Huffman coding

Huffman coding is a lossless data compression algorithm. The idea is to assign variable-length codes to input characters, lengths of the assigned codes are based on the frequencies of corresponding characters. The most frequent character gets the smallest code and the least frequent character gets the largest code.
The variable-length codes assigned to input characters are Prefix Codes, means the codes (bit sequences) are assigned in such a way that the code assigned to one character is not prefix of code assigned to any other character. This is how Huffman Coding makes sure that there is no ambiguity when decoding the generated bit stream.
Let us understand prefix codes with a counter example. Let there be four characters a, b, c and d, and their corresponding variable length codes be 00, 01, 0 and 1. This coding leads to ambiguity because code assigned to c is prefix of codes assigned to a and b. If the compressed bit stream is 0001, the de-compressed output may be “cccd” or “ccb” or “acd” or “ab”.

Author: steffinstanly

Greedy/Huffman coding

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+
+import java.util.PriorityQueue; 
+import java.util.Scanner; 
+import java.util.Comparator; 
+  
+// node class is the basic structure 
+// of each node present in the huffman - tree. 
+class HuffmanNode { 
+  
+    int data; 
+    char c; 
+  
+    HuffmanNode left; 
+    HuffmanNode right; 
+} 
+  
+// comparator class helps to compare the node 
+// on the basis of one of its attribute. 
+// Here we will be compared 
+// on the basis of data values of the nodes. 
+class MyComparator implements Comparator<HuffmanNode> { 
+    public int compare(HuffmanNode x, HuffmanNode y) 
+    { 
+  
+        return x.data - y.data; 
+    } 
+} 
+  
+public class Huffman { 
+  
+    // recursive function to print the 
+    // huffman-code through the tree traversal. 
+    // Here s is the huffman - code generated. 
+    public static void printCode(HuffmanNode root, String s) 
+    { 
+  
+        // base case; if the left and right are null 
+        // then its a leaf node and we print 
+        // the code s generated by traversing the tree. 
+        if (root.left 
+                == null
+            && root.right 
+                   == null
+            && Character.isLetter(root.c)) { 
+  
+            // c is the character in the node 
+            System.out.println(root.c + ":" + s); 
+  
+            return; 
+        } 
+  
+        // if we go to left then add "0" to the code. 
+        // if we go to the right add"1" to the code. 
+  
+        // recursive calls for left and 
+        // right sub-tree of the generated tree. 
+        printCode(root.left, s + "0"); 
+        printCode(root.right, s + "1"); 
+    } 
+  
+    // main function 
+    public static void main(String[] args) 
+    { 
+  
+        Scanner s = new Scanner(System.in); 
+  
+        // number of characters. 
+        int n = 6; 
+        char[] charArray = { 'a', 'b', 'c', 'd', 'e', 'f' }; 
+        int[] charfreq = { 5, 9, 12, 13, 16, 45 }; 
+  
+        // creating a priority queue q. 
+        // makes a min-priority queue(min-heap). 
+        PriorityQueue<HuffmanNode> q 
+            = new PriorityQueue<HuffmanNode>(n, new MyComparator()); 
+  
+        for (int i = 0; i < n; i++) { 
+  
+            // creating a huffman node object 
+            // and adding it to the priority-queue. 
+            HuffmanNode hn = new HuffmanNode(); 
+  
+            hn.c = charArray[i]; 
+            hn.data = charfreq[i]; 
+  
+            hn.left = null; 
+            hn.right = null; 
+  
+            // add functions adds 
+            // the huffman node to the queue. 
+            q.add(hn); 
+        } 
+  
+        // create a root node 
+        HuffmanNode root = null; 
+  
+        // Here we will extract the two minimum value 
+        // from the heap each time until 
+        // its size reduces to 1, extract until 
+        // all the nodes are extracted. 
+        while (q.size() > 1) { 
+  
+            // first min extract. 
+            HuffmanNode x = q.peek(); 
+            q.poll(); 
+  
+            // second min extarct. 
+            HuffmanNode y = q.peek(); 
+            q.poll(); 
+  
+            // new node f which is equal 
+            HuffmanNode f = new HuffmanNode(); 
+  
+            // to the sum of the frequency of the two nodes 
+            // assigning values to the f node. 
+            f.data = x.data + y.data; 
+            f.c = '-'; 
+  
+            // first extracted node as left child. 
+            f.left = x; 
+  
+            // second extracted node as the right child. 
+            f.right = y; 
+  
+            // marking the f node as the root node. 
+            root = f; 
+  
+            // add this node to the priority-queue. 
+            q.add(f); 
+        } 
+  
+        // print the codes by traversing the tree 
+        printCode(root, ""); 
+    } 
+}