Task: Implement slab.cc for CactusNumerical/Slab thorn

thorn_name: CactusNumerical/Slab
src_filename: slab.cc
interface: # Interface definition for thorn Slab

IMPLEMENTS: Slab

INCLUDES HEADER: slab.h IN Slab.h
INCLUDES HEADER: slab.inc IN Slab.inc

USES INCLUDE HEADER: loopcontrol.h

CCTK_POINTER_TO_CONST \
FUNCTION GetMPICommWorld (CCTK_POINTER_TO_CONST IN cctkGH)
USES FUNCTION GetMPICommWorld

schedule: # Schedule definitions for thorn Slab

SCHEDULE Slab_InitMPIDatatypes AT startup after Driver_Startup
{
  LANG: C
} "Create MPI datatypes for complex variables in C"

SCHEDULE Slab_InitTimers AT wragh
{
  LANG: C
} "Initialise timers"

if (timer_output) {
  SCHEDULE Slab_PrintTimers AT shutdown
  {
    LANG: C
  } "Print timers"
}

param: # Parameter definitions for thorn Slab

BOOLEAN timer_output "Print slabbing timings at shutdown time" STEERABLE=always
{
} "no"

BOOLEAN use_alltoallv "Use MPI_Alltoallv for communication?" STEERABLE=always
{
} "no"

configuration: # Configuration definition for thorn Slab

PROVIDES Slab
{
}

OPTIONAL LoopControl MPI
{
}

context: slab.h:
```
#ifndef SLAB_H
#define SLAB_H

#ifdef __cplusplus 
extern "C" 
{
#endif

#include <stdio.h>

#include <cctk.h>

/*
 * Slab_Transfer copies a slab from one array into a slab of another
 * array.
 * 
 * The src and dst variables describe the source and the destination
 * slab, respectively.
 * 
 * The variables gsh, lbnd, ash and lsh describe the shape and
 * distribution of the array containing the slab.  They are equivalent
 * to the corresponding quantities in the cGH structure.
 * 
 * off, str, and len describe the location and shape of the slab
 * within the array.  off is the offset, i.e., the location of the
 * "first" corner of the slab.  str is the stride, i.e., the distance
 * between to grid points in the slab.  The stride can be negative.
 * len is the length, i.e., the number of grid points making up the
 * slab.  len does not include the grid points that are skipped if the
 * stride is larger than one.
 * 
 * xpose describes a possible permutation of the coordinate axes
 * between the slabs.  It is source-axis = xpose[destination-axis].
 * 
 * flip describes a possible inversion of the coordinate axes (from
 * the point of view of the destination slab).  It is source-axis =
 * xpose[flip[destination-axis]].
 * 
 * The corresponding lengths of the source and the destination slabs
 * must be equal, i.e., for all d: src.len[xpose[d]] = dst.len[d].
 * 
 * The slabs are copied according to
 * 
 *     dst[dst.off + I * dst.str] = src[src.off + J * src.str]
 * 
 * where the multi-indices I and J have the ranges specified by
 * dst.len and src.len, respectively, and I and J are related by the
 * transposition
 * 
 *     J = xpose[flip[I]]
 * 
 * 
 * 
 * Restrictions:
 * 
 *     dim >= 0
 * 
 *     gsh >= 0
 *     lbnd >= 0
 *     lsh >= 0
 *     ash >= lsh
 *     lbnd + lsh <= gsh
 *     lbbox and ubbox must be booleans, i.e., either 0 or 1
 *     nghostzones >= 0
 * 
 *     len >= 0
 *     str != 0
 *     off >= 0
 *     off < gsh
 *     off + (len-1) * str >= 0
 *     off + (len-1) * str < gsh
 * 
 *     xpose must be a permutation of 0 ... dim-1
 *     flip must be a boolean, i.e., either 0 or 1
 * 
 * The source and the destination arrays may be the same.
 */

#define SLAB_MAXDIM 3

struct slabinfo {
  int gsh;
  int lbnd, lsh;
  int ash;
  int lbbox, ubbox, nghostzones;
  int off, str, len;
};

struct xferinfo {
  struct slabinfo src, dst;
  int xpose;
  int flip;
};

void
print_slabinfo (FILE                  *          const out,
                struct slabinfo const * restrict const slabinfo);

void
print_xferinfo (FILE                  *          const out,
                struct xferinfo const * restrict const xferinfo);



struct slabsetup;

struct slabsetup *
Slab_MultiTransfer_Init
(cGH             const * restrict const cctkGH,
 int                              const dim,
 struct xferinfo const * restrict const xferinfo,
 int                              const options);

int
Slab_MultiTransfer_Apply
(cGH              const                  * restrict const cctkGH,
 struct slabsetup const                  * restrict const slabsetup,
 int                                                const nvars,
 int              const                  * restrict const srctypes,
 void             const * restrict const * restrict const srcptrs,
 int              const                  * restrict const dsttypes,
 void                   * restrict const * restrict const dstptrs);

int
Slab_MultiTransfer_Finalize
(cGH              const * restrict const cctkGH,
 struct slabsetup       * restrict const slabsetup);

int
Slab_MultiTransfer
(cGH             const                  * restrict const cctkGH,
 int                                               const dim,
 struct xferinfo const                  * restrict const xferinfo,
 int                                               const options,
 int                                               const nvars,
 int             const                  * restrict const srctypes,
 void            const * restrict const * restrict const srcptrs,
 int             const                  * restrict const dsttypes,
 void                  * restrict const * restrict const dstptrs);

int
Slab_Transfer
(cGH             const * restrict const cctkGH,
 int                              const dim,
 struct xferinfo const * restrict const xferinfo,
 int                              const options,
 int                              const srctype,
 void            const * restrict const srcptr,
 int                              const dsttype,
 void                  * restrict const dstptr);

#ifdef __cplusplus 
}
#endif

#endif /* defined SLAB_H */

```

make.code.defn:
```
# Main make.code.defn file for thorn Slab

# Source files in this directory
SRCS = slab.cc

# Subdirectories containing source files
SUBDIRS = 


```

slab.inc:
```
! -*-F90-*-

interface
   subroutine Slab_Transfer (ierr, cctkGH, dim, &
        src_gsh, src_lbnd, src_lsh, src_ash, &
        src_lbbox, src_ubbox, src_nghostzones, &
        src_off, src_str, src_len, &
        dst_gsh, dst_lbnd, dst_lsh, dst_ash, &
        dst_lbbox, dst_ubbox, dst_nghostzones, &
        dst_off, dst_str, dst_len, &
        xpose, flip, &
        options, &
        srctype, srcptr, &
        dsttype, dstptr)
     implicit none
     integer ierr
     CCTK_POINTER cctkGH 
     integer dim
     integer src_gsh(dim), src_lbnd(dim), src_lsh(dim), src_ash(dim)
     integer src_lbbox(dim), src_ubbox(dim), src_nghostzones(dim)
     integer src_off(dim), src_str(dim), src_len(dim)
     integer dst_gsh(dim), dst_lbnd(dim), dst_lsh(dim), dst_ash(dim)
     integer dst_lbbox(dim), dst_ubbox(dim), dst_nghostzones(dim)
     integer dst_off(dim), dst_str(dim), dst_len(dim)
     integer xpose(dim), flip(dim)
     integer options
     integer srctype
     CCTK_REAL srcptr(*)
     integer dsttype
     CCTK_REAL dstptr(*)
   end subroutine Slab_Transfer
end interface

```


combined_doc_context: ## README (README):
```
Cactus Code Thorn Slab
Thorn Author(s)     : Erik Schnetter <schnetter@cct.lsu.edu>
Thorn Maintainer(s) : Erik Schnetter <schnetter@cct.lsu.edu>
--------------------------------------------------------------------------

Purpose of the thorn:

This thorn provides routines that use MPI to transfer slabs of arrays
between processors.

There is a thorn TAT/SlabTest that contains additional routines for
testing this thorn.

