Random.c

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/*
 * This file contains a portable random generator. It will give
 * identical sequences of random integers for any platform with
 * at least 32-bit integers.
 *
 * A version of this generator is described in J. Bentley's column, 
 * "The Software Exploratorium", Unix Review 1991. It is based on 
 * Algorithm A in D. E. Knuth, The Art of Computer Programming, 
 * Vol 2, Section 3.2.2, pp. 172.  
 *  
 * The Random function returns a pseudo-random integer in the range
 * 0...INT_MAX-1.
 *   
 * The SRandom function uses the given seed for a new sequence of
 * pseudo-random numbers.  
 */

unsigned Random(void);
void SRandom(unsigned Seed);

#undef STDLIB_RANDOM
/* #define STDLIB_RANDOM */

#ifdef STDLIB_RANDOM
#include <stdlib.h>
unsigned Random()
{
    return rand();
}

void SRandom(unsigned Seed)
{
    srand(Seed);
}

#else

#include <limits.h>
#define PRANDMAX INT_MAX

static int a = 0, b = 24, arr[55], initialized = 0;

unsigned Random()
{
    int t;

    if (!initialized)
        SRandom(7913);
    if (a-- == 0)
        a = 54;
    if (b-- == 0)
        b = 54;
    if ((t = arr[a] - arr[b]) < 0)
        t += PRANDMAX;
    return (arr[a] = t);
}

void SRandom(unsigned Seed)
{
    int i, ii, last, next;

    Seed %= PRANDMAX;
    arr[0] = last = Seed;
    for (next = i = 1; i < 55; i++) {
        ii = (21 * i) % 55;
        arr[ii] = next;
        if ((next = last - next) < 0)
            next += PRANDMAX;
        last = arr[ii];
    }
    initialized = 1;
    a = 0;
    b = 24;
    for (i = 0; i < 165; i++)
        Random();
}

#endif