// 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

public class SkipList {  // Skiplist class
  private SkipNode head; // Header node for skiplist -- always there
  private int level;     // Level of the current deepest node
  // Convention: A level 0 node has 1 pointer (at array index 0)

  public SkipList() {    // Constructor
    head = new SkipNode(null, 0); // The header is initially at level 0
    level = -1;          // Indicates no nodes of any level
  }

  private void AdjustHead(int newLevel) { // Create a new (larger) head
    SkipNode temp = head;
    head = new SkipNode(null, newLevel);
    for (int i=0; i<=level; i++)
      head.forward[i] = temp.forward[i];
    level = newLevel;
  }

  public void print() {    // Print out contents of a skiplist
    SkipNode temp = head;
    while (temp != null) {
      System.out.print(temp.value() + ": length is "
                       + temp.forward.length + ":");
      for (int i=0; i<temp.forward.length; i++)
        if (temp.forward[i] == null)
          System.out.print("null ");
        else
          System.out.print(temp.forward[i].value() + " ");
      System.out.println();
      temp = temp.forward[0];
    }
    System.out.println();
  }
int randomLevel() { // Pick a level on exponential distribution
  int lev;
  for (lev=0; DSutil.random(2) == 0; lev++); // Do nothing
  return lev;
}
public void insert(Elem newValue) { // Insert into skiplist
  int newLevel = randomLevel();     // Select level for new node
  if (newLevel > level)             // New node will be deepest
    AdjustHead(newLevel);           // Add null pointers to header
  SkipNode[] update = new SkipNode[level+1]; // Track end of level
  SkipNode x = head;                // Start at header node
  for (int i=level; i>=0; i--) {    // Search for insert position
    while((x.forward[i] != null) &&
          (((Elem)x.forward[i].value()).key() < newValue.key()))
      x = x.forward[i];
    update[i] = x;                  // Keep track of end at level i
  }
  x = new SkipNode(newValue, newLevel);  // Create new node
  for (int i=0; i<=newLevel; i++) {      // Splice into list
    x.forward[i] = update[i].forward[i]; // Who x points to
    update[i].forward[i] = x;            // Who y points to
  }
}
public Elem search(int searchKey) { // Skiplist Search
  SkipNode x = head;                // Dummy header node
  for (int i=level; i>=0; i--)      // For each level...
    while ((x.forward[i] != null) &&  // go forward
           (((Elem)x.forward[i].value()).key() < searchKey))
      x = x.forward[i];             // Go one last step
  x = x.forward[0];  // Move to actual record, if it exists
  if ((x != null) && (((Elem)x.value()).key() == searchKey))
    return (Elem)x.value();               // Got it
  else return null;                 // Its not there
}

}