```

## Documentation (.tex files):
Files: documentation.tex

```tex
=== documentation.tex ===
% *======================================================================*
%  Cactus Thorn template for ThornGuide documentation
%  Author: Ian Kelley
%  Date: Sun Jun 02, 2002
%
%  Thorn documentation in the latex file doc/documentation.tex 
%  will be included in ThornGuides built with the Cactus make system.
%  The scripts employed by the make system automatically include 
%  pages about variables, parameters and scheduling parsed from the 
%  relevent thorn CCL files.
%  
%  This template contains guidelines which help to assure that your     
%  documentation will be correctly added to ThornGuides. More 
%  information is available in the Cactus UsersGuide.
%                                                    
%  Guidelines:
%   - Do not change anything before the line
%       % START CACTUS THORNGUIDE",
%     except for filling in the title, author, date etc. fields.
%        - Each of these fields should only be on ONE line.
%        - Author names should be sparated with a \\ or a comma
%   - You can define your own macros are OK, but they must appear after
%     the START CACTUS THORNGUIDE line, and do not redefine standard 
%     latex commands.
%   - To avoid name clashes with other thorns, 'labels', 'citations', 
%     'references', and 'image' names should conform to the following 
%     convention:          
%       ARRANGEMENT_THORN_LABEL
%     For example, an image wave.eps in the arrangement CactusWave and 
%     thorn WaveToyC should be renamed to CactusWave_WaveToyC_wave.eps
%   - Graphics should only be included using the graphix package. 
%     More specifically, with the "includegraphics" command. Do
%     not specify any graphic file extensions in your .tex file. This 
%     will allow us (later) to create a PDF version of the ThornGuide
%     via pdflatex. |
%   - References should be included with the latex "bibitem" command. 
%   - use \begin{abstract}...\end{abstract} instead of \abstract{...}
%   - For the benefit of our Perl scripts, and for future extensions, 
%     please use simple latex.     
%
% *======================================================================* 
% 
% Example of including a graphic image:
%    \begin{figure}[ht]
% 	\begin{center}
%    	   \includegraphics[width=6cm]{MyArrangement_MyThorn_MyFigure}
% 	\end{center}
% 	\caption{Illustration of this and that}
% 	\label{MyArrangement_MyThorn_MyLabel}
%    \end{figure}
%
% Example of using a label:
%   \label{MyArrangement_MyThorn_MyLabel}
%
% Example of a citation:
%    \cite{MyArrangement_MyThorn_Author99}
%
% Example of including a reference
%   \bibitem{MyArrangement_MyThorn_Author99}
%   {J. Author, {\em The Title of the Book, Journal, or periodical}, 1 (1999), 
%   1--16. {\tt http://www.nowhere.com/}}
%
% *======================================================================* 

% If you are using CVS use this line to give version information

\documentclass{article}

% Use the Cactus ThornGuide style file
% (Automatically used from Cactus distribution, if you have a 
%  thorn without the Cactus Flesh download this from the Cactus
%  homepage at www.cactuscode.org)
\usepackage{../../../../doc/latex/cactus}

\begin{document}

% The author of the documentation
\author{Erik Schnetter \textless eschnetter@perimeterinstitute.ca\textgreater}

% The title of the document (not necessarily the name of the Thorn)
\title{Generic Hyperslabbing}

% the date your document was last changed, if your document is in CVS, 
% please use:
\date{$ $Date$ $}

\maketitle

% Do not delete next line
% START CACTUS THORNGUIDE

% Add all definitions used in this documentation here 
%   \def\mydef etc

% Add an abstract for this thorn's documentation
\begin{abstract}
The Slab thorn provides generic slabbing facilities.  A slab is a
sub-array of another array.  Both the array and the slab can be
multidimensional, and the slab can have a non-unit stride.  The Slab
thorn provides a routine to copy a slab from one array into a slab of
another array, while possibly transposing or mirroring the slab.  The
processor distribution of the arrays can be specified freely, so that
the Slab thorn can be used to interface to non-Cactus libraries with
different data layouts.

The Slab thorn is driver independent, i.e., not tied to PUGH or
Carpet, and does not require MPI for single-processor configurations.
\end{abstract}

% The following sections are suggestive only.
% Remove them or add your own.



\section{Introduction}

A \emph{Slab} is a subarray of another array.  This concept is used in
many places with many different names.  Fortran has so-called ``array
subscript triplets'', which represent the same thing.  In BLAS, the
``leading dimension'' arguments are used to described slabs.  Slabs
are sometimes also called ``array views'' in object oriented
applications.  Slabs are rectangular in shape, and can be rotated with
regard to their containing array by multiples of 90 degrees.  They can
also be mirrored, i.e., the direction of axes can be inverted.

It is often necessary to copy slabs from one array into other arrays,
or to copy one slab of an array to another slab of the same array.
This can be used to change the processor distribution of some data, or
to apply symmetry or periodicity boundary conditions, or to collect
data onto a single processor to process it more easily.



\section{Using This Thorn}

\begin{FunctionDescription}{Slab\_Transfer}{}
Transfer a slab contained in one array to a (possibly different) slab
of another (possibly the same) array

\begin{SynopsisSection}
\begin{Synopsis}{C}
\begin{verbatim}
INHERITS: Slab
USES INCLUDE HEADER: Slab.h

#include "cctk.h"
#include "Slab.h"

struct slabinfo {
  int gsh;
  int lbnd, lsh;
  int ash;
  int lbbox, ubbox, nghostzones;
  int off, str, len;
};

struct xferinfo {
  struct slabinfo src, dst;
  int xpose;
  int flip;
};

int Slab_Transfer (cGH                   * restrict const cctkGH,
                   int                              const dim,
                   struct xferinfo const * restrict const xferinfo,
                   int                              const options,
                   int                              const srctype,
                   void            const *          const srcptr,
                   int                              const dsttype,
                   void                  *          const dstptr);
\end{verbatim}
\end{Synopsis}
\end{SynopsisSection}

\begin{ResultSection}
\begin{Result}{0}
Success
\end{Result}
\begin{Result}{nonzero}
Failure
\end{Result}
\end{ResultSection}

\begin{ParameterSection}
\begin{Parameter}{cctkGH}
Pointer to the CCTK grid hierarchy
\end{Parameter}

\begin{Parameter}{dim}
Number of dimensions of the arrays and slabs.  Must be nonnegative.
\end{Parameter}

\begin{Parameter}{xferinfo[dim]}
Describes the layout of the slab transfer, i.e., the shape and
distribution of the source and destination arrays, and the locations
of the source and destination slabs, and a possible transformation
between the slabs.  Each dimension is described separately.  See the
entries \texttt{xferinfo[d].*} below.
\end{Parameter}

\begin{Parameter}{xferinfo[d].src}
Describes the source array and source slab.  See the \textit{slabinfo}
entries below.
\end{Parameter}

\begin{Parameter}{xferinfo[d].dst}
Describes the destination array and destination slab.  See the
\textit{slabinfo} entries below.
\end{Parameter}

\begin{Parameter}{xferinfo[d].xpose}
Describes how to transpose the slab, i.e., possibly permuting the slab
axes, as in $(x,y) \rightarrow (y,x)$.  \texttt{xferinfo[d].xpose}
contains an integer value in the range \texttt{0\ldots dim-1},
specifying the source axis corresponding to the destination axis
\texttt{d}.  Specify \texttt{xferinfo[d].xpose = d} for no
transposition.  No two values of \texttt{xferinfo[*].xpose} may be the
same.
\end{Parameter}

\begin{Parameter}{xferinfo[d].flip}
Describes how to mirror the slab, i.e., possibly inverting the slab
axes, as in $(x) \rightarrow (-x)$.  \texttt{xferinfo[d].flip}
containes a boolean value specifying whether the axis in direction
\texttt{d} should be inverted, i.e., either \texttt{0} or \texttt{1},
where \texttt{0} indicates no inversion, and \texttt{1} indicates
inversion.

When axes are both transposed and inverted while a slab is copied,
then the transposing happens first, and the axis inversion later.
That is, the sequence of ``actions'' is: extract slab, transpose,
invert, insert slab.  For example, when transposing the $x$ and $z$
axes and inverting the $x$ axis, then the destination slab's $x$ axis
is the source slab's flipped $z$ axis, while the destination $z$ axis
is the unflipped source $x$ axis.
\end{Parameter}

\begin{Parameter}{\textit{slab}}
Describes the shape and processor distribution of one dimension of an
array, and the location of this dimension of a slab.  The shape and
distribution is specified in the same manner as in the \texttt{cGH}
structure.  See the entries \texttt{\textit{slab}.*} below.
\end{Parameter}

\begin{Parameter}{\textit{slab}.gsh}
Global shape of the array; the overall number of grid points
\end{Parameter}

\begin{Parameter}{\textit{slab}.lbnd}
Lower boundary of the array; the global index of the lower boundary of
the processor-local part of the array
\end{Parameter}

\begin{Parameter}{\textit{slab}.lsh}
Local shape of the array; the number of grid points on this processor
\end{Parameter}

\begin{Parameter}{\textit{slab}.ash}
Local allocated shape of the array; the number of grid points on this
processor including padding
\end{Parameter}

\begin{Parameter}{\textit{slab}.lbbox}
Lower bounding box of the array; whether the lower boundary of the
array is an outer boundary.  This corresponds to the even entries in
Cactus' \texttt{bbox} array.  Must be \texttt{0} or \texttt{1}.
\end{Parameter}

\begin{Parameter}{\textit{slab}.ubbox}
Upper bounding box of the array; whether the upper boundary of the
array is an outer boundary.  This corresponds to the odd entries in
Cactus' \texttt{bbox} array.  Must be \texttt{0} or \texttt{1}.
\end{Parameter}

\begin{Parameter}{\textit{slab}.nghostzones}
Number of ghost zones of the array; the number of grid points on this
processor that are only copied from neighbouring processors
\end{Parameter}

\begin{Parameter}{\textit{slab}.off}
Slab offset; the global index of the lowest grid point of the slab
\end{Parameter}

\begin{Parameter}{\textit{slab}.str}
Slab stride; the distance between two grid points making up the slab.
Specify \texttt{\textit{slab}.str = 1} for a unit stride.  Must be
positive.
\end{Parameter}

\begin{Parameter}{\textit{slab}.len}
Slab length; the number of grid points that make up the slab.  This
does not count the grid points that are skipped if the slab has a
non-unit stride.  Must be nonnegative.
\end{Parameter}

\begin{Parameter}{options}
Handle of an option table.  Two options are understood,
\texttt{CCTK\_INT useghosts} and \texttt{CCTK\_POINTER comm}.
\texttt{useghosts} (see below) specifies whether values in ghost zones
may be used in the source array.  \texttt{comm} is a pointer to an MPI
communicator that should be used instead of \texttt{MPI\_COMM\_WORLD}
or the communicator returned by the aliased function
\texttt{GetMPICommWorld}.
\end{Parameter}

\begin{Parameter}{srctype}
Type of the source array.  Pass the corresponding
\texttt{CCTK\_VARIABLE\_*} constant.
\end{Parameter}

\begin{Parameter}{srcptr}
Pointer to the source array
\end{Parameter}

\begin{Parameter}{dsttype}
Type of the destination array.  Pass the corresponding
\texttt{CCTK\_VARIABLE\_*} constant.
\end{Parameter}

\begin{Parameter}{dstptr}
Pointer to the destination array
\end{Parameter}
\end{ParameterSection}

\begin{Discussion}
\texttt{Slab\_Transfer} copies one slab from one array onto a possibly
different slab in possibly the same array.  The shape and processor
distribution of the arrays can be specified freely, as long as each
processor holds a rectangular subset of grid points.  The location of
the slab can also be specified freely, and the slab can be rotated (by
multiples of 90 degrees) or inverted before it is copied.  The source
and destination slab are allowed to overlap.

By default, \texttt{Slab\_Transfer} is conservative with regard to
synchronisation.  It assumes that the ghost zones of the source array
are not valid, but will correctly fill in the ghost zones in the
destination slab.  This can be changed by setting the option
\texttt{useghosts} in the option table.

There are currently some restrictions, which can be remedied if the
need arises: The dimension must be 3 (lower dimensions can easily be
padded).  The communication schedule is set up for every slab
transfer, which is expensive.  The number of ghost zones along the
lower and upper boundary is assumed to be the same.  There is no
Fortran interface.
\end{Discussion}

\begin{SeeAlsoSection}
\begin{SeeAlso}{CarpetSlab}
The hyperslabbing thorn of the driver Carpet.
\end{SeeAlso}

\begin{SeeAlso}{PUGHSlab}
The hyperslabbing thorn of the driver CactusPUGH.
\end{SeeAlso}

\begin{SeeAlso}{New hyperslabbing API}
The web page
\texttt{http://www.cactuscode.org/Development/Current.html} contains a
hyperlink to the new proposed hyperslabbing API\@.  This API is slightly
different from the one used here.
\end{SeeAlso}
\end{SeeAlsoSection}

\begin{ExampleSection}
\begin{Example}{C}
The identity transfer: copy a whole 3D grid function without
transforming it

\begin{verbatim}
INHERITS: Slab
USES INCLUDE HEADER: Slab.h

#include <assert.h>
#include "cctk.h"
#include "cctk_Arguments.h"

#include "Slab.h"

DECLARE_CCTK_ARGUMENTS;

struct xferinfo info[3];
int d;
int ierr;

/* Set up the array descriptors */
assert (cctk_dim <= 3);
for (d=0; d<cctk_dim; ++d) {
  info[d].src.gsh = cctk_gsh[d];
  info[d].src.lbnd = cctk_lbnd[d];
  info[d].src.lsh = cctk_lsh[d];
  info[d].src.ash = cctk_ash[d];
  info[d].src.lbbox = cctk_bbox[2*d];
  info[d].src.ubbox = cctk_bbox[2*d+1];
  info[d].src.nghostzones = cctk_nghostzones[d];
  info[d].dst.gsh = cctk_gsh[d];
  info[d].dst.lbnd = cctk_lbnd[d];
  info[d].dst.lsh = cctk_lsh[d];
  info[d].dst.ash = cctk_ash[d];
  info[d].dst.lbbox = cctk_bbox[2*d];
  info[d].dst.ubbox = cctk_bbox[2*d+1];
  info[d].dst.nghostzones = cctk_nghostzones[d];
}

/* Set up the slab and transformation descriptors */
assert (cctk_dim <= 3);
for (d=0; d<cctk_dim; ++d) {
  info[d].src.off = 0;
  info[d].src.len = cctk_gsh[d];
  info[d].src.str = 1;
  info[d].dst.off = 0;
  info[d].dst.len = cctk_gsh[d];
  info[d].dst.str = 1;
  info[d].xpose = d;
  info[d].flip = 0;
}

/* Transfer the slab */
ierr = Slab_Transfer
  (cctkGH, cctk_dim, info, -1,
   CCTK_VARIABLE_REAL, gf1, CCTK_VARIABLE_REAL, gf2);
assert (!ierr);
\end{verbatim}
\end{Example}

\begin{Example}{C}
A complicated transfer: copy a subarray, transposing the $x$ and $z$
axes, and inverting the $x$ axis

\begin{verbatim}
INHERITS: Slab
USES INCLUDE HEADER: Slab.h

#include <assert.h>
#include "cctk.h"
#include "cctk_Arguments.h"

#include "Slab.h"

DECLARE_CCTK_ARGUMENTS;

struct xferinfo info[3];
int d;
int ierr;

/* Set up the array descriptors (same as above) */
assert (cctk_dim <= 3);
for (d=0; d<cctk_dim; ++d) {
  info[d].src.gsh = cctk_gsh[d];
  info[d].src.lbnd = cctk_lbnd[d];
  info[d].src.lsh = cctk_lsh[d];
  info[d].src.ash = cctk_ash[d];
  info[d].src.lbbox = cctk_bbox[2*d];
  info[d].src.ubbox = cctk_bbox[2*d+1];
  info[d].src.nghostzones = cctk_nghostzones[d];
  info[d].dst.gsh = cctk_gsh[d];
  info[d].dst.lbnd = cctk_lbnd[d];
  info[d].dst.lsh = cctk_lsh[d];
  info[d].dst.ash = cctk_ash[d];
  info[d].dst.lbbox = cctk_bbox[2*d];
  info[d].dst.ubbox = cctk_bbox[2*d+1];
  info[d].dst.nghostzones = cctk_nghostzones[d];
}

/* Set up the slab and transformation descriptors */
info[0].src.off = 5;
info[1].src.off = 5;
info[2].src.off = 1;
info[0].src.str = 1;
info[1].src.str = 1;
info[2].src.str = 1;
info[0].src.len = 2;
info[1].src.len = 3;
info[2].src.len = 2;

info[0].dst.off = 2;
info[1].dst.off = 3;
info[2].dst.off = 4;
info[0].dst.str = 1;
info[1].dst.str = 1;
info[2].dst.str = 1;
info[0].dst.len = 2;
info[1].dst.len = 3;
info[2].dst.len = 2;

info[0].xpose = 2;
info[1].xpose = 1;
info[2].xpose = 0;
info[0].flip = 1;
info[1].flip = 0;
info[2].flip = 0;

/* Transfer the slab */
ierr = Slab_Transfer
  (cctkGH, cctk_dim, info, -1,
   CCTK_VARIABLE_REAL, gf1, CCTK_VARIABLE_REAL, gf2);
assert (!ierr);
\end{verbatim}
\end{Example}
\end{ExampleSection}

\end{FunctionDescription}



\section{Obtaining This Thorn}

This thorn is available from the \texttt{CactusNumerical} arrangement in the
\texttt{arrangements} directory on the \texttt{svn.cactuscode.org}
CVS server via anonymous pserver access.


\section{Support and Feedback}

This thorn was written by Erik Schnetter \textless
eschnetter@perimeterinstitute.ca\textgreater\ in 2002.  Contact me for
questions and comments.

% Do not delete next line
% END CACTUS THORNGUIDE

\end{document}

```



Reference solution:
src_code: // TODO:
// Provide facilities for dim > 3
// Set up the slab exchange information in advance
// Slab in several stages
// Test slabbing without MPI
// Allow not setting the ghost zones
// Allow not using / not setting the boundaries
// Allow different numbers of ghost zones at the lower and upper boundary



// Print debug information?
#undef DEBUG

// Perform expensive self-checks?
#undef CHECK

// Omit all self-checks?  (Overrides CHECK)
#undef NDEBUG

// Byte value for poison checks: use 255 for nan, or e.g. 113 for a
// large value
#define POISON_VALUE 254



#include <cassert>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <vector>

#include <cctk.h>
#include <cctk_Arguments.h>
#include <cctk_Parameters.h>
#include <cctk_DefineThorn.h>
#include <util_ErrorCodes.h>
#include <util_Table.h>

