride_plane) {
        in = instart;
        for (iqty = 0; iqty < nqty; iqty++) *(in++) = *(out++);
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, 1 value/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_1(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[slow+mid*nstride_plane]);
      end = begin + nfast*nstride_line;
      for (in = begin; in < end; in += nstride_line)
        *in = *(out++);
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, 2 values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_2(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[2*slow+mid*nstride_plane]);
      end = begin + nfast*nstride_line;
      for (in = begin; in < end; in += nstride_line) {
        *in = *(out++);
        *(in+1) = *(out++);
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, nqty values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_n(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*instart,*begin,*end;
  register int iqty,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,nqty;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;
  nqty = plan->nqty;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[nqty*slow+mid*nstride_plane]);
      end = begin + nfast*nstride_line;
      for (instart = begin; instart < end; instart += nstride_line) {
        in = instart;
        for (iqty = 0; iqty < nqty; iqty++) *(in++) = *(out++);
      }
    }
  }
}

#endif

/* ----------------------------------------------------------------------
   pack/unpack with pointers and memcpy function
   no memcpy version of unpack_permute routines,
     just use PACK_POINTER versions
------------------------------------------------------------------------- */

#ifdef PACK_MEMCPY

/* ----------------------------------------------------------------------
   pack from data -> buf
------------------------------------------------------------------------- */

static void pack_3d(PACK_DATA *data, PACK_DATA *buf, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out;
  register int mid,slow,size;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane,upto;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  size = nfast*sizeof(PACK_DATA);
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_plane;
    upto = slow*nmid*nfast;
    for (mid = 0; mid < nmid; mid++) {
      in = &(buf[upto+mid*nfast]);
      out = &(data[plane+mid*nstride_line]);
      memcpy(in,out,size);
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data
------------------------------------------------------------------------- */

static void unpack_3d(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out;
  register int mid,slow,size;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane,upto;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  size = nfast*sizeof(PACK_DATA);
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_plane;
    upto = slow*nmid*nfast;
    for (mid = 0; mid < nmid; mid++) {
      in = &(data[plane+mid*nstride_line]);
      out = &(buf[upto+mid*nfast]);
      memcpy(in,out,size);
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, 1 value/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_1(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+mid]);
      end = begin + nfast*nstride_plane;
      for (in = begin; in < end; in += nstride_plane)
        *in = *(out++);
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, 2 values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_2(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+2*mid]);
      end = begin + nfast*nstride_plane;
      for (in = begin; in < end; in += nstride_plane) {
        *in = *(out++);
        *(in+1) = *(out++);
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, one axis permutation, nqty values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute1_n(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*instart,*begin,*end;
  register int iqty,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,plane,nqty;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;
  nqty = plan->nqty;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    plane = slow*nstride_line;
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[plane+nqty*mid]);
      end = begin + nfast*nstride_plane;
      for (instart = begin; instart < end; instart += nstride_plane) {
        in = instart;
        for (iqty = 0; iqty < nqty; iqty++) *(in++) = *(out++);
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, 1 value/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_1(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[slow+mid*nstride_plane]);
      end = begin + nfast*nstride_line;
      for (in = begin; in < end; in += nstride_line)
        *in = *(out++);
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, 2 values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_2(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*begin,*end;
  register int mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[2*slow+mid*nstride_plane]);
      end = begin + nfast*nstride_line;
      for (in = begin; in < end; in += nstride_line) {
        *in = *(out++);
        *(in+1) = *(out++);
      }
    }
  }
}

/* ----------------------------------------------------------------------
   unpack from buf -> data, two axis permutation, nqty values/element
------------------------------------------------------------------------- */

static void unpack_3d_permute2_n(PACK_DATA *buf, PACK_DATA *data, struct pack_plan_3d *plan)

{
  register PACK_DATA *in,*out,*instart,*begin,*end;
  register int iqty,mid,slow;
  register int nfast,nmid,nslow,nstride_line,nstride_plane,nqty;

  nfast = plan->nfast;
  nmid = plan->nmid;
  nslow = plan->nslow;
  nstride_line = plan->nstride_line;
  nstride_plane = plan->nstride_plane;
  nqty = plan->nqty;

  out = buf;
  for (slow = 0; slow < nslow; slow++) {
    for (mid = 0; mid < nmid; mid++) {
      begin = &(data[nqty*slow+mid*nstride_plane]);
      end = begin + nfast*nstride_line;
      for (instart = begin; instart < end; instart += nstride_line) {
        in = instart;
        for (iqty = 0; iqty < nqty; iqty++) *(in++) = *(out++);
      }
    }
  }
}

#endif
