g2_create.c

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#include <stdio.h>
#include "grib2_int.h"
 
#define MAPSEC1LEN 13 
#define LENSEC0 16 
g2int
g2_create(unsigned char *cgrib, g2int *listsec0, g2int *listsec1)
{
    g2int zero = 0, one = 1;
 
    /* The mapsec1 array tells us how many bytes are used in the GRIB
     * message by each element of the listsec1 array. For example the
     * first two elements of listsec1 are identifcation of originating
     * section and sub-section - these are each 2-byte entries in the
     * GRIB section 1 table, the IDENTIFICATION SECTION. (See
     * https://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_doc/grib2_sect1.shtml). */
    g2int mapsec1[MAPSEC1LEN] = {2, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1};
    g2int i, lensec1, iofst, ibeg, nbits, len;
 
    /* Only GRIB Edition 2 is acceptable. */
    if (listsec0[1] != 2)
    {
        printf("g2_create: can only code GRIB edition 2.");
        return G2_CREATE_GRIB_VERSION;
    }
 
    /* Pack Section 0 - Indicator Section (except for total length of
     * GRIB message). */
    cgrib[0] = 0x47; /* 'G' */
    cgrib[1] = 0x52; /* 'R' */
    cgrib[2] = 0x49; /* 'I' */
    cgrib[3] = 0x42; /* 'B' */
    sbit(cgrib, &zero, 32, 16); /* reserved for future use */
    sbit(cgrib, listsec0 + 0, 48, 8); /* Discipline */
    sbit(cgrib, listsec0 + 1, 56, 8); /* GRIB edition number */
 
    /* Pack Section 1 - Identification Section. */
    ibeg = LENSEC0 * 8; /* Calculate offset for beginning of section 1. */
    iofst = ibeg + 32; /* Leave space for length of section. */
    sbit(cgrib, &one, iofst, 8); /* Store section number (1). */
    iofst = iofst + 8;
 
    /* Pack up each input value in array listsec1 into the the
     * appropriate number of octets, which are specified in
     * corresponding entries in array mapsec1. */
    for (i = 0; i < MAPSEC1LEN; i++)
    {
        nbits = mapsec1[i] * 8;
        sbit(cgrib, listsec1 + i, iofst, nbits);
        iofst = iofst + nbits;
    }
 
    /* Calculate length of section 1 and store it in octets 1-4 of
     * section 1. */
    lensec1 = (iofst - ibeg) / 8;
    sbit(cgrib, &lensec1, ibeg, 32);
 
    /* Put current byte total of message into Section 0. */
    sbit(cgrib, &zero, 64, 32);
    len = LENSEC0 + lensec1;
    sbit(cgrib, &len, 96, 32);
    return (len);
}