#include <loopcontrol.h>

#ifdef CCTK_MPI
#  include <mpi.h>
#  define HAVE_MPI 1
#else
#  define HAVE_MPI 0
#endif

#include "slab.h"

using namespace std;



#ifdef DEBUG
#  define ifdebug
#else
#  define ifdebug while (0)
#endif

#ifdef CHECK
#  define ifcheck
#else
#  define ifcheck while (0)
#endif

#ifdef NDEBUG
#  define check(x) ((x) ? 0 : CCTK_WARN (0, "internal error"))
#else
#  define check(x) assert (x)
#endif



static int timer_init                  = -1;
static int timer_apply                 = -1;
static int timer_copy_in               = -1;
static int timer_copy_in_noxpose       = -1;
static int timer_copy_in_xposexy       = -1;
static int timer_copy_in_xposegeneral  = -1;
static int timer_copy_in_general       = -1;
static int timer_xfer                  = -1;
static int timer_copy_back             = -1;
static int timer_copy_back_noflip      = -1;
static int timer_copy_back_flipx       = -1;
static int timer_copy_back_flipy       = -1;
static int timer_copy_back_flipxy      = -1;
static int timer_copy_back_flipgeneral = -1;
static int timer_copy_back_general     = -1;



extern "C"
void
Slab_InitTimers (CCTK_ARGUMENTS)
{
  DECLARE_CCTK_ARGUMENTS;
  timer_init                  = CCTK_TimerCreate ("Slab/init"                 );
  timer_apply                 = CCTK_TimerCreate ("Slab/apply"                );
  timer_copy_in               = CCTK_TimerCreate ("Slab/copy_in"              );
  timer_copy_in_noxpose       = CCTK_TimerCreate ("Slab/copy_in_noxpose"      );
  timer_copy_in_xposexy       = CCTK_TimerCreate ("Slab/copy_in_xposexy"      );
  timer_copy_in_xposegeneral  = CCTK_TimerCreate ("Slab/copy_in_xposegeneral" );
  timer_copy_in_general       = CCTK_TimerCreate ("Slab/copy_in_general"      );
  timer_xfer                  = CCTK_TimerCreate ("Slab/xfer"                 );
  timer_copy_back             = CCTK_TimerCreate ("Slab/copy_back"            );
  timer_copy_back_noflip      = CCTK_TimerCreate ("Slab/copy_back_noflip"     );
  timer_copy_back_flipx       = CCTK_TimerCreate ("Slab/copy_back_flipx"      );
  timer_copy_back_flipy       = CCTK_TimerCreate ("Slab/copy_back_flipy"      );
  timer_copy_back_flipxy      = CCTK_TimerCreate ("Slab/copy_back_flipxy"     );
  timer_copy_back_flipgeneral = CCTK_TimerCreate ("Slab/copy_back_flipgeneral");
  timer_copy_back_general     = CCTK_TimerCreate ("Slab/copy_back_general"    );
}

extern "C"
int
Slab_PrintTimers ()
{
  CCTK_TimerPrintDataI (timer_init                 , -1);
  CCTK_TimerPrintDataI (timer_apply                , -1);
  CCTK_TimerPrintDataI (timer_copy_in              , -1);
  CCTK_TimerPrintDataI (timer_copy_in_noxpose      , -1);
  CCTK_TimerPrintDataI (timer_copy_in_xposexy      , -1);
  CCTK_TimerPrintDataI (timer_copy_in_xposegeneral , -1);
  CCTK_TimerPrintDataI (timer_copy_in_general      , -1);
  CCTK_TimerPrintDataI (timer_xfer                 , -1);
  CCTK_TimerPrintDataI (timer_copy_back            , -1);
  CCTK_TimerPrintDataI (timer_copy_back_noflip     , -1);
  CCTK_TimerPrintDataI (timer_copy_back_flipx      , -1);
  CCTK_TimerPrintDataI (timer_copy_back_flipy      , -1);
  CCTK_TimerPrintDataI (timer_copy_back_flipxy     , -1);
  CCTK_TimerPrintDataI (timer_copy_back_flipgeneral, -1);
  CCTK_TimerPrintDataI (timer_copy_back_general    , -1);
  return 0;
}



// Find out which driver to use
#ifdef CCTK_MPI
#  if defined CARPET_CARPET
#    include "Carpet/Carpet/src/carpet_public.h"
#  endif
#  if defined CACTUSPUGH_PUGH
#    include "CactusPUGH/PUGH/src/include/pugh.h"
#  endif
#endif



#ifdef CCTK_MPI
// Determine MPI type sizes

#  define CACTUS_MPI_BYTE MPI_CHAR

#  define CACTUS_MPI_INT1 MPI_CHAR

#  if SIZEOF_SHORT_INT == 2
#    define CACTUS_MPI_INT2 MPI_SHORT
#  elif SIZEOF_INT == 2
#    define CACTUS_MPI_INT2 MPI_INT
#  elif SIZEOF_LONG_INT == 2
#    define CACTUS_MPI_INT2 MPI_LONG
#  elif SIZEOF_LONG_LONG == 2
#    define CACTUS_MPI_INT2 MPI_LONG_LONG_INT
#  endif

#  if SIZEOF_SHORT_INT == 4
#    define CACTUS_MPI_INT4 MPI_SHORT
#  elif SIZEOF_INT == 4
#    define CACTUS_MPI_INT4 MPI_INT
#  elif SIZEOF_LONG_INT == 4
#    define CACTUS_MPI_INT4 MPI_LONG
#  elif SIZEOF_LONG_LONG == 4
#    define CACTUS_MPI_INT4 MPI_LONG_LONG_INT
#  endif

#  if SIZEOF_SHORT_INT == 8
#    define CACTUS_MPI_INT8 MPI_SHORT
#  elif SIZEOF_INT == 8
#    define CACTUS_MPI_INT8 MPI_INT
#  elif SIZEOF_LONG_INT == 8
#    define CACTUS_MPI_INT8 MPI_LONG
#  elif SIZEOF_LONG_LONG == 8
#    define CACTUS_MPI_INT8 MPI_LONG_LONG_INT
#  endif

#  if SIZEOF_SHORT_INT == 16
#    define CACTUS_MPI_INT16 MPI_SHORT
#  elif SIZEOF_INT == 16
#    define CACTUS_MPI_INT16 MPI_INT
#  elif SIZEOF_LONG_INT == 16
#    define CACTUS_MPI_INT16 MPI_LONG
#  elif SIZEOF_LONG_LONG == 16
#    define CACTUS_MPI_INT16 MPI_LONG_LONG_INT
#  endif

#  if SIZEOF_FLOAT == 4
#    define CACTUS_MPI_REAL4 MPI_FLOAT
#  elif SIZEOF_DOUBLE == 4
#    define CACTUS_MPI_REAL4 MPI_DOUBLE
#  elif SIZEOF_LONG_DOUBLE == 4
#    define CACTUS_MPI_REAL4 MPI_LONG_DOUBLE
#  endif

#  if SIZEOF_FLOAT == 8
#    define CACTUS_MPI_REAL8 MPI_FLOAT
#  elif SIZEOF_DOUBLE == 8
#    define CACTUS_MPI_REAL8 MPI_DOUBLE
#  elif SIZEOF_LONG_DOUBLE == 8
#    define CACTUS_MPI_REAL8 MPI_LONG_DOUBLE
#  endif

#  if SIZEOF_FLOAT == 16
#    define CACTUS_MPI_REAL16 MPI_FLOAT
#  elif SIZEOF_DOUBLE == 16
#    define CACTUS_MPI_REAL16 MPI_DOUBLE
#  elif SIZEOF_LONG_DOUBLE == 16
#    define CACTUS_MPI_REAL16 MPI_LONG_DOUBLE
#  endif

static MPI_Datatype CACTUS_MPI_COMPLEX8;
static MPI_Datatype CACTUS_MPI_COMPLEX16;
static MPI_Datatype CACTUS_MPI_COMPLEX32;

#endif



// Replace MPI functions if MPI is disabled
#ifndef CCTK_MPI

typedef int MPI_Comm;

typedef enum {
  CACTUS_MPI_BYTE      = CCTK_VARIABLE_BYTE,
  CACTUS_MPI_INT       = CCTK_VARIABLE_INT,
  CACTUS_MPI_INT1      = CCTK_VARIABLE_INT1,
  CACTUS_MPI_INT2      = CCTK_VARIABLE_INT2,
  CACTUS_MPI_INT4      = CCTK_VARIABLE_INT4,
  CACTUS_MPI_INT8      = CCTK_VARIABLE_INT8,
  CACTUS_MPI_INT16     = CCTK_VARIABLE_INT16,
  CACTUS_MPI_REAL      = CCTK_VARIABLE_REAL,
  CACTUS_MPI_REAL4     = CCTK_VARIABLE_REAL4,
  CACTUS_MPI_REAL8     = CCTK_VARIABLE_REAL8,
  CACTUS_MPI_REAL16    = CCTK_VARIABLE_REAL16,
  CACTUS_MPI_COMPLEX   = CCTK_VARIABLE_COMPLEX,
  CACTUS_MPI_COMPLEX8  = CCTK_VARIABLE_COMPLEX8,
  CACTUS_MPI_COMPLEX16 = CCTK_VARIABLE_COMPLEX16,
  CACTUS_MPI_COMPLEX32 = CCTK_VARIABLE_COMPLEX32
} MPI_Datatype;

static MPI_Datatype MPI_INT;

typedef enum { MPI_MIN, MPI_MAX } MPI_Op;

static int
MPI_Barrier (MPI_Comm comm)
{
  return 0;
}

static int
MPI_Bcast (void * buffer, int count,
           MPI_Datatype datatype, int root, MPI_Comm comm)
{
  assert (buffer);
  assert (count >= 0);
  assert (root == 0);
  return 0;
}

static int
MPI_Comm_size (MPI_Comm comm, int * size)
{
  *size = 1;
  return 0;
}

static int
MPI_Comm_rank (MPI_Comm comm, int * rank)
{
  *rank = 0;
  return 0;
}

static int
MPI_Allgather (void * sendbuf, int sendcnt, int sendtype,
	       void * recvbuf, int recvcnt, int recvtype,
	       MPI_Comm comm)
{
  int recvsize;
  assert (sendbuf);
  assert (recvbuf);
  assert (sendcnt == recvcnt);
  assert (recvcnt >= 0);
  assert (sendtype == recvtype);
  recvsize = CCTK_VarTypeSize (recvtype);
  assert (recvsize > 0);
  memcpy (recvbuf, sendbuf, recvcnt * recvsize);
  return 0;
}

static int
MPI_Alltoall (void * sendbuf, int sendcnt, int sendtype,
	      void * recvbuf, int recvcnt, int recvtype,
	      MPI_Comm comm)
{
  int recvsize;
  assert (sendbuf);
  assert (recvbuf);
  assert (sendcnt == recvcnt);
  assert (recvcnt >= 0);
  assert (sendtype == recvtype);
  recvsize = CCTK_VarTypeSize (recvtype);
  assert (recvsize > 0);
  memcpy (recvbuf, sendbuf, recvcnt * recvsize);
  return 0;
}

static int
MPI_Alltoallv (void * sendbuf, int * sendcnt, int * sendoff, int sendtype,
	       void * recvbuf, int * recvcnt, int * recvoff, int recvtype,
	       MPI_Comm comm)
{
  int recvsize;
  assert (sendbuf);
  assert (recvbuf);
  assert (sendcnt);
  assert (recvcnt);
  assert (*sendcnt == *recvcnt);
  assert (*recvcnt >= 0);
  assert (sendoff);
  assert (recvoff);
  assert (*sendoff == 0);
  assert (*recvoff == 0);
  assert (sendtype == recvtype);
  recvsize = CCTK_VarTypeSize (recvtype);
  assert (recvsize > 0);
  memcpy (recvbuf, sendbuf, *recvcnt * recvsize);
  return 0;
}

static int
MPI_Allreduce (void * sendbuf, void * recvbuf, int count,
               MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)
{
  int recvsize;
  assert (sendbuf);
  assert (recvbuf);
  assert (count >= 0);
  recvsize = CCTK_VarTypeSize (datatype);
  assert (recvsize > 0);
  memcpy (recvbuf, sendbuf, count * recvsize);
  return 0;
}

typedef int MPI_Request;        // dummy
typedef int MPI_Status;         // dummy
#define MPI_REQUEST_NULL 0      // dummy
#define MPI_STATUSES_IGNORE 0   // dummy

static int
MPI_Irecv (void *buf, int count, MPI_Datatype datatype,
           int source, int tag, MPI_Comm comm, MPI_Request *request)
{
  abort();
}

static int
MPI_Isend (void *buf, int count, MPI_Datatype datatype, int dest,
           int tag, MPI_Comm comm, MPI_Request *request)
{
  abort();
}

static int
MPI_Waitall (int count, MPI_Request *array_of_requests,
             MPI_Status *array_of_statuses)
{
  abort();
}

#endif



// Get the MPI COMM_WOLRD communicator from the driver
static MPI_Comm
get_mpi_comm (cGH const * restrict const cctkGH)
{
#ifdef CCTK_MPI
  if (CCTK_IsFunctionAliased ("GetMPICommWorld")) {
    return * (MPI_Comm const *) GetMPICommWorld (cctkGH);
  }
#  if defined CACTUSPUGH_PUGH
  {
    static int PUGH_active = -1;
    if (PUGH_active == -1) PUGH_active = CCTK_IsThornActive ("PUGH");
    assert (PUGH_active >= 0);
    if (PUGH_active) return PUGH_pGH(cctkGH)->PUGH_COMM_WORLD;
  }
#  endif
  return MPI_COMM_WORLD;
#else
  return 0;
#endif
}



