Task: Huffman coding





Java: import java.util.*;

abstract class HuffmanTree implements Comparable<HuffmanTree> {
    public final int frequency; 
    public HuffmanTree(int freq) { frequency = freq; }

    
    public int compareTo(HuffmanTree tree) {
        return frequency - tree.frequency;
    }
}

class HuffmanLeaf extends HuffmanTree {
    public final char value; 
   
    public HuffmanLeaf(int freq, char val) {
        super(freq);
        value = val;
    }
}

class HuffmanNode extends HuffmanTree {
    public final HuffmanTree left, right; 
   
    public HuffmanNode(HuffmanTree l, HuffmanTree r) {
        super(l.frequency + r.frequency);
        left = l;
        right = r;
    }
}

public class HuffmanCode {
    
    public static HuffmanTree buildTree(int[] charFreqs) {
        PriorityQueue<HuffmanTree> trees = new PriorityQueue<HuffmanTree>();
        
        
        for (int i = 0; i < charFreqs.length; i++)
            if (charFreqs[i] > 0)
                trees.offer(new HuffmanLeaf(charFreqs[i], (char)i));

        assert trees.size() > 0;
        
        while (trees.size() > 1) {
            
            HuffmanTree a = trees.poll();
            HuffmanTree b = trees.poll();

            
            trees.offer(new HuffmanNode(a, b));
        }
        return trees.poll();
    }

    public static void printCodes(HuffmanTree tree, StringBuffer prefix) {
        assert tree != null;
        if (tree instanceof HuffmanLeaf) {
            HuffmanLeaf leaf = (HuffmanLeaf)tree;

            
            System.out.println(leaf.value + "\t" + leaf.frequency + "\t" + prefix);

        } else if (tree instanceof HuffmanNode) {
            HuffmanNode node = (HuffmanNode)tree;

            
            prefix.append('0');
            printCodes(node.left, prefix);
            prefix.deleteCharAt(prefix.length()-1);

            
            prefix.append('1');
            printCodes(node.right, prefix);
            prefix.deleteCharAt(prefix.length()-1);
        }
    }

    public static void main(String[] args) {
        String test = "this is an example for huffman encoding";

        
        
        int[] charFreqs = new int[256];
        
        for (char c : test.toCharArray())
            charFreqs[c]++;

        
        HuffmanTree tree = buildTree(charFreqs);

        
        System.out.println("SYMBOL\tWEIGHT\tHUFFMAN CODE");
        printCodes(tree, new StringBuffer());
    }
}



Python: from heapq import heappush, heappop, heapify
from collections import defaultdict

def encode(symb2freq):
    
    heap = [[wt, [sym, ""]] for sym, wt in symb2freq.items()]
    heapify(heap)
    while len(heap) > 1:
        lo = heappop(heap)
        hi = heappop(heap)
        for pair in lo[1:]:
            pair[1] = '0' + pair[1]
        for pair in hi[1:]:
            pair[1] = '1' + pair[1]
        heappush(heap, [lo[0] + hi[0]] + lo[1:] + hi[1:])
    return sorted(heappop(heap)[1:], key=lambda p: (len(p[-1]), p))

txt = "this is an example for huffman encoding"
symb2freq = defaultdict(int)
for ch in txt:
    symb2freq[ch] += 1


huff = encode(symb2freq)
print "Symbol\tWeight\tHuffman Code"
for p in huff:
    print "%s\t%s\t%s" % (p[0], symb2freq[p[0]], p[1])