/* 6dd.c  2000 rolf.freitag at email.de */
// An example function to convert the true random bytes from /dev/rp1m,
// which are equally distributed in the range 0...255, into a number which is
// equally distributed in the range 0...999999.
// If a guaranteed maximum answer time is needed this funktion can be enhanced with a
// timeout and an instant conversion via <4 byte random number> modulo 1000000 (which will
// produce an not perfect equally distributed random number) or a pseudorandom number
// generator (usually this is not needed).

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

int
main (void)
{
  unsigned int ui1 = 0;                           // random variable
  int i2 = 0;                                     // return value
  union                                           // Union of one integer and sizeof(int) random bytes (both without sign).
  {
    unsigned int ui;
    unsigned char uc[sizeof (int)];
  } u =
  {
    0
  };
  int lc = 0;                                     // local counter
  FILE *fp = NULL;                                // file pointer to the random device

  fp = fopen ("/dev/rp1m", "rb");
  if (fp == NULL)
  {
    perror ("\aerror: can't open device ");
    exit (errno);
  }
  while (lc < 2)                                  // produce 2 times 3 decimal digits; this loop is done 1.024 times at average
  {
    fscanf (fp, "%c", u.uc);                      // read a true random byte
    fscanf (fp, "%c", &u.uc[1]);                  // read a true random byte
    ui1 = (u.uc[1] >> 2);                         // store the six for u unused bits
    if ((u.ui &= 0x3ff) < 1000)                   // Is it a 3 decimal digit number?
    {
      i2 += u.ui * (lc ? 1000 : 1);               // the last or the first 3 decimal digits
      lc++;                                       // three decimal digits done
    }
    if (lc < 2)
    {
      fscanf (fp, "%c", u.uc);                    // read a true random byte
      fscanf (fp, "%c", &u.uc[1]);                // read a true random byte
      ui1 |= (u.uc[1] >> 2) << 6;                 // store the six for u unused bits
      if ((u.ui &= 0x3ff) < 1000)                 // Is it a 3 decimal digit number?
      {
        i2 += u.ui * (lc ? 1000 : 1);             // the last or the first 3 decimal digits
        lc++;                                     // three decimal digits done
      }
    }
    if ((lc < 2) && ((ui1 &= 0x3ff) < 1000))      // use 10 of the 12 unused true random bits if
    {                                             // it is a 3 decimal digit number
      i2 += ui1 * (lc ? 1000 : 1);                // the last or the first 3 decimal digits
      lc++;                                       // three decimal digits done
    }
  }
  fclose (fp);
  fprintf (stdout, "Wirklich zufaellige Zufallszahl zwischen 0 und 999999, ");
  fprintf (stdout, "die mit einer\nWahrscheinlichkeit ");
  fprintf (stdout, "von genau 1/1000000 einen der Werte 0...999999 annimmt: %u\n", i2);
  return (0);
}