[C] Generatore random tra 0-1

[killercode]

Header

#define Norm_Factor   2.328306437080797e-10

#ifdef MACRO
#define Ranf()  (((n1=n1->next)->Y += (n2=n2->next)->Y), (double)n1->Y * Norm_Factor)
#else
#define Ranf() rand55()
double rand55();
#endif

void rand_init(long);

typedef struct Lnk_List *Lnk_List_Ptr;
struct Lnk_List {
	unsigned long Y;
	Lnk_List_Ptr next;
} ;

File

#include <stdio.h>
#include <stdlib.h> 
#include "rand55.h" 

long Seed=161803398L;

Lnk_List_Ptr Ran, n1, n2;

#ifdef INLINE
inline
#endif
double rand55()
{
    n1=n1->next; 
    n2=n2->next;
    n1->Y += n2->Y;

    return  (double)n1->Y * Norm_Factor ;
}

void rand_init(long idum)
{
    int i,j;
    long tmp,aux;
    double null;

    if((Ran = (Lnk_List_Ptr)calloc(55,sizeof(struct Lnk_List)))==NULL)
	exit(1);
    for(i=0;i<54;++i) {
	(Ran+i)->next = Ran+i+1;
    }
    (Ran+54)->next = Ran;
    tmp = Seed + idum;
    Ran[54].Y = tmp;
    aux=1;
    j=20;
    for(i=0;i<54;++i,j+=21){
	j%=55;
	Ran[j].Y=aux;
	aux+=tmp;
	tmp=Ran[j].Y;
    }

    n1=Ran;
    n2=Ran+31;

    for(j=0;j<55*4;++j) {
	null += rand55();
    }
    fprintf(stderr,"Rand55 initialized - chksum=%f\n",null);

}

Il file contenente la main semplicemente chiama fa funzione Ranf() e stampa il risultato. Le marco sono tutte a zero

Qualcuno mi spiegherebbe a cosa serve la funzione rand_init?
Grazie