Add LZW and Arithmetic Coding algorithms in compression category

src/main/java/com/thealgorithms/compression/ArithmeticCoding.java

@@ -0,0 +1,39 @@
+package com.thealgorithms.compression;
+
+import java.util.Map;
+
+/**
+ * Implementation of Arithmetic Coding algorithm.
+ * Reference: https://en.wikipedia.org/wiki/Arithmetic_coding
+ */
+
+public final class ArithmeticCoding {
+
+ private ArithmeticCoding() {
+ throw new UnsupportedOperationException("Utility class");
+ }
+
+ public static double encode(String input, Map<Character, Double> probabilities) {
+ double low = 0.0;
+ double high = 1.0;
+
+ for (char symbol : input.toCharArray()) {
+ double range = high - low;
+ double cumProb = 0.0;
+
+ for (Map.Entry<Character, Double> entry : probabilities.entrySet()) {
+ char current = entry.getKey();
+ double prob = entry.getValue();
+ double next = cumProb + prob;
+
+ if (symbol == current) {
+ high = low + range * next;
+ low = low + range * cumProb;
+ break;
+ }
+ cumProb = next;
+ }
+ }
+ return (low + high) / 2.0;
+ }
+}

src/main/java/com/thealgorithms/compression/LZW.java

@@ -0,0 +1,46 @@
+package com.thealgorithms.compression;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Implementation of LZW (Lempel–Ziv–Welch) compression algorithm.
+ * Reference: https://en.wikipedia.org/wiki/Lempel–Ziv–Welch
+ */
+
+public final class LZW {
+
+ private LZW() {
+ throw new UnsupportedOperationException("Utility class");
+ }
+
+ public static List<Integer> compress(String input) {
+ int dictSize = 256;
+ Map<String, Integer> dictionary = new HashMap<>();
+ for (int i = 0; i < 256; i++) {
+ dictionary.put("" + (char) i, i);
+ }
+
+ String w = "";
+ List<Integer> result = new ArrayList<>();
+
+ for (char c : input.toCharArray()) {
+ String wc = w + c;
+ if (dictionary.containsKey(wc)) {
+ w = wc;
+ } else {
+ result.add(dictionary.get(w));
+ dictionary.put(wc, dictSize++);
+ w = "" + c;
+ }
+ }
+
+ if (!w.isEmpty()) {
+ result.add(dictionary.get(w));
+ }
+
+ return result;
+ }
+}