#include <stdio.h>
#include <stdlib.h>
#include "grib2.h"

void seekgb(FILE *lugb,g2int iseek,g2int mseek,g2int *lskip,g2int *lgrib)
//$$$  SUBPROGRAM DOCUMENTATION BLOCK
//
// SUBPROGRAM: seekgb         Searches a file for the next GRIB message.
//   PRGMMR: Gilbert          ORG: W/NP11      DATE: 2002-10-28
//
// ABSTRACT: This subprogram searches a file for the next GRIB Message.
//   The search is done starting at byte offset iseek of the file referenced
//   by lugb for mseek bytes at a time.
//   If found, the starting position and length of the message are returned
//   in lskip and lgrib, respectively.
//   The search is terminated when an EOF or I/O error is encountered.
//
// PROGRAM HISTORY LOG:
// 2002-10-28  GILBERT   Modified from Iredell's skgb subroutine
//
// USAGE:    seekgb(FILE *lugb,g2int iseek,g2int mseek,int *lskip,int *lgrib)
//   INPUT ARGUMENTS:
//     lugb       - FILE pointer for the file to search.  File must be
//                  opened before this routine is called.
//     iseek      - number of bytes in the file to skip before search
//     mseek      - number of bytes to search at a time
//   OUTPUT ARGUMENTS:
//     lskip      - number of bytes to skip from the beginning of the file
//                  to where the GRIB message starts
//     lgrib      - number of bytes in message (set to 0, if no message found)
//
// ATTRIBUTES:
//   LANGUAGE: C
//
//$$$
{
      // g2int  ret;
      g2int k,k4,ipos,nread,lim,start,vers,end,lengrib;
      unsigned char *cbuf;

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
      *lgrib=0;
      cbuf=(unsigned char *)malloc(mseek);
      nread=mseek;
      ipos=iseek;

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
//  LOOP UNTIL GRIB MESSAGE IS FOUND

      while (*lgrib==0 && nread==mseek) {

//  READ PARTIAL SECTION

        /* ret= */ fseek(lugb,ipos,SEEK_SET);
        nread=(int)fread(cbuf,sizeof(unsigned char),mseek,lugb);
        lim=nread-8;

//  LOOK FOR 'GRIB...' IN PARTIAL SECTION

        for (k=0;k<lim;k++) {
          gbit(cbuf,&start,(k+0)*8,4*8);
          gbit(cbuf,&vers,(k+7)*8,1*8);
          if (start==1196575042 && (vers==1 || vers==2)) {
//  LOOK FOR '7777' AT END OF GRIB MESSAGE
            if (vers == 1) gbit(cbuf,&lengrib,(k+4)*8,3*8);
            if (vers == 2) gbit(cbuf,&lengrib,(k+12)*8,4*8);
            /* ret= */ fseek(lugb,ipos+k+lengrib-4,SEEK_SET);
            k4=(int)fread(&end,sizeof(g2int),1,lugb);
            if (k4 == 1 && end == 926365495) {      //GRIB message found
                *lskip=ipos+k;
                *lgrib=lengrib;
                break;
            }
          }
        }
        ipos=ipos+lim;
      }

      free(cbuf);
}