// Initialise the MPI datatypes for complex variables
extern "C"
int
Slab_InitMPIDatatypes ()
{
#ifdef CCTK_MPI
#  ifdef HAVE_CCTK_REAL4
  assert (CACTUS_MPI_REAL4 != MPI_DATATYPE_NULL);
  MPI_Type_contiguous (2, CACTUS_MPI_REAL4, &CACTUS_MPI_COMPLEX8);
  MPI_Type_commit (&CACTUS_MPI_COMPLEX8);
#  endif
#  ifdef HAVE_CCTK_REAL8
  assert (CACTUS_MPI_REAL8 != MPI_DATATYPE_NULL);
  MPI_Type_contiguous (2, CACTUS_MPI_REAL8, &CACTUS_MPI_COMPLEX16);
  MPI_Type_commit (&CACTUS_MPI_COMPLEX16);
#  endif
#  ifdef HAVE_CCTK_REAL16
  if (CACTUS_MPI_REAL16 != MPI_DATATYPE_NULL) {
    MPI_Type_contiguous (2, CACTUS_MPI_REAL16, &CACTUS_MPI_COMPLEX32);
    MPI_Type_commit (&CACTUS_MPI_COMPLEX32);
  } else {
    // CCTK_REAL16 is not supported by MPI
    CCTK_WARN (CCTK_WARN_ALERT,
               "CCTK_REAL16 support is enabled in Cactus, but is not supported by MPI. All MPI operations with this datatype will fail.");
    CACTUS_MPI_COMPLEX32 = MPI_DATATYPE_NULL;
  }
#  endif
#endif
  
#ifndef CCTK_MPI
  switch (sizeof(int)) {
#ifdef HAVE_CCTK_INT1
  case sizeof(CCTK_INT1): MPI_INT = CACTUS_MPI_INT1; break;
#endif
#ifdef HAVE_CCTK_INT2
  case sizeof(CCTK_INT2): MPI_INT = CACTUS_MPI_INT2; break;
#endif
#ifdef HAVE_CCTK_INT4
  case sizeof(CCTK_INT4): MPI_INT = CACTUS_MPI_INT4; break;
#endif
#ifdef HAVE_CCTK_INT8
  case sizeof(CCTK_INT8): MPI_INT = CACTUS_MPI_INT8; break;
#endif
  default: CCTK_BUILTIN_UNREACHABLE();
  }
#endif
  
  return 0;
}



// Normalise a Cactus datatype
static int
normal_type (int cactustype)
{
  switch (cactustype) {
  case CCTK_VARIABLE_INT:
#ifdef CCTK_INTEGER_PRECISION_1
    return CCTK_VARIABLE_INT1;
#endif
#ifdef CCTK_INTEGER_PRECISION_2
    return CCTK_VARIABLE_INT2;
#endif
#ifdef CCTK_INTEGER_PRECISION_4
    return CCTK_VARIABLE_INT4;
#endif
#ifdef CCTK_INTEGER_PRECISION_8
    return CCTK_VARIABLE_INT8;
#endif
#ifdef CCTK_INTEGER_PRECISION_16
    return CCTK_VARIABLE_INT16;
#endif
    CCTK_BUILTIN_UNREACHABLE();
  case CCTK_VARIABLE_REAL:
#ifdef CCTK_REAL_PRECISION_4
    return CCTK_VARIABLE_REAL4;
#endif
#ifdef CCTK_REAL_PRECISION_8
    return CCTK_VARIABLE_REAL8;
#endif
#ifdef CCTK_REAL_PRECISION_16
    return CCTK_VARIABLE_REAL16;
#endif
    CCTK_BUILTIN_UNREACHABLE();
  case CCTK_VARIABLE_COMPLEX:
#ifdef CCTK_REAL_PRECISION_4
    return CCTK_VARIABLE_COMPLEX8;
#endif
#ifdef CCTK_REAL_PRECISION_8
    return CCTK_VARIABLE_COMPLEX16;
#endif
#ifdef CCTK_REAL_PRECISION_16
    return CCTK_VARIABLE_COMPLEX32;
#endif
    CCTK_BUILTIN_UNREACHABLE();
  }
  return cactustype;
}



// Find the MPI datatype corresponding to a Cactus datatype
static MPI_Datatype
mpi_type (int const cactustype)
{
  int const normaltype = normal_type (cactustype);
  switch (normaltype) {
  case CCTK_VARIABLE_BYTE: return CACTUS_MPI_BYTE;
#ifdef HAVE_CCTK_INT1
  case CCTK_VARIABLE_INT1: return CACTUS_MPI_INT1;
#endif
#ifdef HAVE_CCTK_INT2
  case CCTK_VARIABLE_INT2: return CACTUS_MPI_INT2;
#endif
#ifdef HAVE_CCTK_INT4
  case CCTK_VARIABLE_INT4: return CACTUS_MPI_INT4;
#endif
#ifdef HAVE_CCTK_INT8
  case CCTK_VARIABLE_INT8: return CACTUS_MPI_INT8;
#endif
// #ifdef HAVE_CCTK_INT16
//   case CCTK_VARIABLE_INT16: return CACTUS_MPI_INT16;
// #endif
#ifdef HAVE_CCTK_REAL4
  case CCTK_VARIABLE_REAL4: return CACTUS_MPI_REAL4;
  case CCTK_VARIABLE_COMPLEX8: return CACTUS_MPI_COMPLEX8;
#endif
#ifdef HAVE_CCTK_REAL8
  case CCTK_VARIABLE_REAL8: return CACTUS_MPI_REAL8;
  case CCTK_VARIABLE_COMPLEX16: return CACTUS_MPI_COMPLEX16;
#endif
#ifdef HAVE_CCTK_REAL16
  case CCTK_VARIABLE_REAL16: return CACTUS_MPI_REAL16;
  case CCTK_VARIABLE_COMPLEX32: return CACTUS_MPI_COMPLEX32;
#endif
  }
  CCTK_ERROR("Unsupported datatype");
}



////////////////////////////////////////////////////////////////////////////////



struct bbox {
  int off;                      // offset
  int len;                      // length
  int alen;                     // allocated length
  int str;                      // stride
};

struct arrays {
  bbox global;                  // global region (all processors)
  bbox local;                   // this processor's region
  bbox active;                  // ???
  bbox slab;                    // the slab that should be transferred
};

struct xfer {
  arrays src;                   // source region
  arrays dst;                   // destination region
  int xpose;                    // exchange axes
  int flip;                     // change directions of axes
};



static inline int
roundup (int const x, int const y)
{
  assert (x >= 0);
  assert (y > 0);
  return (x + y - 1) / y * y;
}



static void
bbox_print (bbox const * restrict const bbox)
{
  assert (bbox);
  printf
    ("[%d:%d:%d]",
     bbox->off, bbox->off + (bbox->len - 1) * bbox->str, bbox->str);
}

static void
bbox_check (bbox const * restrict const bbox)
{
  assert (bbox);
  assert (bbox->len >= 0);
  assert (bbox->alen >= bbox->len);
  assert (bbox->str > 0);
}

static void
global2bbox (slabinfo const * restrict const slab,
             bbox           * restrict const bbox)
{
  assert (slab);
  assert (bbox);
  assert (slab->gsh >= 0);
  bbox->off = 0;
  bbox->len = slab->gsh;
  bbox->alen = slab->gsh;
  bbox->str = 1;
  bbox_check (bbox);
}

static void
local2bbox (slabinfo const * restrict const slab,
            bbox           * restrict const bbox)
{
  assert (slab);
  assert (bbox);
  assert (slab->lbnd >= 0);
  assert (slab->lsh >= 0);
  assert (slab->ash >= slab->lsh);
  assert (slab->lbnd + slab->lsh <= slab->gsh);
  bbox->off = slab->lbnd;
  bbox->len = slab->lsh;
  bbox->alen = slab->ash;
  bbox->str = 1;
  bbox_check (bbox);
}

static void
active2bbox (slabinfo const * restrict const slab,
             bbox           * restrict const bbox,
             int                       const useghosts)
{
  assert (slab);
  assert (bbox);
  assert (useghosts == 0 or useghosts == 1);
  assert (slab->lbnd >= 0);
  assert (slab->lsh >= 0);
  assert (slab->ash >= slab->lsh);
  assert (slab->lbnd + slab->lsh <= slab->gsh);
  assert (slab->lbbox == 0 or slab->lbbox == 1);
  assert (slab->ubbox == 0 or slab->ubbox == 1);
  assert (slab->nghostzones >= 0);
  int const nlghostzones = slab->lbbox or useghosts ? 0 : slab->nghostzones;
  int const nughostzones = slab->ubbox or useghosts ? 0 : slab->nghostzones;
  bbox->off = slab->lbnd + nlghostzones;
  bbox->len = slab->lsh - nlghostzones - nughostzones;
  bbox->alen = slab->ash;
  bbox->str = 1;
  bbox_check (bbox);
}

static void
slab2bbox (slabinfo const * restrict const slab,
           bbox           * restrict const bbox)
{
  assert (slab);
  assert (bbox);
  bbox->off = slab->off;
  bbox->len = slab->len;
  bbox->alen = slab->len;
  bbox->str = slab->str;
  bbox_check (bbox);
}

static int
bbox_iscontained (bbox const * restrict const inner,
                  bbox const * restrict const outer)
{
  bbox_check (inner);
  bbox_check (outer);
  int const inner_last = inner->off + (inner->len - 1) * inner->str;
  int const outer_last = outer->off + (outer->len - 1) * outer->str;
  return inner->off >= outer->off and inner_last <= outer_last;
}

static void
bbox_clip (bbox       * restrict const inner,
           bbox const * restrict const outer)
{
  bbox_check (inner);
  bbox_check (outer);
  int inner_last = inner->off + (inner->len - 1) * inner->str;
  int const outer_last = outer->off + (outer->len - 1) * outer->str;
  if (inner->off < outer->off) {
    inner->off += roundup (outer->off - inner->off, inner->str);
  }
  if (inner_last > outer_last) {
    inner_last -= roundup (inner_last - outer_last, inner->str);
  }
  assert ((inner_last - inner->off) % inner->str == 0);
  if (inner_last >= inner->off) {
    inner->len = (inner_last - inner->off + inner->str) / inner->str;
  } else {
    inner->len = 0;
  }
  bbox_check (inner);
}

// ydst = xdst + Flip (ysrc - xsrc)]
// This function is its own inverse.
static void
bbox_xform (bbox       * restrict const ydst,
            bbox const * restrict const ysrc,
            bbox const * restrict const xdst,
            bbox const * restrict const xsrc,
            int                   const flip)
{
  assert (ydst);
  bbox_check (ysrc);
  bbox_check (xdst);
  bbox_check (xsrc);
  assert (ysrc->str == xsrc->str);
  /* int const xsrc_last = xsrc->off + (xsrc->len - 1) * xsrc->str; */
  int const xdst_last = xdst->off + (xdst->len - 1) * xdst->str;
  int const ysrc_last = ysrc->off + (ysrc->len - 1) * ysrc->str;
  ydst->str = xdst->str;
  assert ((ysrc->off - xsrc->off) % ysrc->str == 0);
  ydst->off = xdst->off + (ysrc->off - xsrc->off) / ysrc->str * ydst->str;
  int ydst_last =
    xdst->off + (ysrc_last - xsrc->off) / ysrc->str * ydst->str;
  if (flip) {
    int const off = ydst->off;
    int const last = ydst_last;
    ydst->off = xdst->off + xdst_last - last;
    ydst_last = xdst_last - (off - xdst->off);
  }
  assert ((ysrc_last - xsrc->off) % ysrc->str == 0);
  assert (ydst_last - ydst->off + ydst->str >= 0);
  ydst->len = (ydst_last - ydst->off + ydst->str) / ydst->str;
  ydst->alen = xdst->alen;
  bbox_check (ydst);
}



extern "C"
void
print_slabinfo (FILE           *          const out,
                slabinfo const * restrict const slabinfo)
{
  fprintf (out, "    gsh: %d\n", slabinfo->gsh);
  fprintf (out, "    lbnd: %d, lsh: %d\n", slabinfo->lbnd, slabinfo->lsh);
  fprintf (out, "    ash: %d\n", slabinfo->ash);
  fprintf (out, "    lbbox: %d, ubbox: %d, nghostzones: %d\n",
           slabinfo->lbbox, slabinfo->ubbox, slabinfo->nghostzones);
  fprintf (out, "    off: %d, str: %d, len: %d\n",
           slabinfo->off, slabinfo->str, slabinfo->len);
}

extern "C"
void
print_xferinfo (FILE           *          const out,
                xferinfo const * restrict const xferinfo)
{
  fprintf (out, "  src:\n");
  print_slabinfo (out, & xferinfo->src);
  fprintf (out, "  dst:\n");
  print_slabinfo (out, & xferinfo->dst);
  fprintf (out, "  xpose: %d\n", xferinfo->xpose);
  fprintf (out, "  flip: %d\n", xferinfo->flip);
}                

