simpack.c

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#include <stdlib.h>
#include <math.h>
#include "grib2.h"
 
void
simpack(g2float *fld, g2int ndpts, g2int *idrstmpl, 
    unsigned char *cpack, g2int *lcpack)
{
    static g2int zero = 0;
    g2int *ifld;
    g2int j, nbits, imin, imax, maxdif, nbittot, left;
    g2float bscale, dscale, rmax, rmin, temp;
    double maxnum;
    static g2float alog2 = 0.69314718;       /*  ln(2.0) */
 
    bscale = int_power(2.0, -idrstmpl[1]);
    dscale = int_power(10.0, idrstmpl[2]);
    if (idrstmpl[3] <= 0 || idrstmpl[3] > 31)
        nbits = 0;
    else
        nbits = idrstmpl[3];
 
    /* Find max and min values in the data. */
    rmax = fld[0];
    rmin = fld[0];
    for (j = 1; j < ndpts; j++) {
        if (fld[j] > rmax)
        rmax = fld[j];
        if (fld[j] < rmin)
        rmin = fld[j];
    }
 
    ifld = calloc(ndpts, sizeof(g2int));
 
    /* If max and min values are not equal, pack up field. If they are
     * equal, we have a constant field, and the reference value (rmin)
     * is the value for each point in the field and set nbits to 0. */
    if (rmin != rmax) {
 
        /* Determine which algorithm to use based on user-supplied
     * binary scale factor and number of bits. */
        if (nbits == 0 && idrstmpl[1] == 0) {
 
            /* No binary scaling and calculate minumum number of bits
         * in which the data will fit. */
            imin = (g2int)rint(rmin * dscale);
            imax = (g2int)rint(rmax * dscale);
            maxdif = imax - imin;
            temp = log((double)(maxdif + 1)) / alog2;
            nbits = (g2int)ceil(temp);
            rmin = (g2float)imin;
            /*   scale data */
            for(j = 0; j < ndpts; j++)
                ifld[j] = (g2int)rint(fld[j] * dscale) - imin;
        }
        else if (nbits != 0 && idrstmpl[1] == 0) {
 
            /* Use minimum number of bits specified by user and adjust
         * binary scaling factor to accomodate data. */
            rmin = rmin * dscale;
            rmax = rmax * dscale;
            maxnum = int_power(2.0, nbits) - 1;
            temp = log(maxnum / (rmax - rmin)) / alog2;
            idrstmpl[1] = (g2int)ceil(-1.0 * temp);
            bscale = int_power(2.0, -idrstmpl[1]);
            /*   scale data */
            for (j = 0; j < ndpts; j++)
                ifld[j] = (g2int)rint(((fld[j] * dscale) - rmin) * bscale);
        }
        else if (nbits == 0 && idrstmpl[1] != 0) {
 
            /* Use binary scaling factor and calculate minumum number
         * of bits in which the data will fit. */
            rmin = rmin * dscale;
            rmax = rmax * dscale;
            maxdif = (g2int)rint((rmax - rmin) * bscale);
            temp = log((double)(maxdif + 1)) / alog2;
            nbits = (g2int)ceil(temp);
            /*   scale data */
            for (j = 0; j < ndpts; j++)
                ifld[j] = (g2int)rint(((fld[j] * dscale) - rmin) * bscale);
        }
        else if (nbits != 0 && idrstmpl[1] != 0) {
 
            /* Use binary scaling factor and use minumum number of
         * bits specified by user. Dangerous - may loose
         * information if binary scale factor and nbits not set
         * properly by user. */
            rmin = rmin * dscale;
            /*   scale data */
            for (j = 0; j < ndpts; j++)
                ifld[j] = (g2int)rint(((fld[j] * dscale) - rmin) * bscale);
        }
 
        /* Pack data, Pad last octet with Zeros, if necessary, and
     * calculate the length of the packed data in bytes. */
        sbits(cpack, ifld, 0, nbits, 0, ndpts);
        nbittot = nbits * ndpts;
        left = 8 - (nbittot % 8);
        if (left != 8) {
            sbit(cpack, &zero, nbittot, left);   /* Pad with zeros to fill Octet. */
            nbittot = nbittot + left;
        }
        *lcpack = nbittot / 8;
    }
    else {
        nbits = 0;
        *lcpack = 0;
    }
 
    /* Fill in ref value and number of bits in Template 5.0. */
    mkieee(&rmin, idrstmpl, 1);   /* ensure reference value is IEEE format. */
    idrstmpl[3] = nbits;
    idrstmpl[4] = 0;         /* original data were reals. */
 
    free(ifld);
}