/* SList.java */

/**
 *  The SList class is a singly-linked implementation of the linked list
 *  abstraction.  SLists are mutable data structures, which can grow at either
 *  end.
 *
 *  @author Kathy Yelick and Jonathan Shewchuk
 **/

public class SList {

  private SListNode head;
  private SListNode tail;
  private int size;

  /**
   *  SList() constructs an empty list.
   **/

  public SList() {
    size = 0;
    head = null;
    tail=null;
  }

  /**
   *  isEmpty() indicates whether the list is empty.
   *  @return true if the list is empty, false otherwise.
   **/

  public boolean isEmpty() {
    return size == 0;
  }

  /**
   *  length() returns the length of this list.
   *  @return the length of this list.
   **/

  public int length() {
    return size;
  }

  /**
   *  insertFront() inserts item "obj" at the beginning of this list.
   *  @param obj the item to be inserted.
   **/

  public void insertFront(Object obj) {
    head = new SListNode(obj, head);
    size++;
    if(size==1){tail=head;
      }
  }

  /**
   *  insertEnd() inserts item "obj" at the end of this list.
   *  @param obj the item to be inserted.
   **/

  public void insertEnd(Object obj) {
    if (tail == null) {
      insertFront(obj);
    } else {
      tail.next=new SListNode(obj,null);
      tail=tail.next;
      size++;}
    
  }

  /**
   *  nth() returns the item at the specified position.  If position < 1 or
   *  position > this.length(), null is returned.  Otherwise, the item at
   *  position "position" is returned.  The list does not change.
   *  @param position the desired position, from 1 to length(), in the list.
   *  @return the item at the given position in the list.
   **/

  public Object nth(int position) {
    SListNode currentNode;

    if ((position < 1) || (head == null)) {
      return null;
    } else {
      currentNode = head;
      while (position > 1) {
        currentNode = currentNode.next;
        if (currentNode == null) {
          return null;
        }
        position--;
      }
      return currentNode.item;
    }
  } 

  /**
   *  toString() converts the list to a String.
   *  @return a String representation of the list.
   **/

  public String toString() {
    int i;
    Object obj;
    String result = "[  ";

    SListNode cur = head;

    while (cur != null) {
      obj = cur.item;
      result = result + obj.toString() + "  ";
      cur = cur.next;
    }
    result = result + "]";
    return result;
  }


  /**
   *  main() runs test cases on the SList class.  Prints summary
   *  information on basic operations and halts with an error (and a stack
   *  trace) if any of the tests fail.
   **/

  public static void main (String[] args) {
    SList a=new SList();
    a.insertFront(12);
    a.insertFront(9);
    a.insertFront(6);
    a.insertFront(3);
    a.insertEnd(15);
    
    System.out.println(a.toString());
    // Fill in your solution for Part I here.

    testEmpty();
    testAfterInsertFront();
    testAfterInsertEnd();
  }

    
  /**
   *  testEmpty() tests toString(), isEmpty(), length(), insertFront(), and
   *  insertEnd() on an empty list.  Prints summary information of the tests
   *  and halts the program if errors are detected.
   **/

  private static void testEmpty() {
    SList lst1 = new SList();
    SList lst2 = new SList();
    System.out.println();
    System.out.println("Here is a list after construction: "
               + lst1.toString());
    TestHelper.verify(lst1.toString().equals("[  ]"),
              "toString on newly constructed list failed");

    System.out.println("isEmpty() should be true. It is: " +
               lst1.isEmpty());
    TestHelper.verify(lst1.isEmpty() == true,
              "isEmpty() on newly constructed list failed");    

    System.out.println("length() should be 0. It is: " +
               lst1.length());
    TestHelper.verify(lst1.length() == 0, 
              "length on newly constructed list failed");    
    lst1.insertFront(new Integer(3));
    System.out.println("Here is a list after insertFront(3) to an empty list: "
               + lst1.toString());
    TestHelper.verify(lst1.toString().equals("[  3  ]"),
              "InsertFront on empty list failed");
    lst2.insertEnd(new Integer(5));
    System.out.println("Here is a list after insertEnd(5) on an empty list: "
               + lst2.toString());
    TestHelper.verify(lst2.toString().equals("[  5  ]"),
              "insertEnd on empty list failed");
  }

  /**
   *  testAfterInsertFront() tests toString(), isEmpty(), length(),
   *  insertFront(), and insertEnd() after insertFront().  Prints summary
   *  information of the tests and halts the program if errors are detected.
   **/

  private static void testAfterInsertFront() {
    SList lst1 = new SList();
    lst1.insertFront(new Integer(3));
    lst1.insertFront(new Integer(2));
    lst1.insertFront(new Integer(1));
    System.out.println();
    System.out.println("Here is a list after insertFront 3, 2, 1: "
               + lst1.toString());
    TestHelper.verify(lst1.toString().equals("[  1  2  3  ]"),
              "InsertFronts on non-empty list failed");
    System.out.println("isEmpty() should be false. It is: " +
               lst1.isEmpty());
    TestHelper.verify(lst1.isEmpty() == false,
              "isEmpty() after insertFront failed");    
    System.out.println("length() should be 3. It is: " +
               lst1.length());
    TestHelper.verify(lst1.length() == 3, 
              "length() after insertFront failed");
    lst1.insertEnd(new Integer(4));
    System.out.println("Here is the same list after insertEnd(4): "
               + lst1.toString());
    TestHelper.verify(lst1.toString().equals("[  1  2  3  4  ]"),
              "insertEnd on non-empty list failed");
  }
    
  /**
   *  testAfterInsertEnd() tests toString(), isEmpty(), length(),
   *  insertFront(), and insertEnd() after insertEnd().  Prints summary
   *  information of the tests and halts the program if errors are detected.
   **/

  private static void testAfterInsertEnd() {
    SList lst1 = new SList();
    lst1.insertEnd(new Integer(6));
    lst1.insertEnd(new Integer(7));
    System.out.println();
    System.out.println("Here is a list after insertEnd 6, 7: "
               + lst1.toString());
    System.out.println("isEmpty() should be false. It is: " +
               lst1.isEmpty());
    TestHelper.verify(lst1.isEmpty() == false,
              "isEmpty() after insertEnd failed");    
    System.out.println("length() should be 2. It is: " +
               lst1.length());
    TestHelper.verify(lst1.length() == 2, 
              "length() after insertEndfailed");
    lst1.insertFront(new Integer(5));
    System.out.println("Here is the same list after insertFront(5): "
               + lst1.toString());
    TestHelper.verify(lst1.toString().equals("[  5  6  7  ]"),
              "insertFront after insertEnd failed");
  }
}