/* addthis.c
   
This is a sample user defined plug-in function for use with GrADS.  
It does a very basic task: it adds a number to all the non-missing
values in a GrADS expression, which may be for gridded or station data.

The function takes two arguments: the first is the GrADS expression 
and the second is the number to add to the result of the expression. 

Example: 
'define tempf=addthis(tempc*9/5,32)'

************************************************************************

Instructions for compiling this sample program are in the documentation:
http://cola.gmu.edu/grads/gadoc/udp.html#compile

************************************************************************

GrADS plug-in functions are passed pointers to three data structures :
  1. gafunc -- contains info about the function call including
     the number of arguments and pointers to the argument strings 
  2. gastat -- the GrADS status structure contains pointers to file 
     structures and defined variables, the current dimension environment,
     and results from expression evaluations
  3. gaudpinfo -- used for keeping track of plug-in compatibility

These structures are defined in the main GrADS include file, grads.h,
which is why that file must be included here. 

************************************************************************

This plug-in also uses some internal GrADS functions: 
  1. gaprnt -- prints messages that can be captured by the scripting language
  2. getdbl -- parses a string and converts it to a double-precision number
  3. gaexpr -- evaluates the GrADS expression and does the required I/O

************************************************************************/

#include "grads.h"    

gaint addthis (struct gafunc *pfc, struct gastat *pst, struct gaudpinfo *pinfo) {
gaint i,rc;
struct gagrid *pgr;
struct gastn *stn;
struct garpt *rpt;
gadouble *val,addend;
char *valu;

  /* Check if this plug-in is still compatible with the GrADS interface */
  if (pinfo->version != UDPVERS) printf("Error: version mismatch. Recompile this plug-in\n");

  /* Check for the proper number of arguments */
  if (pfc->argnum!=2) {
    gaprnt (0,"Error from ADDTHIS: Two arguments expected. \n");
    return (1);
  }

  /* Parse the second argument, which should be a number */
  if (getdbl(pfc->argpnt[1],&addend) == NULL) {
    gaprnt (0,"Error from ADDTHIS: the user-provided addend is not a number\n");
    return (1);
  }
 
  /* Invoke gaexpr to evaluate the first argument, which should be a GrADS expression.
     The reuslt gets stored in the gastat structure 'pst' */
  rc = gaexpr(pfc->argpnt[0],pst);
  if (rc) return (rc);

  /* If the expression is a grid ... */
  if (pst->type==1) {
    /* Set up pointers to the gridded data structure, the grid of data, and the undef mask  */
    pgr = pst->result.pgr;
    val = pgr->grid;
    valu = pgr->umask;
    /* Loop over all grid points */ 
    for (i=0; i<pgr->isiz*pgr->jsiz; i++) {
      /* Add the user-provided number to the non-missing grid values */
      if (*valu!=0) *val = addend+(*val);
      val++; valu++;
    }
  } 
  /* ...the expression is station data */
  else {              
    /* Set up pointers to the station data structure and the report chain */
    stn = pst->result.stn;
    rpt = stn->rpt;
    /* Loop over all reports */
    while (rpt!=NULL) {
      /* Add the user-provided number to the non-missing station values */
      if (rpt->umask!=0) rpt->val = addend+(rpt->val);
      rpt=rpt->rpt;
    }
  }

  /* finish without errors */
  return (0);
}