SUBROUTINE RPN_COMM_adj_halo (g,minx,maxx,miny,maxy,ni,nj,nk,halox,haloy,periodx,periody,gni,npol_row)

ARGUMENTS

g	Array containing the local tile	real or integer	I/O
minx,maxx, miny, maxy	Total dimensions of array g (nk for z axis)	integer	I
ni,nj,nk	Dimensions of local array	integer	I
halox,haloy	Halo width for x and y	integer	I
periodx,periody	Periodicity for x and y	logical	I
gni	Global array size(=problem size) along x	integer	I
npol_row	Number of polar rows of processes (semi-lag case)	integer	I

Local array size: g(minx:maxx,miny:maxy,nk)
Local problem size: g(ni,nj,nk)
Local problem size + halos: g(1-halox,ni+halox,1-haloy,nj+haloy,nk)
Row size (semi-lag case): g(gni,nj,nk)

                       maxx
minx +-------------------+ maxy
     |     halo          |
     | +---------------+ |
     | |     ni        | |
     | |             nj| |
     | |(1,1,k)        | |
     | +---------------+ |
     |                   |
     +-------------------+ miny

Size of the problem and decomposition of the domain for 3x3 topology :

      1        gni
      +--+--+--+
      |P6|P7|P8|
      +--+--+--+
      |P3|P4|P5|
      +--+--+--+
      |P0|P1|P2|
      +--+--+--+
      1         gni

East-west exchange example: The "x" column is added to the "y" column on the local array of the rigth neighbor

            P0                                                                    P1

+-----------+                  +-------------+
| halo     x|                  |  y+x        |
| +-------+x|                  | +---------+ |
| |       |x|    --------->    | |y+x      | |
| |       |x|                  | |y+x      | |
| +-------+x|                  | +---------+ |
| halo     x|                  |  y+x        |
+-----------+                  +-------------+

Semi-lagrangian case, halo exchange example: here, we gather local arrays of a row of processes. At the end of the exchange, each process of the row has a copy of the "global" row.

            P0                            P1                                P2                                P0 and P1 and P2
                                                                                                                      (a=x+y, b=y+z)
+----------+   +----------+   +----------+    +---------------------+
|xxxxzzxxxx|   |yyyyyyyyyy|   |zzzzzzzzzz|    |xxxxxxaayyyybbzzzzzzz|
|x+------+x|   |y+------+y|   |z+------+z|    |x+-----------------+z|
|x|xxxxxx|x| + |y|yyyyyy|y| + |z|zzzzzz|z| -> |x|xxxxaayyyybbzzzzz|z|
|x|xxxxxx|x|   |y|yyyyyy|y|   |z|zzzzzz|z|    |x|xxxxaayyyybbzzzzz|z|
|x+------+x|   |y+------+y|   |z+------+z|    |x+-----------------+z|
|xxxxxxxxxx|   |yyyyyyyyyy|   |zzzzzzzzzz|    |xxxxxxaayyyybbzzzzzzz|
+----------+   +----------+   +----------+    +---------------------+
 

N.B.: Obviously, we need, minx <= 1-halox < ni+halox <= maxx (same for y)