alpar@6: /*** This is a portable random number generator whose origins are
alpar@6:  *** unknown.  As far as can be told, this is public domain software.
alpar@6: 
alpar@6: 
alpar@6: /*** portable random number generator */
alpar@6: 
alpar@6: /*** Note that every variable used here must have at least 31 bits
alpar@6:  *** of precision, exclusive of sign.  Long integers should be enough.
alpar@6:  *** The generator is the congruential:  i = 7**5 * i mod (2^31-1).
alpar@6:  ***/
alpar@6: 
alpar@6: #define MULTIPLIER 16807
alpar@6: #define MODULUS    2147483647
alpar@6: 
alpar@6: static long saved_seed;
alpar@6: 
alpar@6: 
alpar@6: /*** set_random - initialize constants and seed */
alpar@6: 
alpar@6: void set_random(seed)
alpar@6: long seed;
alpar@6: {
alpar@6:   saved_seed = seed;
alpar@6: }
alpar@6: 
alpar@6: 
alpar@6: /*** ng_random - generate a random integer in the interval [a,b] (b >= a >= 0) */
alpar@6: 
alpar@6: long ng_random(a, b)
alpar@6: long a, b;
alpar@6: {
alpar@6:   register long hi, lo;
alpar@6: 
alpar@6:   hi = MULTIPLIER * (saved_seed >> 16);
alpar@6:   lo = MULTIPLIER * (saved_seed & 0xffff);
alpar@6:   hi += (lo>>16);
alpar@6:   lo &= 0xffff;
alpar@6:   lo += (hi>>15);
alpar@6:   hi &= 0x7fff;
alpar@6:   lo -= MODULUS;
alpar@6:   if ((saved_seed = (hi<<16) + lo) < 0)
alpar@6:     saved_seed += MODULUS;
alpar@6: 
alpar@6:   if (b <= a)
alpar@6:     return b;
alpar@6:   return a + saved_seed % (b - a + 1);
alpar@6: }