package GCB;
/*
    Define the head of a linked list
        the head contains a pointer to the first list entry
                                       the last list entry
                          the count of entries in the list
        getCount()       returns the count of entries on the list
        getFirst()       returns first entry on the list
        getLast()        returns last entry on the list
        setHead(entry)   places entry on the head of the list
        getHead()        returns (and removes) the head of the list
        setTail(entry)   places entry on the tail of the list
        getTail()        returns (and removes) the tail of the list
        remove(entry)    removes the entry from the list
*/
public class ListHead {

//  interal attributes
static final String CODE_FILE = "ListHead";
static final int CODE_REVISION = 2;

    private ListEntry first,                 // start of the linked list
                      last;                  // end of the linked list
    private int count;                       // number of entries on linked list

//  internal methods
    void display() {
        ListEntry e = (ListEntry) getFirst();
        while(e!=null) {
            System.out.println(e.toString());
            e = (ListEntry) e.getNext();
        }
    }

//  external methods
//  constructors
    public ListHead() {
        first = last = null;
        count = 0;
    }
    public int getCount() {return count;}       // get number of entries on list
    public ListEntry getFirst() {return first;} // get first entry
    public ListEntry getLast() {return last;}   // get last entry
    public void setHead(ListEntry e) {          // place at head of list
        if(count==0)                            // list is empty
            first = last = e;
        else {                                  // put head after new entry
            first.setPrior(e);
            e.setNext(first);
            first = e;
        }
        count++;
        e.setHead(this);
    }
    public ListEntry getHead() {                // get the head of the list
        if(count==0) return null;               // list is empty
        ListEntry e = first;                    // pop first entry
        first = e.getNext();
        count--;
        if(count==0) last = null;
        else first.setPrior(null);
        e.clean();
        return e;
    }
    public void setTail(ListEntry e) {          // place at tail of list
        if(count==0)                            // list is empty
            first = last = e;
        else {                                  // put tail before new entry
            last.setNext(e);
            e.setPrior(last);
            last = e;
        }
        count++;
        e.setHead(this);
    }
    public ListEntry getTail() {                // get the tail of the list
        if(count==0) return null;               // list is empty
        ListEntry e = last;                     // cut last entry
        last = e.getPrior();
        count--;
        if(count==0) first = null;
        else last.setNext(null);
        e.clean();
        return e;
    }
    public void remove(ListEntry e) {           // remove the entry from a list
        if(e.getPrior()==null) {                // head of the list
            if(first!=e) {
                System.out.println(CODE_FILE+
                                   " attempt to remove entry (head?) from wrong list!");
                System.out.println(CODE_FILE+" entry="+e.toString());
                System.out.println(CODE_FILE+" entry="+e.getClass());
                System.out.println(CODE_FILE+" head="+this.toString());
                System.out.println(CODE_FILE+" head="+this.getClass());
                Thread.dumpStack();
                return;
            }
            getHead();                          // pop this off the list
            return;
        }
        if(e.getNext()==null) {                 // end of the list
            if(last!=e) {
                System.out.println(CODE_FILE+
                                   " attempt to remove entry (tail?) from wrong list!");
                System.out.println(CODE_FILE+" entry="+e.toString());
                System.out.println(CODE_FILE+" entry="+e.getClass());
                System.out.println(CODE_FILE+" head="+this.toString());
                System.out.println(CODE_FILE+" head="+this.getClass());
                Thread.dumpStack();
                return;
            }
            getTail();                          // pull this off the list
            return;
        }
        ListEntry l = getFirst();               // find on the list
        while(l!=null) {
            if(l==e) {
                (e.getPrior()).setNext(e.getNext());  // correct forward linkage
                (e.getNext()).setPrior(e.getPrior()); // correct backward linkage
                e.clean();
                count--;
                return;
            }
            l = l.getNext();
        }
        System.out.println(CODE_FILE+
                           " attempt to remove entry (middle?) from wrong list!");
        System.out.println(CODE_FILE+" entry="+e.toString());
        System.out.println(CODE_FILE+" entry="+e.getClass());
        System.out.println(CODE_FILE+" head="+this.toString());
        System.out.println(CODE_FILE+" head="+this.getClass());
        Thread.dumpStack();
    }
}