Huffman coding in C#

Huffman coding is an encoding algorithm used for data compression. So called Huffman trees are used for encoding and decoding of data. The leaves of these trees contain the symbols and their frequencies. Each parent node contains the sum of the frequencies of its children.

Example of a Huffman tree

Huffman trees are constructed as follows:

1. Create a list of nodes where each node holds a symbol and its frequency. Sort the list according to frequency.

Ordered list of Huffman nodes for the input string "foo bar"

2. Take the first two nodes in the list and create a new node (also known as the parent node) which holds the sum of the frequencies of those nodes and the nodes as its children.

3. Add the new node to the list of nodes and remove the two nodes you’ve used to create the parent node.

4. Repeat these steps until the list contains one node. This is the root of the Huffman tree.

Node class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace HuffmanTest
{
    public class Node
    {
        public char Symbol { get; set; }
        public int Frequency { get; set; }
        public Node Right { get; set; }
        public Node Left { get; set; }

        public List<bool> Traverse(char symbol,List<bool> data)
        {
            // Leaf
            if (Right == null && Left == null)
            {
                if (symbol.Equals(this.Symbol))
                {
                    return data;
                }
                else
                {
                    return null;
                }
            }
            else
            {
                List<bool> left = null;
                List<bool> right = null;

                if (Left != null)
                {
                    List<bool> leftPath = new List<bool>();
                    leftPath.AddRange(data);
                    leftPath.Add(false);

                    left = Left.Traverse(symbol, leftPath);
                }

                if (Right != null)
                {
                    List<bool> rightPath = new List<bool>();
                    rightPath.AddRange(data);
                    rightPath.Add(true);
                    right = Right.Traverse(symbol, rightPath);
                }

                if (left != null)
                {
                    return left;
                }
                else
                {
                    return right;
                }
            }
        }
    }
}

HuffmanTree class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace HuffmanTest
{
    public class HuffmanTree
    {
        private List<Node> nodes = new List<Node>();
        public Node Root { get; set; }
        public Dictionary<char, int> Frequencies = new Dictionary<char, int>();

        public void Build(string source)
        {
            for (int i = 0; i < source.Length; i++)
            {
                if (!Frequencies.ContainsKey(source[i]))
                {
                    Frequencies.Add(source[i], 0);
                }

                Frequencies[source[i]]++;
            }

            foreach (KeyValuePair<char, int> symbol in Frequencies)
            {
                nodes.Add(new Node() { Symbol = symbol.Key, Frequency = symbol.Value });
            }

            while (nodes.Count > 1)
            {
                List<Node> orderedNodes = nodes.OrderBy(node => node.Frequency).ToList<Node>();

                if (orderedNodes.Count >= 2)
                {
                    // Take first two items
                    List<Node> taken = orderedNodes.Take(2).ToList<Node>();

                    // Create a parent node by combining the frequencies
                    Node parent = new Node()
                    {
                        Symbol = '*',
                        Frequency = taken[0].Frequency + taken[1].Frequency,
                        Left = taken[0],
                        Right = taken[1]
                    };

                    nodes.Remove(taken[0]);
                    nodes.Remove(taken[1]);
                    nodes.Add(parent);
                }

                this.Root = nodes.FirstOrDefault();

            }

        }

        public BitArray Encode(string source)
        {
            List<bool> encodedSource = new List<bool>();

            for (int i = 0; i < source.Length; i++)
            {
                List<bool> encodedSymbol = this.Root.Traverse(source[i], new List<bool>());
                encodedSource.AddRange(encodedSymbol);
            }

            BitArray bits = new BitArray(encodedSource.ToArray()); 

            return bits;
        }

        public string Decode(BitArray bits)
        {
            Node current = this.Root;
            string decoded = "";

            foreach(bool bit in bits){
                if (bit)
                {
                    if (current.Right != null)
                    {
                        current = current.Right;
                    }
                }
                else
                {
                    if(current.Left != null)
                    {
                        current = current.Left;
                    }
                }

                if (IsLeaf(current))
                {
                    decoded += current.Symbol;
                    current = this.Root;
                }
            }

            return decoded;
        }

        public bool IsLeaf(Node node)
        {
            return (node.Left == null && node.Right == null);
        }

    }
}

The main program

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace HuffmanTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = "foo bar";
            HuffmanTree huffmanTree = new HuffmanTree();

            // Build the Huffman tree
            huffmanTree.Build(input);

            // Encode
            BitArray encoded = huffmanTree.Encode(input);

            Console.Write("Encoded: ");
            foreach (bool bit in encoded)
            {
                Console.Write((bit ? 1 : 0) + "");
            }
            Console.WriteLine();

            // Decode
            string decoded = huffmanTree.Decode(encoded);

            Console.WriteLine("Decoded: " + decoded);

            Console.ReadLine();
        }
    }
}