// workhorse routines for the actual copying transposing, and flipping
// of data
template<typename T>
inline void
copy_data (const vector<xfer> &info,
           const vector<bbox> &srcdetail,
           const vector<int> &srcoffset,
           const vector<int> &srcelems,
           vector<char> &srcdata,
           void  const * restrict const * restrict const srcptrs,
           const int n,
           const vector<int> &varis,
           const int nvaris,
           const int xpose_x,
           const int xpose_y,
           const int xpose_z)
{
  assert (srcptrs);
  
  int const srcoffi = info[0].src.local.off;
  int const srcoffj = info[1].src.local.off;
  int const srcoffk = info[2].src.local.off;
  
  int const srcleni = info[0].src.local.len;
  int const srclenj = info[1].src.local.len;
  int const srclenk = info[2].src.local.len;
  
  int const srcaleni = info[0].src.local.alen;
  int const srcalenj = info[1].src.local.alen;
  int const srcalenk = info[2].src.local.alen;
  
  int const srcdetailoffi = srcdetail[n*SLAB_MAXDIM+0].off;
  int const srcdetailoffj = srcdetail[n*SLAB_MAXDIM+1].off;
  int const srcdetailoffk = srcdetail[n*SLAB_MAXDIM+2].off;
  
  int const srcdetailleni = srcdetail[n*SLAB_MAXDIM+0].len;
  int const srcdetaillenj = srcdetail[n*SLAB_MAXDIM+1].len;
  int const srcdetaillenk = srcdetail[n*SLAB_MAXDIM+2].len;
  
  int const dstdetailleni = srcdetail[n*SLAB_MAXDIM+xpose_x].len;
  int const dstdetaillenj = srcdetail[n*SLAB_MAXDIM+xpose_y].len;
  int const dstdetaillenk = srcdetail[n*SLAB_MAXDIM+xpose_z].len;
  
  if (n==0) assert (srcoffset[n]==0);
  // TODO: This does not take nvaris into account
  // if (n<size-1) assert (srcoffset[n+1]==srcoffset[n]+srcdetailleni*srcdetaillenj*srcdetaillenk);
  
  ifcheck {
    const int xpose[SLAB_MAXDIM] = {xpose_x, xpose_y, xpose_z};
    for (int i = 0; i < SLAB_MAXDIM; ++i) {
      for (int j = i+1; j < SLAB_MAXDIM; ++j) {
        assert(xpose[i] != xpose[j]);
      }
    }
  }
  
  assert (dstdetailleni*dstdetaillenj*dstdetaillenk == srcelems[n]);
  if (srcelems[n] == 0) return;
  
  for (int vari=0; vari<nvaris; ++vari) {
    T * restrict const srcdataptr =
      (T *)&srcdata.front() + srcoffset[n] + vari * srcelems[n];
    T const * restrict const srcptr = (T const *)srcptrs[varis[vari]];
    assert(srcptr);
    
    if (xpose_x==0 and xpose_y==1 and xpose_z==2) {
      // no transposition
      
      CCTK_TimerStartI (timer_copy_in_noxpose);
#     pragma omp parallel
      CCTK_LOOP3(Slab_copy_in_noxpose, i,j,k,
                 0,0,0, srcdetailleni,srcdetaillenj,srcdetaillenk,
                 srcaleni,srcalenj,srcalenk)
      {
        int const srcindi = srcdetailoffi + i - srcoffi;
        int const srcindj = srcdetailoffj + j - srcoffj;
        int const srcindk = srcdetailoffk + k - srcoffk;
        ifcheck assert (srcindi>=0 and srcindi<srcleni);
        ifcheck assert (srcindj>=0 and srcindj<srclenj);
        ifcheck assert (srcindk>=0 and srcindk<srclenk);
        size_t const srcind =
          srcindi + srcaleni * (srcindj + srcalenj * srcindk);
        size_t const bufind =
          i + dstdetailleni * (j + dstdetaillenj * k);
        srcdataptr[bufind] = srcptr[srcind];
      } CCTK_ENDLOOP3(Slab_copy_in_noxpose);
      CCTK_TimerStopI (timer_copy_in_noxpose);
      
    } else if (xpose_x==1 and xpose_y==0 and xpose_z==2) {
      // transpose x and y
      
      CCTK_TimerStartI (timer_copy_in_xposexy);
#     pragma omp parallel
      // Interchange i and j loops
      CCTK_LOOP3(Slab_copy_in_xposexy, j,i,k,
                 0,0,0, srcdetaillenj,srcdetailleni,srcdetaillenk,
                 srcalenj,srcaleni,srcalenk)
      {
        int const srcindi = srcdetailoffi + i - srcoffi;
        int const srcindj = srcdetailoffj + j - srcoffj;
        int const srcindk = srcdetailoffk + k - srcoffk;
        ifcheck assert (srcindi>=0 and srcindi<srcleni);
        ifcheck assert (srcindj>=0 and srcindj<srclenj);
        ifcheck assert (srcindk>=0 and srcindk<srclenk);
        size_t const srcind =
          srcindi + srcaleni * (srcindj + srcalenj * srcindk);
        size_t const bufind =
          j + dstdetailleni * (i + dstdetaillenj * k);
        srcdataptr[bufind] = srcptr[srcind];
      } CCTK_ENDLOOP3(Slab_copy_in_xposexy);
      CCTK_TimerStopI (timer_copy_in_xposexy);
      
    } else {
      // general transposition
      
      CCTK_TimerStartI (timer_copy_in_xposegeneral);
#     pragma omp parallel
      CCTK_LOOP3(Slab_copy_in_xposegeneral, i,j,k,
                 0,0,0, srcdetailleni,srcdetaillenj,srcdetaillenk,
                 srcaleni,srcalenj,srcalenk)
      {
        int ipos[SLAB_MAXDIM];
        ipos[0] = i;
        ipos[1] = j;
        ipos[2] = k;
        int const srcindi = srcdetailoffi + i - srcoffi;
        int const srcindj = srcdetailoffj + j - srcoffj;
        int const srcindk = srcdetailoffk + k - srcoffk;
        ifcheck assert (srcindi>=0 and srcindi<srcleni);
        ifcheck assert (srcindj>=0 and srcindj<srclenj);
        ifcheck assert (srcindk>=0 and srcindk<srclenk);
        size_t const srcind =
          srcindi + srcaleni * (srcindj + srcalenj * srcindk);
        // TODO: rewrite this, moving the index onto dstdetaillen and
        // outside the loop; then collapse all loop specialisations
        size_t const bufind =
          (ipos[xpose_x] + dstdetailleni *
           (ipos[xpose_y] + dstdetaillenj * ipos[xpose_z]));
        srcdataptr[bufind] = srcptr[srcind];
      } CCTK_ENDLOOP3(Slab_copy_in_xposegeneral);
      CCTK_TimerStopI (timer_copy_in_xposegeneral);
      
    }
    
  } // for vari
}

// workhorse routine responsible for the actual copying/flipping of data
template<typename T> inline void
copy_data_back (const vector<xfer> &info,
                const vector<bbox> &dstdetail,
                const vector<int> &dstoffset,
                const vector<int> &dstelems,
                const vector<char> &dstdata,
                void * restrict const * restrict const dstptrs,
                const int n,
                const vector<int> &varis,
                const int nvaris,
                const bool flip_x,
                const bool flip_y,
                const bool flip_z)
{
  assert (dstptrs);
  
  int const dstoffi = info[0].dst.local.off;
  int const dstoffj = info[1].dst.local.off;
  int const dstoffk = info[2].dst.local.off;
  
  int const dstleni = info[0].dst.local.len;
  int const dstlenj = info[1].dst.local.len;
  int const dstlenk = info[2].dst.local.len;
  
  int const dstaleni = info[0].dst.local.alen;
  int const dstalenj = info[1].dst.local.alen;
  int const dstalenk = info[2].dst.local.alen;
  
  int const dstdetailoffi = dstdetail[n*SLAB_MAXDIM+0].off;
  int const dstdetailoffj = dstdetail[n*SLAB_MAXDIM+1].off;
  int const dstdetailoffk = dstdetail[n*SLAB_MAXDIM+2].off;
  
  int const dstdetailleni = dstdetail[n*SLAB_MAXDIM+0].len;
  int const dstdetaillenj = dstdetail[n*SLAB_MAXDIM+1].len;
  int const dstdetaillenk = dstdetail[n*SLAB_MAXDIM+2].len;
  
  assert (dstdetailleni*dstdetaillenj*dstdetaillenk == dstelems[n]);
  if (dstelems[n] == 0) return;
  
  for (int vari=0; vari<nvaris; ++vari) {
    T * restrict const dstptr = (T * restrict)dstptrs[varis[vari]];
    assert (dstptr);
    T const * restrict const dstdataptr =
      (T const *)&dstdata.front() + dstoffset[n] + vari * dstelems[n];
    
    if (not flip_x and not flip_y and not flip_z) {
      // no flipping
      
      CCTK_TimerStartI (timer_copy_back_noflip);
#     pragma omp parallel
      CCTK_LOOP3(Slab_copy_back_noflip, i,j,k,
                 0,0,0, dstdetailleni,dstdetaillenj,dstdetaillenk,
                 dstaleni,dstalenj,dstalenk)
      {
        int const dstindi = dstdetailoffi + i - dstoffi;
        int const dstindj = dstdetailoffj + j - dstoffj;
        int const dstindk = dstdetailoffk + k - dstoffk;
        ifcheck assert (dstindi>=0 and dstindi<dstleni);
        ifcheck assert (dstindj>=0 and dstindj<dstlenj);
        ifcheck assert (dstindk>=0 and dstindk<dstlenk);
        size_t const dstind =
          dstindi + dstaleni * (dstindj + dstalenj * dstindk);
        size_t const bufind =
          i + dstdetailleni * (j + dstdetaillenj * k);
        dstptr[dstind] = dstdataptr[bufind];
      } CCTK_ENDLOOP3(Slab_copy_back_noflip);
      CCTK_TimerStopI (timer_copy_back_noflip);
      
    } else if (flip_x and not flip_y and not flip_z) {
      // flip in x direction
      
      CCTK_TimerStartI (timer_copy_back_flipx);
#     pragma omp parallel
      CCTK_LOOP3(Slab_copy_back_flipx, i,j,k,
                 0,0,0, dstdetailleni,dstdetaillenj,dstdetaillenk,
                 dstaleni,dstalenj,dstalenk)
      {
        int const dstindi = dstdetailoffi + (dstdetailleni - 1 - i) - dstoffi;
        int const dstindj = dstdetailoffj + j - dstoffj;
        int const dstindk = dstdetailoffk + k - dstoffk;
        ifcheck assert (dstindi>=0 and dstindi<dstleni);
        ifcheck assert (dstindj>=0 and dstindj<dstlenj);
        ifcheck assert (dstindk>=0 and dstindk<dstlenk);
        size_t const dstind =
          dstindi + dstaleni * (dstindj + dstalenj * dstindk);
        size_t const bufind =
          i + dstdetailleni * (j + dstdetaillenj * k);
        dstptr[dstind] = dstdataptr[bufind];
      } CCTK_ENDLOOP3(Slab_copy_back_flipx);
      CCTK_TimerStopI (timer_copy_back_flipx);
      
    } else if (not flip_x and flip_y and not flip_z) {
      // flip in y direction
      
      CCTK_TimerStartI (timer_copy_back_flipy);
#     pragma omp parallel
      CCTK_LOOP3(Slab_copy_back_flipy, i,j,k,
                 0,0,0, dstdetailleni,dstdetaillenj,dstdetaillenk,
                 dstaleni,dstalenj,dstalenk)
      {
        int const dstindi = dstdetailoffi + i - dstoffi;
        int const dstindj = dstdetailoffj + (dstdetaillenj - 1 - j) - dstoffj;
        int const dstindk = dstdetailoffk + k - dstoffk;
        ifcheck assert (dstindi>=0 and dstindi<dstleni);
        ifcheck assert (dstindj>=0 and dstindj<dstlenj);
        ifcheck assert (dstindk>=0 and dstindk<dstlenk);
        size_t const dstind =
          dstindi + dstaleni * (dstindj + dstalenj * dstindk);
        size_t const bufind =
          i + dstdetailleni * (j + dstdetaillenj * k);
        dstptr[dstind] = dstdataptr[bufind];
      } CCTK_ENDLOOP3(Slab_copy_back_flipy);
      CCTK_TimerStopI (timer_copy_back_flipy);
      
    } else if (flip_x and flip_y and not flip_z) {
      // flip in both x and y direction
      
      CCTK_TimerStartI (timer_copy_back_flipxy);
#     pragma omp parallel
      CCTK_LOOP3(Slab_copy_back_flipxy, i,j,k,
                 0,0,0, dstdetailleni,dstdetaillenj,dstdetaillenk,
                 dstaleni,dstalenj,dstalenk)
      {
        int const dstindi = dstdetailoffi + (dstdetailleni - 1 - i) - dstoffi;
        int const dstindj = dstdetailoffj + (dstdetaillenj - 1 - j) - dstoffj;
        int const dstindk = dstdetailoffk + k - dstoffk;
        ifcheck assert (dstindi>=0 and dstindi<dstleni);
        ifcheck assert (dstindj>=0 and dstindj<dstlenj);
        ifcheck assert (dstindk>=0 and dstindk<dstlenk);
        size_t const dstind =
          dstindi + dstaleni * (dstindj + dstalenj * dstindk);
        size_t const bufind =
          i + dstdetailleni * (j + dstdetaillenj * k);
        dstptr[dstind] = dstdataptr[bufind];
      } CCTK_ENDLOOP3(Slab_copy_back_flipxy);
      CCTK_TimerStopI (timer_copy_back_flipxy);
      
