Chapter 25 Exercise 16, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

25.16 (Data compression: Huffman coding) Write a program that prompts the user to enter a file name, then displays the frequency table of the characters in the file and displays the Huffman code for each character.

import java.io.BufferedInputStream;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.Scanner;

public class Exercise16 {
 public static void main(String[] args) throws IOException {
  @SuppressWarnings("resource")
  Scanner input = new Scanner(System.in);
  System.out.print("Enter a file name: ");
  String filename = input.nextLine();

  File file = new File(filename);
  if (!file.exists()) {
   System.out.println("File " + filename + " does not exist");
   System.exit(1);
  }
  
  DataInputStream inputFile = new DataInputStream(new BufferedInputStream(new FileInputStream(file)));
  int size = inputFile.available();
  byte[] b = new byte[size];
  inputFile.read(b);
  inputFile.close();
  
  int[] counts = getCharacterFrequency(new String(b));

  System.out.printf("%-15s%-15s%-15s%-15s\n", "ASCII Code", "Character",
    "Frequency", "Code");

  Tree tree = getHuffmanTree(counts); // Create a Huffman tree
  String[] codes = getCode(tree.root); // Get codes

  for (int i = 0; i < codes.length; i++)
   if (counts[i] != 0) // (char)i is not in text if counts[i] is 0
    System.out.printf("%-15d%-15s%-15d%-15s\n", i, (char) i + "",
      counts[i], codes[i]);
 }

 /**
  * Get Huffman codes for the characters This method is called once after a
  * Huffman tree is built
  */
 public static String[] getCode(Tree.Node root) {
  if (root == null)
   return null;
  String[] codes = new String[2 * 128];
  assignCode(root, codes);
  return codes;
 }

 /* Recursively get codes to the leaf node */
 private static void assignCode(Tree.Node root, String[] codes) {
  if (root.left != null) {
   root.left.code = root.code + "0";
   assignCode(root.left, codes);

   root.right.code = root.code + "1";
   assignCode(root.right, codes);
  } else {
   codes[(int) root.element] = root.code;
  }
 }

 /** Get a Huffman tree from the codes */
 public static Tree getHuffmanTree(int[] counts) {
  // Create a heap to hold trees
  Heap<Tree> heap = new Heap<Tree>(); // Defined in Listing 24.10
  for (int i = 0; i < counts.length; i++) {
   if (counts[i] > 0)
    heap.add(new Tree(counts[i], (char) i)); // A leaf node tree
  }

  while (heap.getSize() > 1) {
   Tree t1 = heap.remove(); // Remove the smallest weight tree
   Tree t2 = heap.remove(); // Remove the next smallest weight
   heap.add(new Tree(t1, t2)); // Combine two trees
  }

  return heap.remove(); // The final tree
 }

 /** Get the frequency of the characters */
 public static int[] getCharacterFrequency(String text) {
  int[] counts = new int[256]; // 256 ASCII characters

  for (int i = 0; i < text.length(); i++)
   counts[(int) text.charAt(i)]++; // Count the character in text

  return counts;
 }

 /** Define a Huffman coding tree */
 public static class Tree implements Comparable<Tree> {
  Node root; // The root of the tree

  /** Create a tree with two subtrees */
  public Tree(Tree t1, Tree t2) {
   root = new Node();
   root.left = t1.root;
   root.right = t2.root;
   root.weight = t1.root.weight + t2.root.weight;
  }

  /** Create a tree containing a leaf node */
  public Tree(int weight, char element) {
   root = new Node(weight, element);
  }

  @Override
  /** Compare trees based on their weights */
  public int compareTo(Tree t) {
   if (root.weight < t.root.weight) // Purposely reverse the order
    return 1;
   else if (root.weight == t.root.weight)
    return 0;
   else
    return -1;
  }

  public class Node {
   char element; // Stores the character for a leaf node
   int weight; // weight of the subtree rooted at this node
   Node left; // Reference to the left subtree
   Node right; // Reference to the right subtree
   String code = ""; // The code of this node from the root

   /** Create an empty node */
   public Node() {
   }

   /** Create a node with the specified weight and character */
   public Node(int weight, char element) {
    this.weight = weight;
    this.element = element;
   }
  }
 }

 static class Heap<E extends Comparable<E>> {
  private java.util.ArrayList<E> list = new java.util.ArrayList<E>();

  /** Create a default heap */
  public Heap() {
  }

  /** Create a heap from an array of objects */
  public Heap(E[] objects) {
   for (int i = 0; i < objects.length; i++)
    add(objects[i]);
  }

  /** Add a new object into the heap */
  public void add(E newObject) {
   list.add(newObject); // Append to the heap
   int currentIndex = list.size() - 1; // The index of the last node

   while (currentIndex > 0) {
    int parentIndex = (currentIndex - 1) / 2;
    // Swap if the current object is greater than its parent
    if (list.get(currentIndex).compareTo(list.get(parentIndex)) > 0) {
     E temp = list.get(currentIndex);
     list.set(currentIndex, list.get(parentIndex));
     list.set(parentIndex, temp);
    } else
     break; // the tree is a heap now

    currentIndex = parentIndex;
   }
  }

  /** Remove the root from the heap */
  public E remove() {
   if (list.size() == 0)
    return null;

   E removedObject = list.get(0);
   list.set(0, list.get(list.size() - 1));
   list.remove(list.size() - 1);

   int currentIndex = 0;
   while (currentIndex < list.size()) {
    int leftChildIndex = 2 * currentIndex + 1;
    int rightChildIndex = 2 * currentIndex + 2;

    // Find the maximum between two children
    if (leftChildIndex >= list.size())
     break; // The tree is a heap
    int maxIndex = leftChildIndex;
    if (rightChildIndex < list.size()) {
     if (list.get(maxIndex).compareTo(list.get(rightChildIndex)) < 0) {
      maxIndex = rightChildIndex;
     }
    }

    // Swap if the current node is less than the maximum
    if (list.get(currentIndex).compareTo(list.get(maxIndex)) < 0) {
     E temp = list.get(maxIndex);
     list.set(maxIndex, list.get(currentIndex));
     list.set(currentIndex, temp);
     currentIndex = maxIndex;
    } else
     break; // The tree is a heap
   }

   return removedObject;
  }

  /** Get the number of nodes in the tree */
  public int getSize() {
   return list.size();
  }
 }
}