external/io_grib1/WGRIB/ibm2flt.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3
   4 /* ibm2flt       wesley ebisuzaki
   5  *
   6  * v1.1 .. faster
   7  * v1.1 .. if mant == 0 -> quick return
   8  *
   9  */
  10
  11
  12 double ibm2flt(unsigned char *ibm) {
  13
  14         int positive, power;
  15         unsigned int abspower;
  16         long int mant;
  17         double value, exp;
  18
  19         mant = (ibm[1] << 16) + (ibm[2] << 8) + ibm[3];
  20         if (mant == 0) return 0.0;
  21
  22         positive = (ibm[0] & 0x80) == 0;
  23         power = (int) (ibm[0] & 0x7f) - 64;
  24         abspower = power > 0 ? power : -power;
  25
  26
  27         /* calc exp */
  28         exp = 16.0;
  29         value = 1.0;
  30         while (abspower) {
  31                 if (abspower & 1) {
  32                         value *= exp;
  33                 }
  34                 exp = exp * exp;
  35                 abspower >>= 1;
  36         }
  37
  38         if (power < 0) value = 1.0 / value;
  39         value = value * mant / 16777216.0;
  40         if (positive == 0) value = -value;
  41         return value;
  42 }
  43