    } else {
      // general flipping
      
      CCTK_TimerStartI (timer_copy_back_flipgeneral);
#     pragma omp parallel
      CCTK_LOOP3(Slab_copy_back_flipgeneral, i,j,k,
                 0,0,0, dstdetailleni,dstdetaillenj,dstdetaillenk,
                 dstaleni,dstalenj,dstalenk)
      {
        // TODO: rewrite this, moving the flipping onto dstlen and
        // outside the loop; then collapse all loop specialisations
        int const dstindi =
          dstdetailoffi + (flip_x ? dstdetailleni - 1 - i : i) - dstoffi;
        int const dstindj =
          dstdetailoffj + (flip_y ? dstdetaillenj - 1 - j : j) - dstoffj;
        int const dstindk =
          dstdetailoffk + (flip_z ? dstdetaillenk - 1 - k : k) - dstoffk;
        ifcheck assert (dstindi>=0 and dstindi<dstleni);
        ifcheck assert (dstindj>=0 and dstindj<dstlenj);
        ifcheck assert (dstindk>=0 and dstindk<dstlenk);
        size_t const dstind =
          dstindi + dstaleni * (dstindj + dstalenj * dstindk);
        size_t const bufind =
          i + dstdetailleni * (j + dstdetaillenj * k);
        dstptr[dstind] = dstdataptr[bufind];
      } CCTK_ENDLOOP3(Slab_copy_back_flipgeneral);
      CCTK_TimerStopI (timer_copy_back_flipgeneral);
      
    }
    
  } // for vari
}



struct slabsetup {
  MPI_Comm comm;
  vector<xfer> info;
  vector<xfer> allinfo;
  vector<bbox> srcdetail, dstdetail;
  size_t srclentot, dstlentot;
};



extern "C"
slabsetup *
Slab_MultiTransfer_Init
(cGH       const* restrict const cctkGH,
 int                       const dim,
 xferinfo  const* restrict const xferinfo,
 int                       const options)
{
  DECLARE_CCTK_PARAMETERS;
  
  // Check arguments
  check (cctkGH);
  check (dim >= 0);
  check (xferinfo);
  
  
  
  CCTK_TimerStartI (timer_init);
  
  slabsetup * restrict const slabsetup = new struct slabsetup;
  
  
  
  bool useghosts;
  {
    CCTK_INT tmp;
    int const iret = Util_TableGetInt (options, &tmp, "useghosts");
    if (iret == 1) {
      // There was an entry, use it
      useghosts = tmp;
    } else if (iret == UTIL_ERROR_BAD_HANDLE or
               iret == UTIL_ERROR_TABLE_NO_SUCH_KEY)
    {
      // There was no entry, use a default
      useghosts = false;
    } else {
      // Something went wrong, abort
      check (0);
    }
  }
  
  check (dim <= SLAB_MAXDIM);
  vector<xfer>& info = slabsetup->info;
  info.resize (SLAB_MAXDIM);
  for (int d=0; d<dim; ++d) {
    global2bbox (&xferinfo[d].src, &info[d].src.global);
    local2bbox  (&xferinfo[d].src, &info[d].src.local);
    active2bbox (&xferinfo[d].src, &info[d].src.active, useghosts);
    slab2bbox   (&xferinfo[d].src, &info[d].src.slab);
    check (bbox_iscontained (&info[d].src.active, &info[d].src.local));
    check (bbox_iscontained (&info[d].src.local, &info[d].src.global));
    
    global2bbox (&xferinfo[d].dst, &info[d].dst.global);
    local2bbox  (&xferinfo[d].dst, &info[d].dst.local);
    active2bbox (&xferinfo[d].dst, &info[d].dst.active, 1); // fill ghosts
    slab2bbox   (&xferinfo[d].dst, &info[d].dst.slab);
    check (bbox_iscontained (&info[d].dst.active, &info[d].dst.local));
    check (bbox_iscontained (&info[d].dst.local, &info[d].dst.global));
    
    info[d].xpose = xferinfo[d].xpose;
    check (info[d].xpose >= 0 and info[d].xpose < dim);
    info[d].flip = xferinfo[d].flip;
    check (info[d].flip == 0 or info[d].flip == 1);
  }
  for (int d=dim; d<SLAB_MAXDIM; ++d) {
    static bbox const fake_bbox = { 0, 1, 1 };
    static arrays const fake_arrays =
      { { 0, 1, 1 }, { 0, 1, 1 }, { 0, 1, 1 }, { 0, 1, 1 } };
    
    bbox_check (&fake_bbox);
    
    info[d].src = fake_arrays;
    check (bbox_iscontained (&info[d].src.active, &info[d].src.local));
    check (bbox_iscontained (&info[d].src.local, &info[d].src.global));
    
    info[d].dst = fake_arrays;
    check (bbox_iscontained (&info[d].dst.active, &info[d].dst.local));
    check (bbox_iscontained (&info[d].dst.local, &info[d].dst.global));
    
    info[d].xpose = d;
    check (info[d].xpose >= 0 and info[d].xpose < SLAB_MAXDIM);
    info[d].flip = 0;
    check (info[d].flip == 0 or info[d].flip == 1);
  }
  
  ifcheck {
    ifdebug printf ("srcinfo:\n");
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      printf ("   src.global d=%d ", d);
      bbox_print (&info[d].src.global);
      printf ("\n");
      printf ("   src.local  d=%d ", d);
      bbox_print (&info[d].src.local);
      printf ("\n");
      printf ("   src.active d=%d ", d);
      bbox_print (&info[d].src.active);
      printf ("\n");
      printf ("   src.slab   d=%d ", d);
      bbox_print (&info[d].src.slab);
      printf ("\n");
    }
    ifdebug printf ("dstinfo:\n");
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      printf ("   dst.global d=%d ", d);
      bbox_print (&info[d].dst.global);
      printf ("\n");
      printf ("   dst.local  d=%d ", d);
      bbox_print (&info[d].dst.local);
      printf ("\n");
      printf ("   dst.active d=%d ", d);
      bbox_print (&info[d].dst.active);
      printf ("\n");
      printf ("   dst.slab   d=%d ", d);
      bbox_print (&info[d].dst.slab);
      printf ("\n");
    }
    ifdebug printf ("info:\n");
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      printf ("   xpose      d=%d %d\n", d, info[d].xpose);
      printf ("   flip       d=%d %d\n", d, info[d].flip);
    }
  }
  
  {
    bool iflag[SLAB_MAXDIM];
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      iflag[d] = false;
    }
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      assert (not iflag[info[d].xpose]);
      iflag[info[d].xpose] = true;
    }
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      assert (iflag[d]);
    }
    // Allow non-contributing processors to be non-knowledgeable
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      if (info[info[d].xpose].src.slab.len and info[d].dst.slab.len > 0) {
        assert (info[info[d].xpose].src.slab.len == info[d].dst.slab.len);
      }
    }
  }
  
  size_t& srclentot = slabsetup->srclentot;
  size_t& dstlentot = slabsetup->dstlentot;
  srclentot = 1;
  dstlentot = 1;
  for (int d=0; d<SLAB_MAXDIM; ++d) {
    srclentot *= info[d].src.local.len;
    dstlentot *= info[d].dst.local.len;
  }
  
  
  
  MPI_Comm& comm = slabsetup->comm;
  {
    CCTK_POINTER_TO_CONST tmp1;
    int const iret1 = Util_TableGetPointerToConst (options, &tmp1, "comm");
    if (iret1 == 1) {
      // There was an entry, use it
      comm = * (MPI_Comm const *) tmp1;
    } else if (iret1 == UTIL_ERROR_TABLE_WRONG_DATA_TYPE) {
      // Entry has wrong type, fall back
      CCTK_POINTER tmp2;
      int const iret2 = Util_TableGetPointer (options, &tmp2, "comm");
      if (iret2 == 1) {
        // There was an entry, use it
        comm = * (MPI_Comm const *) tmp2;
      } else {
        // Something went wrong, abort
        check (0);
      }
    } else if (iret1 == UTIL_ERROR_BAD_HANDLE or
               iret1 == UTIL_ERROR_TABLE_NO_SUCH_KEY)
    {
      // There was no entry, use a default
      comm = get_mpi_comm (cctkGH);
    } else {
      // Something went wrong, abort
      check (0);
    }
  }
  
  ifcheck {
    ifdebug fflush (stdout);
    MPI_Barrier (comm);
  }
  
  int size, rank;
  MPI_Comm_size (comm, &size);
  MPI_Comm_rank (comm, &rank);
  
  ifcheck {
    static int count = 424242;
    int mycount = count;
    ifdebug fflush (stdout);
    MPI_Bcast (&mycount, 1, MPI_INT, 0, comm);
    assert (mycount == count);
    ++ count;
  }
  
  
  
  vector<xfer>& allinfo = slabsetup->allinfo;
  allinfo.resize (size * SLAB_MAXDIM);
  {
    int const info_nints = sizeof(xfer) / sizeof(int);
    ifdebug fflush (stdout);
    MPI_Allgather
      (&info.front(),    SLAB_MAXDIM * info_nints, MPI_INT,
       &allinfo.front(), SLAB_MAXDIM * info_nints, MPI_INT, comm);
  }
  
  for (int n = 0; n < size; ++n) {
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      // Allow non-contributing processors to be non-knowledgeable
      if (allinfo[n*SLAB_MAXDIM+d].src.slab.len > 0 and
          info[d].src.slab.len > 0)
      {
        assert
          (allinfo[n*SLAB_MAXDIM+d].src.global.off == info[d].src.global.off);
        assert
          (allinfo[n*SLAB_MAXDIM+d].src.global.len == info[d].src.global.len);
        assert
          (allinfo[n*SLAB_MAXDIM+d].src.global.str == info[d].src.global.str);
        assert
          (allinfo[n*SLAB_MAXDIM+d].src.local.str == info[d].src.local.str);
        assert
          (allinfo[n*SLAB_MAXDIM+d].src.active.str == info[d].src.active.str);
        // 2003-03-01 eschnett: I don't know why the following should
        // be necessary
        assert
          (allinfo[n*SLAB_MAXDIM+d].src.slab.str == info[d].src.slab.str);
      }
      if (allinfo[n*SLAB_MAXDIM+d].dst.slab.len > 0 and
          info[d].dst.slab.len > 0)
      {
        assert
          (allinfo[n*SLAB_MAXDIM+d].dst.global.off == info[d].dst.global.off);
        assert
          (allinfo[n*SLAB_MAXDIM+d].dst.global.len == info[d].dst.global.len);
        assert
          (allinfo[n*SLAB_MAXDIM+d].dst.global.str == info[d].dst.global.str);
        assert
          (allinfo[n*SLAB_MAXDIM+d].dst.local.str == info[d].dst.local.str);
        assert
          (allinfo[n*SLAB_MAXDIM+d].dst.active.str == info[d].dst.active.str);
        assert
          (allinfo[n*SLAB_MAXDIM+d].dst.slab.str == info[d].dst.slab.str);
      }
      assert (allinfo[n*SLAB_MAXDIM+d].xpose == info[d].xpose);
      assert (allinfo[n*SLAB_MAXDIM+d].flip == info[d].flip);
    }
  }
  
  
  
  vector<bbox>& srcdetail = slabsetup->srcdetail;
  srcdetail.resize (size * SLAB_MAXDIM);
  for (int n = 0; n < size; ++n) {
    ifdebug printf ("srcdetail n=%d:\n", n);
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      srcdetail[n*SLAB_MAXDIM+d] = allinfo[n*SLAB_MAXDIM+d].src.slab;
      ifdebug printf ("   src.slab                d=%d ", d);
      ifdebug bbox_print (&srcdetail[n*SLAB_MAXDIM+d]);
      ifdebug printf ("\n");
      bbox_clip (&srcdetail[n*SLAB_MAXDIM+d], &info[d].src.active);
      ifdebug printf ("   clipped with src.active d=%d ", d);
      ifdebug bbox_print (&srcdetail[n*SLAB_MAXDIM+d]);
      ifdebug printf ("\n");
    }
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      bbox whereto;
      bbox wherefrom;
      whereto = allinfo[n*SLAB_MAXDIM+d].dst.slab;
      ifdebug printf ("   dst.slab                d=%d ", info[d].xpose);
      ifdebug bbox_print (&whereto);
      ifdebug printf ("\n");
      bbox_clip (&whereto, &allinfo[n*SLAB_MAXDIM+d].dst.active);
      ifdebug printf ("   whereto                 d=%d ", info[d].xpose);
      ifdebug bbox_print (&whereto);
      ifdebug printf ("\n");
      bbox_xform
	(&wherefrom, &whereto,
	 &allinfo[n*SLAB_MAXDIM+info[d].xpose].src.slab,
         &allinfo[n*SLAB_MAXDIM+d].dst.slab,
	 info[d].flip);
      ifdebug printf ("   wherefrom               d=%d ", info[d].xpose);
      ifdebug bbox_print (&wherefrom);
      ifdebug printf ("\n");
      bbox_clip (&srcdetail[n*SLAB_MAXDIM+info[d].xpose], &wherefrom);
      ifdebug printf ("   clipped with wherefrom  d=%d ", info[d].xpose);
      ifdebug bbox_print (&srcdetail[n*SLAB_MAXDIM+info[d].xpose]);
      ifdebug printf ("\n");
    }
  }
  
