Add Compression Algorithm

Compression/src/HEncoder.java

@@ -0,0 +1,105 @@
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+
+public class HEncoder {
+
+	public HashMap<Character, String> encoder = new HashMap<>();
+	public HashMap<String, Character> decoder = new HashMap<>();
+
+	private static class Node {
+
+		Character ch;
+		Integer freq;
+		Node left;
+		Node right;
+
+		public static final Nodecomparator Ctor = new Nodecomparator();
+
+		public static class Nodecomparator implements Comparator<Node> {
+
+			@Override
+			public int compare(Node o1, Node o2) {
+				return o2.freq - o1.freq;
+			}
+
+		}
+	}
+
+	public HEncoder(String feeder) {
+		// 1. freq map
+		HashMap<Character, Integer> freqmap = new HashMap<>();
+		for (int i = 0; i < feeder.length(); ++i) {
+			char ch = feeder.charAt(i);
+			if (freqmap.containsKey(ch)) {
+				freqmap.put(ch, freqmap.get(ch) + 1);
+			} else {
+				freqmap.put(ch, 1);
+			}
+		}
+
+		// 2. prepare the heap from keyset
+		genericheap<Node> heap = new genericheap<Node>(Node.Ctor);
+		ArrayList<Character> k = new ArrayList<>(freqmap.keySet());
+		for (Character c : k) {
+			Node n = new Node();
+			n.ch = c;
+			n.left = null;
+			n.right = null;
+			n.freq = freqmap.get(c);
+			heap.add(n);
+		}
+
+		// 3.Prepare tree, remove two , merge, add it back
+		Node fn = new Node();
+		while (heap.size() != 1) {
+			Node n1 = heap.removeHP();
+			Node n2 = heap.removeHP();
+			fn = new Node();
+
+			fn.freq = n1.freq + n2.freq;
+			fn.left = n1;
+			fn.right = n2;
+
+			heap.add(fn);
+		}
+
+		// 4. traverse
+
+		traverse(heap.removeHP(), "");
+	}
+
+	private void traverse(Node node, String osf) {
+
+		if (node.left == null && node.right == null) {
+			encoder.put(node.ch, osf);
+			decoder.put(osf, node.ch);
+			return;
+		}
+		traverse(node.left, osf + "0");
+		traverse(node.right, osf + "1");
+
+	}
+
+	public String compress(String str) {
+		String rv = "";
+		for (int i = 0; i < str.length(); ++i) {
+			rv += encoder.get(str.charAt(i));
+		}
+		return rv;
+	}
+
+	public String decompress(String str) {
+		String s = "";
+		String code = "";
+		for (int i = 0; i < str.length(); ++i) {
+			code += str.charAt(i);
+			if (decoder.containsKey(code)) {
+				s += decoder.get(code);
+				code = "";
+			}
+		}
+
+		return s;
+	}
+}