source: lmdz_wrf/WRFV3/external/io_grib1/WGRIB/ibm2flt.c @ 1

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

/* ibm2flt       wesley ebisuzaki
 *
 * v1.1 .. faster
 * v1.1 .. if mant == 0 -> quick return
 *
 */


double ibm2flt(unsigned char *ibm) {

        int positive, power;
        unsigned int abspower;
        long int mant;
        double value, exp;

        mant = (ibm[1] << 16) + (ibm[2] << 8) + ibm[3];
        if (mant == 0) return 0.0;

        positive = (ibm[0] & 0x80) == 0;
        power = (int) (ibm[0] & 0x7f) - 64;
        abspower = power > 0 ? power : -power;


        /* calc exp */
        exp = 16.0;
        value = 1.0;
        while (abspower) {
                if (abspower & 1) {
                        value *= exp;
                }
                exp = exp * exp;
                abspower >>= 1;
        }

        if (power < 0) value = 1.0 / value;
        value = value * mant / 16777216.0;
        if (positive == 0) value = -value;
        return value;
}