  
  
  vector<bbox>& dstdetail = slabsetup->dstdetail;
  dstdetail.resize (size * SLAB_MAXDIM);
  for (int n = 0; n < size; ++n) {
    ifdebug printf ("dstdetail n=%d:\n", n);
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      // dstdetail[n*SLAB_MAXDIM+d] = allinfo[n*SLAB_MAXDIM+d].dst.slab;
      dstdetail[n*SLAB_MAXDIM+d] = info[d].dst.slab;
      ifdebug printf ("   dst.slab                d=%d ", d);
      ifdebug bbox_print (&dstdetail[n*SLAB_MAXDIM+d]);
      ifdebug printf ("\n");
      bbox_clip (&dstdetail[n*SLAB_MAXDIM+d], &info[d].dst.active);
      ifdebug printf ("   clipped with dst.active d=%d ", d);
      ifdebug bbox_print (&dstdetail[n*SLAB_MAXDIM+d]);
      ifdebug printf ("\n");
    }
    for (int d=0; d<SLAB_MAXDIM; ++d) {
      bbox wherefrom;
      bbox whereto;
      // wherefrom = allinfo[n*SLAB_MAXDIM+info[d].xpose].src.slab;
      wherefrom = info[info[d].xpose].src.slab;
      ifdebug printf ("   src.slab                d=%d ", d);
      // ifdebug bbox_print (&dstdetail[n*SLAB_MAXDIM+d]);
      ifdebug bbox_print (&wherefrom);
      ifdebug printf ("\n");
      bbox_clip (&wherefrom, &allinfo[n*SLAB_MAXDIM+info[d].xpose].src.active);
      ifdebug printf ("   wherefrom               d=%d ", d);
      // ifdebug bbox_print (&dstdetail[n*SLAB_MAXDIM+d]);
      ifdebug bbox_print (&wherefrom);
      ifdebug printf ("\n");
      bbox_xform
	(&whereto, &wherefrom,
	 &allinfo[n*SLAB_MAXDIM+d].dst.slab,
         // &allinfo[n*SLAB_MAXDIM+info[d].xpose].src.slab,
         &info[info[d].xpose].src.slab,
	 info[d].flip);
      ifdebug printf ("   whereto                 d=%d ", d);
      // ifdebug bbox_print (&dstdetail[n*SLAB_MAXDIM+d]);
      ifdebug bbox_print (&whereto);
      ifdebug printf ("\n");
      bbox_clip (&dstdetail[n*SLAB_MAXDIM+d], &whereto);
      ifdebug printf ("   clipped with whereto    d=%d ", d);
      ifdebug bbox_print (&dstdetail[n*SLAB_MAXDIM+d]);
      ifdebug printf ("\n");
    }
  }
  
  
  
  CCTK_TimerStopI (timer_init);
  
  return slabsetup;
}



extern "C"
int
Slab_MultiTransfer_Apply
(cGH       const                  * restrict const cctkGH,
 slabsetup const                  * restrict const slabsetup,
 int                                         const nvars,
 int       const                  * restrict const srctypes,
 void      const * restrict const * restrict const srcptrs,
 int       const                  * restrict const dsttypes,
 void            * restrict const * restrict const dstptrs)
{
  DECLARE_CCTK_PARAMETERS;
  
  // Check arguments
  check (cctkGH);
  check (slabsetup);
  check (nvars >= 0);
  check (nvars==0 or srctypes);
  for (int var=0; var<nvars; ++var) check (srctypes[var] >= 0);
  check (nvars==0 or srcptrs);
  size_t const& srclentot = slabsetup->srclentot;
  for (int var=0; var<nvars; ++var) if (srclentot > 0) assert (srcptrs[var]);
  check (nvars==0 or dsttypes);
  for (int var=0; var<nvars; ++var) check (dsttypes[var] >= 0);
  check (nvars==0 or dstptrs);
  size_t const& dstlentot = slabsetup->dstlentot;
  for (int var=0; var<nvars; ++var) if (dstlentot > 0) assert (dstptrs[var]);
  
  if (nvars==0) return 0;
  
  
  
  CCTK_TimerStartI (timer_apply);
  
  
  
  MPI_Comm const& comm = slabsetup->comm;
  
  ifcheck {
    ifdebug fflush (stdout);
    MPI_Barrier (comm);
  }
  
  int size, rank;
  MPI_Comm_size (comm, &size);
  MPI_Comm_rank (comm, &rank);
  
  ifcheck {
    static int count = 424242;
    int mycount = count;
    ifdebug fflush (stdout);
    MPI_Bcast (&mycount, 1, MPI_INT, 0, comm);
    assert (mycount == count);
    ++ count;
  }
  
  
  
  vector<xfer> const& info = slabsetup->info;
  vector<xfer> const& allinfo = slabsetup->allinfo;
  vector<bbox> const& srcdetail = slabsetup->srcdetail;
  vector<bbox> const& dstdetail = slabsetup->dstdetail;
  
  vector<int> srcelems (size);
  vector<int> srccount (size);
  vector<int> srcoffset (size + 1);
  
  vector<int> dstelems (size);
  vector<int> dstcount (size);
  vector<int> dstoffset (size + 1);
  
  
  
  int nvartypes = 0;
  vector<int> vartypes (nvars);
  vector<int> vartypecount (nvars);
  for (int var=0; var<nvars; ++var) {
    int const srctype = srctypes[var];
    int const dsttype = dsttypes[var];
    check (srctype == dsttype);
    int vartypei;
    for (vartypei=0; vartypei<nvartypes; ++vartypei) {
      if (srctype == vartypes[vartypei]) break;
    }
    if (vartypei>=nvartypes) {
      vartypes[nvartypes] = srctype;
      vartypecount[nvartypes] = 0;
      ++nvartypes;
    }
    assert (vartypei<nvartypes);
    ++ vartypecount[vartypei];
  }
  
  for (int vartypei=0; vartypei<nvartypes; ++vartypei) {
    int const vartype = vartypes[vartypei];
    int nvaris = 0;
    vector<int> varis (nvars);
    for (int var=0; var<nvars; ++var) {
      if (srctypes[var] == vartype) {
        varis[nvaris] = var;
        ++nvaris;
      }
    }
    assert (nvaris == vartypecount[vartypei]);
    assert (nvaris > 0);
    
    int const vartypesize = CCTK_VarTypeSize (vartype);
    check (vartypesize > 0);
    MPI_Datatype const vardatatype = mpi_type (vartype);
    
    
    
    srcoffset[0] = 0;
    for (int n = 0; n < size; ++n) {
      srcelems[n] = 1;
      for (int d=0; d<SLAB_MAXDIM; ++d) {
        srcelems[n] *= srcdetail[n*SLAB_MAXDIM+d].len;
      }
      srccount[n] = nvaris * srcelems[n];
      ifdebug printf
        ("srccnt n=%d offset=%d count=%d\n", n, srcoffset[n], srccount[n]);
      srcoffset[n+1] = srcoffset[n] + srccount[n];
    }
    vector<char> srcdata (srcoffset[size] * vartypesize);
    ifcheck {
      if (vartype == CCTK_VARIABLE_REAL) {
        CCTK_REAL * restrict const srcdataptr = (CCTK_REAL *)&srcdata.front();
        CCTK_REAL marker;
        memset (&marker, POISON_VALUE, sizeof marker);
        for (int i = 0; i < srcoffset[size]; ++i) {
          memcpy (&srcdataptr[i], &marker, sizeof marker);
        }
      }
    }
    
    
    
    dstoffset[0] = 0;
    for (int n = 0; n < size; ++n) {
      dstelems[n] = 1;
      for (int d=0; d<SLAB_MAXDIM; ++d) {
        dstelems[n] *= dstdetail[n*SLAB_MAXDIM+d].len;
      }
      dstcount[n] = nvaris * dstelems[n];
      ifdebug printf
        ("dstcnt n=%d offset=%d count=%d\n", n, dstoffset[n], dstcount[n]);
      dstoffset[n+1] = dstoffset[n] + dstcount[n];
    }
    vector<char> dstdata (dstoffset[size] * vartypesize);
    ifcheck {
      if (vartype == CCTK_VARIABLE_REAL) {
        CCTK_REAL * restrict const dstdataptr = (CCTK_REAL *)&dstdata.front();
        CCTK_REAL marker;
        memset (&marker, POISON_VALUE, sizeof marker);
        for (int i = 0; i < dstoffset[size]; ++i) {
          memcpy (&dstdataptr[i], &marker, sizeof marker);
        }
      }
    }
    
    check (srccount[rank] == dstcount[rank]);
    
    
    
    ifcheck {
      vector<int> src2count (size);
      vector<int> dst2count (size);
      ifdebug fflush (stdout);
      MPI_Alltoall
        (&srccount.front(), 1, MPI_INT, &src2count.front(), 1, MPI_INT, comm);
      MPI_Alltoall
        (&dstcount.front(), 1, MPI_INT, &dst2count.front(), 1, MPI_INT, comm);
      for (int n = 0; n < size; ++n) {
        check (src2count[n] == dstcount[n]);
        check (dst2count[n] == srccount[n]);
      }
    }



    CCTK_TimerStartI (timer_copy_in);
    
    for (int n = 0; n < size; ++n) {
      check (SLAB_MAXDIM == 3);
      
      if (srcdetail[n*SLAB_MAXDIM  ].str==1 and
          srcdetail[n*SLAB_MAXDIM+1].str==1 and
          srcdetail[n*SLAB_MAXDIM+2].str==1 and
          vartype == CCTK_VARIABLE_REAL)
      {
        // Optimised for stride 1 and CCTK_REAL
        copy_data<CCTK_REAL>
          (info, srcdetail, srcoffset, srcelems, srcdata, srcptrs, 
           n, varis, nvaris, info[0].xpose, info[1].xpose, info[2].xpose);
      } else if (srcdetail[n*SLAB_MAXDIM  ].str==1 and
                 srcdetail[n*SLAB_MAXDIM+1].str==1 and
                 srcdetail[n*SLAB_MAXDIM+2].str==1 and
                 vartype == CCTK_VARIABLE_COMPLEX)
      {
        // Optimised for stride 1 and CCTK_COMPLEX
        copy_data<CCTK_COMPLEX>
          (info, srcdetail, srcoffset, srcelems, srcdata, srcptrs, 
           n, varis, nvaris, info[0].xpose, info[1].xpose, info[2].xpose);
      } else if (srcdetail[n*SLAB_MAXDIM  ].str==1 and
                 srcdetail[n*SLAB_MAXDIM+1].str==1 and
                 srcdetail[n*SLAB_MAXDIM+2].str==1 and
                 vartype == CCTK_VARIABLE_INT)
      {
        // Optimised for stride 1 and CCTK_INT
        copy_data<CCTK_INT>
          (info, srcdetail, srcoffset, srcelems, srcdata, srcptrs, 
           n, varis, nvaris, info[0].xpose, info[1].xpose, info[2].xpose);
      } else {
        // Generic, unoptimised version
        CCTK_TimerStartI (timer_copy_in_general);
        
        int const srcdetailleni = srcdetail[n*SLAB_MAXDIM+info[0].xpose].len;
        int const srcdetaillenj = srcdetail[n*SLAB_MAXDIM+info[1].xpose].len;
        int const srcdetaillenk = srcdetail[n*SLAB_MAXDIM+info[2].xpose].len;
        
        for (int vari=0; vari<nvaris; ++vari) {
          char * restrict const srcdataptr =
            &srcdata[vartypesize * (srcoffset[n] + vari * srcelems[n])];
          char const * restrict const srcptr =
            (char const *)srcptrs[varis[vari]];
          
#         pragma omp parallel for
          for (int k = 0; k < srcdetaillenk; ++k) {
            for (int j = 0; j < srcdetaillenj; ++j) {
              for (int i = 0; i < srcdetailleni; ++i) {
                int ipos[SLAB_MAXDIM];
                ipos[0] = i;
                ipos[1] = j;
                ipos[2] = k;
                int srcipos[SLAB_MAXDIM];
                int bufipos[SLAB_MAXDIM];
                for (int d=0; d<SLAB_MAXDIM; ++d) {
                  int const c = info[d].xpose;
                  srcipos[c] = srcdetail[n*SLAB_MAXDIM+c].off + ipos[d] * srcdetail[n*SLAB_MAXDIM+c].str;
                  assert (srcipos[c] >= info[c].src.local.off and
                          srcipos[c] < info[c].src.local.off + info[c].src.local.len);
                  assert (srcipos[c] >= allinfo[n*SLAB_MAXDIM+c].src.slab.off and
                          srcipos[c] <= allinfo[n*SLAB_MAXDIM+c].src.slab.off + (allinfo[n*SLAB_MAXDIM+c].src.slab.len - 1) * allinfo[n*SLAB_MAXDIM+c].src.slab.str);
                  assert ((srcipos[c] - allinfo[n*SLAB_MAXDIM+c].src.slab.off) % allinfo[n*SLAB_MAXDIM+c].src.slab.str == 0);
                  bufipos[d] = ipos[d];
                  assert (bufipos[d] >= 0 and bufipos[d] < srcdetail[n*SLAB_MAXDIM+c].len);
                }
                size_t srcind = 0;
                size_t bufind = 0;
                for (int d=SLAB_MAXDIM-1; d>=0; --d) {
                  int const c = info[d].xpose;
                  srcind = srcind * info[d].src.local.alen + srcipos[d] - info[d].src.local.off;
                  bufind = bufind * srcdetail[n*SLAB_MAXDIM+c].len + bufipos[d];
                }
                assert (srcind < srclentot);
                assert (bufind < (size_t)srccount[n]);
                memcpy (srcdataptr + vartypesize * bufind,
                        srcptr + vartypesize * srcind,
                        vartypesize);
              }
            }
          }
          
