// Source code example for "A Practical Introduction
// to Data Structures and Algorithm Analysis"
// by Clifford A. Shaffer, Prentice Hall, 1998.
// Copyright 1998 by Clifford A. Shaffer

// A modified version of the linked list implementation
// that uses a freelist.  Only a few methods are actually different.
class LList implements List { // Linked list class (freelist)
private Link head;            // Pointer to list header
private Link tail;            // Pointer to last Object in list 
private Link curr;            // Position of current Object
LList(int sz) { setup(); }    // Constructor -- Ignore sz
LList() { setup(); }          // Constructor

private void setup()          // Do the initializations
{ tail = head = curr = Link.get(null, null); } // Create header node

public void clear() {         // Remove all Elems from list
  head.setNext(null);
  curr = tail = head;         // Reinitialize
}

// The following three methods, assert, append, and remove, are the
// only ones affected by the use of the freelist methods.

// Insert Object at current position
public void insert(Object it) {
  Assert.notNull(curr, "No current element");
  curr.setNext(Link.get(it, curr.next())); // Get Link
  if (tail == curr)             // Appended new Object
    tail = curr.next();
}

public void append(Object it) { // Insert Object at tail of list
  tail.setNext(Link.get(it, null));        // Get Link
  tail = tail.next();
}

public Object remove() {          // Remove and return current Elem
  Assert.notFalse(isInList(), "No current element");
  Object it = curr.next().element();    // Remember value
  if (tail == curr.next()) tail = curr; // Removed last: set tail
  Link tempptr = curr.next();
  curr.setNext(curr.next().next());     // Remove from list
  tempptr.release();                    // Release Link
  return it;                            // Return value removed
}

public void setFirst()          // Set curr to first position
{ curr = head; }

public void next()              // Move curr to next position
{ if (curr != null)  curr = curr.next(); }

public void prev() {            // Move curr to previous position
  Link temp = head;
  if ((curr == null) || (curr == head)) // No previous Object
  { curr = null;  return; }             //   so just return
  while ((temp != null) && (temp.next() != curr))
	temp = temp.next();
  curr = temp;
}

public int length() {           // Return current length of list
  int cnt = 0;
  for (Link temp = head.next(); temp != null; temp = temp.next())
     cnt++;                     // Count the number of Objects
  return cnt;
}

public void setPos(int pos) { // Set curr to specified position
  curr = head;
  for(int i=0; (curr!=null) && (i<pos); i++)
    curr = curr.next();
}

public void setValue(Object it)  // Replace current Object
{ Assert.notFalse(isInList());  curr.next().setElement(it); }

public Object currValue()         // Return current Object
{ Assert.notFalse(isInList());  return curr.next().element(); }

public boolean isEmpty()        // Return true if list is empty
{ return head.next() == null; }

public boolean isInList()       // true if curr is within list
  { return (curr != null) && (curr.next() != null); }


public void print() {          // Print out the list's elements
  if (isEmpty()) System.out.println("()");
  else {
    System.out.print("( ");
	for (setFirst(); isInList(); next())
	  System.out.print(currValue() + " ");
    System.out.println(")");
  }
}

} // class List