i have problem in understanding the v_unshift function as it checks for v->ofs as 0 and creates a sublist ,it means sublist is full , but in the initial instance of vlist creation it has a sublist and ofs as 0 ,then why it needs to create a sublist in first insertion.
also please explain the ofs and buf as i want to understand it more better.

struct sublist
{
    struct sublist* next;
    int *buf;
};
/*
 * Vlist Node Declaration
 */
typedef struct vlist_t 
{
    sublist* head;
    int last_size, ofs;
}*vlist;
/*
 * Vlist Class Declaration
 */
class v_list
{
    public:
        sublist *sublist_new(int);
        vlist v_new();
        void v_del(vlist);
        int v_size(vlist);
        int* v_addr(vlist, int);
        int v_elem(vlist, int);
        int* v_unshift(vlist, int);
        int v_shift(vlist);
        v_list()
        {}
};

*
 * Creating new sublist
 */
sublist *v_list::sublist_new(int s)
{
    sublist* sub = (sublist *)malloc(sizeof(sublist) + sizeof(int) * s);
    sub->buf = (int*)(sub + 1);
    sub->next = 0;
    return sub;
}
/*
 * Creating Vlist from Sublist
 */
vlist v_list::v_new()
{
    vlist v = new(vlist_t);
    v->head = sublist_new(1);
    v->last_size = 1;
    v->ofs = 0;
    return v;
}
 
/*
 * Deleting Vlist
 */
void v_list::v_del(vlist v)
{
    sublist *s;
    while (v->head) 
    {
        s = v->head->next;
        free(v->head);
        v->head = s;
    }
    free(v);
}
 
/*
 * Compute Length of Vlist
 */
int v_list::v_size(vlist v)
{
    return v->last_size * 2 - v->ofs - 2;
}
 
int *v_list::v_addr(vlist v, int idx)
{
    sublist *s = v->head;
    int top, i;
    top = v->last_size; 
    i = idx + v->ofs;
    if (i + 2 >= (top << 1)) 
    {
        cout<<"!: idx "<<idx<<" out of range"<<endl;
        abort();
    }
    while (s && i >= top) 
    {
        s = s->next;
        i ^= top;
        top >>= 1;
    }
    return s->buf + i;
}
 
/*
 * Locate Element at any position in the Vlist 
 */
int v_list::v_elem(vlist v, int idx)
{
    return *v_addr(v, idx);
}
 
/*
 * Add Element in the Vlist 
 */
int *v_list::v_unshift(vlist v, int x)
{
    sublist* s;
    int *p;
    if (!v->ofs) 
    {
        if (!(s = sublist_new(v->last_size << 1))) 
        {
            cout<<"allocation failure"<<endl;
            return 0;
        }
        v->ofs = (v->last_size <<= 1);  
        s->next = v->head;
        v->head = s;
    }
    *(p = v->head->buf + --v->ofs) = x;
    return p;
}
 
/*
 * Remove Element from the Vlist 
 */
int v_list::v_shift(vlist v)
{
    sublist* s;
    int x;
    if (v->last_size == 1 && v->ofs == 1) 
    {
        cout<<"empty list"<<endl;
        abort();
    }
    x = v->head->buf[v->ofs++];
    if (v->ofs == v->last_size) 
    {
        v->ofs = 0;
        if (v->last_size > 1) 
        {
            s = v->head;
            v->head = s->next;
            v->last_size >>= 1;
            free(s);
        }
    }
    return x;
}