        } // for vari
        CCTK_TimerStopI (timer_copy_in_general);
        
      }
    } // for n
    
    ifcheck {
      if (vartype == CCTK_VARIABLE_REAL) {
        CCTK_REAL const * restrict const srcdataptr =
          (CCTK_REAL const *)&srcdata.front();
        CCTK_REAL marker;
        memset (&marker, POISON_VALUE, sizeof marker);
        for (int i = 0; i < srcoffset[size]; ++i) {
          assert (memcmp(&srcdataptr[i], &marker, sizeof marker) != 0);
        }
      }
    }
    CCTK_TimerStopI (timer_copy_in);
    
    
    
    CCTK_TimerStartI (timer_xfer);
    ifdebug fflush (stdout);
    if (not HAVE_MPI or use_alltoallv) {
      MPI_Alltoallv
        (&srcdata.front(), &srccount.front(), &srcoffset.front(), vardatatype,
         &dstdata.front(), &dstcount.front(), &dstoffset.front(), vardatatype,
         comm);
    } else {
      vector<MPI_Request> requests;
      requests.reserve (2 * size);
      // Start receive
      for (int n = 0; n < size; ++n) {
        if (n != rank and dstcount[n] > 0) {
          MPI_Request req;
          MPI_Irecv
            (&dstdata[vartypesize * dstoffset[n]], dstcount[n], vardatatype,
             n, 0, comm, &req);
          requests.push_back (req);
        }
      }
      // Start send
      for (int n = 0; n < size; ++n) {
        if (n != rank and srccount[n] > 0) {
          MPI_Request req;
          MPI_Isend
            (&srcdata[vartypesize * srcoffset[n]], srccount[n], vardatatype,
             n, 0, comm, &req);
          requests.push_back (req);
        }
      }
      // Self communication
      {
        int const n = rank;
        assert (dstcount[n] == srccount[n]);
        memcpy (&dstdata[vartypesize * dstoffset[n]],
                &srcdata[vartypesize * srcoffset[n]],
                dstcount[n] * vartypesize);
      }
      // Wait
      MPI_Waitall (requests.size(), &requests.front(), MPI_STATUSES_IGNORE);
    }
    
    ifcheck {
      if (vartype == CCTK_VARIABLE_REAL) {
        for (int vari=0; vari<nvaris; ++vari) {
          CCTK_REAL const * restrict const dstdataptr =
            (CCTK_REAL const *)&dstdata.front();
          CCTK_REAL marker;
          memset (&marker, POISON_VALUE, sizeof marker);
          for (int i = 0; i < dstoffset[size]; ++i) {
            assert (memcmp(&dstdataptr[i], &marker, sizeof marker) != 0);
          }
        }
      }
    }
    CCTK_TimerStopI (timer_xfer);
    
    
    
    CCTK_TimerStartI (timer_copy_back);
    for (int n = 0; n < size; ++n) {
      check (SLAB_MAXDIM == 3);
      
      if (dstdetail[n*SLAB_MAXDIM  ].str==1 and
          dstdetail[n*SLAB_MAXDIM+1].str==1 and
          dstdetail[n*SLAB_MAXDIM+2].str==1 and
          vartype == CCTK_VARIABLE_REAL)
      {
        // Optimised version for stride 1 and CCTK_REAL
        copy_data_back<CCTK_REAL>
          (info, dstdetail, dstoffset, dstelems, dstdata, dstptrs, 
           n, varis, nvaris, info[0].flip, info[1].flip, info[2].flip);
      } else if (dstdetail[n*SLAB_MAXDIM  ].str==1 and
                 dstdetail[n*SLAB_MAXDIM+1].str==1 and
                 dstdetail[n*SLAB_MAXDIM+2].str==1 and
                 vartype == CCTK_VARIABLE_COMPLEX)
      {
        // Optimised version for stride 1 and CCTK_COMPLEX
        copy_data_back<CCTK_COMPLEX>
          (info, dstdetail, dstoffset, dstelems, dstdata, dstptrs, 
           n, varis, nvaris, info[0].flip, info[1].flip, info[2].flip);
      } else if (dstdetail[n*SLAB_MAXDIM  ].str==1 and
                 dstdetail[n*SLAB_MAXDIM+1].str==1 and
                 dstdetail[n*SLAB_MAXDIM+2].str==1 and
                 vartype == CCTK_VARIABLE_INT)
      {
        // Optimised version for stride 1 and CCTK_INT
        copy_data_back<CCTK_INT>
          (info, dstdetail, dstoffset, dstelems, dstdata, dstptrs, 
           n, varis, nvaris, info[0].flip, info[1].flip, info[2].flip);
      } else {
        // Generic, unoptimised version
        CCTK_TimerStartI (timer_copy_back_general);
        
        int const dstdetailleni = dstdetail[n*SLAB_MAXDIM+0].len;
        int const dstdetaillenj = dstdetail[n*SLAB_MAXDIM+1].len;
        int const dstdetaillenk = dstdetail[n*SLAB_MAXDIM+2].len;
        
        for (int vari=0; vari<nvaris; ++vari) {
          char * restrict const dstptr = (char * restrict)dstptrs[varis[vari]];
          char const * restrict const dstdataptr =
            &dstdata[vartypesize * (dstoffset[n] + vari * dstelems[n])];
          
#         pragma omp parallel for
          for (int k = 0; k < dstdetaillenk; ++k) {
            for (int j = 0; j < dstdetaillenj; ++j) {
              for (int i = 0; i < dstdetailleni; ++i) {
                int ipos[SLAB_MAXDIM];
                ipos[0] = i;
                ipos[1] = j;
                ipos[2] = k;
                int bufipos[SLAB_MAXDIM];
                int dstipos[SLAB_MAXDIM];
                for (int d=0; d<SLAB_MAXDIM; ++d) {
                  if (not info[d].flip) {
                    bufipos[d] = ipos[d];
                  } else {
                    bufipos[d] = dstdetail[n*SLAB_MAXDIM+d].len - 1 - ipos[d];
                  }
                  ifcheck assert (bufipos[d] >= 0 and bufipos[d] < dstdetail[n*SLAB_MAXDIM+d].len);
                  dstipos[d] = dstdetail[n*SLAB_MAXDIM+d].off + ipos[d] * info[d].dst.slab.str;
                  ifcheck assert (dstipos[d] >= info[d].dst.local.off and
                                  dstipos[d] < info[d].dst.local.off + info[d].dst.local.len);
                  ifcheck assert (dstipos[d] >= info[d].dst.slab.off and
                                  dstipos[d] <= info[d].dst.slab.off + (info[d].dst.slab.len - 1) * info[d].dst.slab.str);
                  ifcheck assert ((dstipos[d] - info[d].dst.slab.off) % info[d].dst.slab.str == 0);
                }
                size_t bufind = 0;
                size_t dstind = 0;
                for (int d=SLAB_MAXDIM-1; d>=0; --d) {
                  bufind = bufind * dstdetail[n*SLAB_MAXDIM+d].len + bufipos[d];
                  dstind = dstind * info[d].dst.local.alen + dstipos[d] - info[d].dst.local.off;
                }
                ifcheck assert (bufind < (size_t)dstcount[n]);
                ifcheck assert (dstind < dstlentot);
                memcpy (dstptr + vartypesize * dstind,
                        dstdataptr + vartypesize * bufind,
                        vartypesize);
              }
            }
          }
          
        } // for vari
        CCTK_TimerStopI (timer_copy_back_general);
        
      }
      
    } // for n
    CCTK_TimerStopI (timer_copy_back);
    
  } // for vartypei
  
  
  
  ifcheck {
    ifdebug fflush (stdout);
    MPI_Barrier (comm);
  }
  
  CCTK_TimerStopI (timer_apply);
  
  return 0;
}



extern "C"
int
Slab_MultiTransfer_Finalize
(cGH       const * restrict const cctkGH,
 slabsetup       * restrict const slabsetup)
{
  DECLARE_CCTK_PARAMETERS;
  
  // Check arguments
  check (cctkGH);
  check (slabsetup);
  
  delete slabsetup;
  
  return 0;
}



// Interface for transferring a variable in one go
extern "C"
int
Slab_MultiTransfer (cGH      const                  * restrict const cctkGH,
                    int                                        const dim,
                    xferinfo const                  * restrict const xferinfo,
                    int                                        const options,
                    int                                        const nvars,
                    int      const                  * restrict const srctypes,
                    void     const * restrict const * restrict const srcptrs,
                    int      const                  * restrict const dsttypes,
                    void           * restrict const * restrict const dstptrs)
{
  slabsetup * restrict const slabsetup =
    Slab_MultiTransfer_Init (cctkGH, dim, xferinfo, options);
  Slab_MultiTransfer_Apply (cctkGH, slabsetup,
                            nvars, srctypes, srcptrs, dsttypes, dstptrs);
  Slab_MultiTransfer_Finalize (cctkGH, slabsetup);
  return 0;
}



// Old interface for transferring a single variable
extern "C"
int
Slab_Transfer (cGH      const * restrict const cctkGH,
               int                       const dim,
               xferinfo const * restrict const xferinfo,
               int                       const options,
               int                       const srctype,
               void     const * restrict const srcptr,
               int                       const dsttype,
               void           * restrict const dstptr)
{
  int const nvars = 1;
  int const srctypes[] = { srctype };
  void const * restrict const srcptrs[] = { srcptr };
  int const dsttypes[] = { dsttype };
  void * restrict const dstptrs[] = { dstptr };
  return Slab_MultiTransfer (cctkGH, dim, xferinfo, options,
                             nvars, srctypes, srcptrs, dsttypes, dstptrs);
}



// Fortran wrapper
extern "C"
void CCTK_FCALL
CCTK_FNAME(Slab_Transfer) (int                * restrict const ierr,
                           cGH  const * const * restrict const cctkGH,
                           int  const         * restrict const dim,
                           int  const         * restrict const src_gsh,
                           int  const         * restrict const src_lbnd,
                           int  const         * restrict const src_lsh,
                           int  const         * restrict const src_ash,
                           int  const         * restrict const src_lbbox,
                           int  const         * restrict const src_ubbox,
                           int  const         * restrict const src_nghostzones,
                           int  const         * restrict const src_off,
                           int  const         * restrict const src_str,
                           int  const         * restrict const src_len,
                           int  const         * restrict const dst_gsh,
                           int  const         * restrict const dst_lbnd,
                           int  const         * restrict const dst_lsh,
                           int  const         * restrict const dst_ash,
                           int  const         * restrict const dst_lbbox,
                           int  const         * restrict const dst_ubbox,
                           int  const         * restrict const dst_nghostzones,
                           int  const         * restrict const dst_off,
                           int  const         * restrict const dst_str,
                           int  const         * restrict const dst_len,
                           int  const         * restrict const xpose,
                           int  const         * restrict const flip,
                           int  const         * restrict const options,
                           int  const         * restrict const srctype,
                           void const         * restrict const srcptr,
                           int  const         * restrict const dsttype,
                           void               * restrict const dstptr)
{
  vector<xferinfo> xferinfo (*dim);
  
  for (int d=0; d<*dim; ++d) {
    xferinfo[d].src.gsh         = src_gsh[d];
    xferinfo[d].src.lbnd        = src_lbnd[d];
    xferinfo[d].src.lsh         = src_lsh[d];
    xferinfo[d].src.ash         = src_ash[d];
    xferinfo[d].src.lbbox       = src_lbbox[d];
    xferinfo[d].src.ubbox       = src_ubbox[d];
    xferinfo[d].src.nghostzones = src_nghostzones[d];
    xferinfo[d].src.off         = src_off[d];
    xferinfo[d].src.str         = src_str[d];
    xferinfo[d].src.len         = src_len[d];
    
    xferinfo[d].dst.gsh         = dst_gsh[d];
    xferinfo[d].dst.lbnd        = dst_lbnd[d];
    xferinfo[d].dst.lsh         = dst_lsh[d];
    xferinfo[d].dst.ash         = dst_ash[d];
    xferinfo[d].dst.lbbox       = dst_lbbox[d];
    xferinfo[d].dst.ubbox       = dst_ubbox[d];
    xferinfo[d].dst.nghostzones = dst_nghostzones[d];
    xferinfo[d].dst.off         = dst_off[d];
    xferinfo[d].dst.str         = dst_str[d];
    xferinfo[d].dst.len         = dst_len[d];
    
    xferinfo[d].xpose           = xpose[d];
    xferinfo[d].flip            = flip[d];
  }
  
  *ierr = Slab_Transfer (*cctkGH, *dim, &xferinfo.front(), *options,
                         *srctype, srcptr, *dsttype, dstptr);
}