// Immutable linked lists of Objects
// Use the name "ObjectList" rather than "List" to avoid confusion with "List" in the AWT

import java.util.Vector;

public class ObjectList {
    
  // Instance variables
    
  private Object head;
  private ObjectList tail;
  private boolean empty;
  
  // Constructor method:
  
  private ObjectList () {
    this.empty = true;
  }
  
  private ObjectList (Object head, ObjectList tail) {
    this.head = head;
    this.tail = tail;
    this.empty = false;
  }
  
  // Instance methods: 
  
  private boolean isEmpty() {
    return empty;
  }
  
  private ObjectList prepend(Object n) {
    return new ObjectList (n, this);
  }
  
  private Object head () {
    if (empty) {
      throw new RuntimeException("Attempt to get the head of an empty ObjectList");
    } else {
      return head;
    }
  }
  
  private ObjectList tail() {
    if (empty) {
      throw new RuntimeException("Attempt to get the tail of an empty ObjectList");
    } else {
      return tail;
    }
  }
  
  public String toString () {
    if (empty) {
      return "[]";
    } else {
      StringBuffer sb = new StringBuffer();
      sb.append("[");
      sb.append(head);
      ObjectList toDo = tail;
      while (! toDo.empty) {
	sb.append(",");
	sb.append(toDo.head);
	toDo = toDo.tail;
      }
      sb.append("]");
      return sb.toString();
    }
  }

  public boolean equals (Object x) {
    if (x instanceof ObjectList) {
      ObjectList L = (ObjectList) x; 
      if (empty) {
	return L.isEmpty();
      } else if (L.isEmpty()) {
	return false;
      } else {
	return this.head().equals(L.head())
	       && this.tail().equals(L.tail());
      }
    } else {
      return false; 
    }
  }
  
  public static void prettyPrint1 (ObjectList L) {
    // Only expand outermost list to multiple lines.
    // Useful for printing list of lists. 
    ppIndent1(L);
  }
  
  private static void ppIndent1 (ObjectList L) {
    if (isEmpty(L)) {
      System.out.print("[]");
    } else {
      System.out.print("[");
      for (ObjectList elts = L; ! isEmpty(elts); elts = tail(elts)) {
	System.out.print(head(elts).toString());
	if (isEmpty(tail(elts))) {
	  System.out.println();
	} else {
	  System.out.println(",");
	}
	System.out.print(" ");
      }
      System.out.println("]");
    }
  }
  
  
  // Class Methods: 
  
  public static ObjectList empty() {
    return new ObjectList();
  }
  
  public static boolean isEmpty(ObjectList L) {
    return L.empty;
  }
  
  public static ObjectList prepend(Object n, ObjectList L) {
    return new ObjectList(n, L);
  }
  
  public static Object head(ObjectList L) {
    return L.head();
  }
  
  public static ObjectList tail(ObjectList L) {
    return L.tail();
  }

  public static String toString (ObjectList L) {
    return L.toString();
  }

  public static boolean equals (ObjectList L1, ObjectList L2) {
    return L1.equals(L2);
  }

  public static ObjectList fromString (String s) {
    return fromVector(VectorOps.fromStringHelper(s,'[',']')); 
  }

  public static ObjectList fromVector (Vector v) {
    ObjectList result = empty();
    for (int i = v.size() - 1; i >=0; i--) {
      result = prepend(v.get(i), result);
    }
    return result;
  }
  
  public static ObjectList fromArray(Object [] a) {
    ObjectList result = empty();
    for (int i = a.length - 1; i >=0; i--) {
      result = prepend(a[i], result);
    }
    return result;
  }
  
  public static void main (String [] args) {
    if (args.length == 2 && args[0].equals("fromString")) {
      System.out.println(fromString(args[1]));
    } else {
      System.out.println("unrecognized option");
    }

  }

    
}