feat: add HuffmanCoding with fail-fast validation and immutable design

singhc7 · singhc7 · commit ce781829dd0f · 2026-02-24T21:55:37.000-06:00
diff --git a/src/main/java/com/thealgorithms/compression/HuffmanCoding.java b/src/main/java/com/thealgorithms/compression/HuffmanCoding.java
@@ -0,0 +1,242 @@
+package com.thealgorithms.compression;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+
+/**
+ * Huffman Coding Compression Algorithm Implementation.
+ * <p>
+ * Huffman Coding is a popular greedy algorithm used for lossless data compression.
+ * It reduces the overall size of data by assigning variable-length, prefix-free
+ * binary codes to input characters, ensuring that more frequent characters receive
+ * the shortest possible codes.
+ * </p>
+ * <p>
+ * <strong>Key Features:</strong>
+ * <ul>
+ * <li>Uses a PriorityQueue (min-heap) to efficiently construct the optimal prefix tree.</li>
+ * <li>Fail-fast design throws exceptions for unsupported characters and malformed binary payloads.</li>
+ * <li>Immutable internal dictionary state prevents external tampering with generated codes.</li>
+ * <li>Robust handling of edge cases, including single-character strings and incomplete sequences.</li>
+ * </ul>
+ * </p>
+ * @author Chahat Sandhu, <a href="https://github.com/singhc7">singhc7</a>
+ * @see <a href="https://en.wikipedia.org/wiki/Huffman_coding">Huffman Coding (Wikipedia)</a>
+ */
+public class HuffmanCoding {
+
+    private Node root;
+    private final Map<Character, String> huffmanCodes;
+
+    /**
+     * Represents a node within the Huffman Tree.
+     * Implements {@link Comparable} to allow sorting by frequency in a PriorityQueue.
+     */
+    private static class Node implements Comparable<Node> {
+        final char ch;
+        final int freq;
+        final Node left, right;
+
+        /**
+         * Constructs a leaf node containing a specific character and its frequency.
+         *
+         * @param ch   The character stored in this leaf.
+         * @param freq The frequency of occurrence of the character.
+         */
+        Node(char ch, int freq) {
+            this.ch = ch;
+            this.freq = freq;
+            this.left = null;
+            this.right = null;
+        }
+
+        /**
+         * Constructs an internal node that merges two child nodes.
+         * The character is defaulted to the null character ('\0').
+         *
+         * @param freq  The combined frequency of the left and right child nodes.
+         * @param left  The left child node.
+         * @param right The right child node.
+         */
+        Node(int freq, Node left, Node right) {
+            this.ch = '\0';
+            this.freq = freq;
+            this.left = left;
+            this.right = right;
+        }
+
+        /**
+         * Determines if the current node is a leaf (contains no children).
+         *
+         * @return {@code true} if both left and right children are null, {@code false} otherwise.
+         */
+        boolean isLeaf() {
+            return left == null && right == null;
+        }
+
+        /**
+         * Compares this node with another node based on their frequencies.
+         * Used by the PriorityQueue to maintain the min-heap property.
+         *
+         * @param other The other Node to compare against.
+         * @return A negative integer, zero, or a positive integer as this node's frequency
+         * is less than, equal to, or greater than the specified node's frequency.
+         */
+        @Override
+        public int compareTo(Node other) {
+            return Integer.compare(this.freq, other.freq);
+        }
+    }
+
+    /**
+     * Initializes the Huffman Tree and generates immutable prefix-free codes
+     * based on the character frequencies in the provided text.
+     *
+     * @param text The input string used to calculate frequencies and build the optimal tree.
+     * If null or empty, an empty tree and dictionary are created.
+     */
+    public HuffmanCoding(String text) {
+        if (text == null || text.isEmpty()) {
+            this.huffmanCodes = Collections.emptyMap();
+            return;
+        }
+
+        Map<Character, String> tempCodes = new HashMap<>();
+        buildTree(text);
+        generateCodes(root, "", tempCodes);
+
+        if (tempCodes.size() == 1) {
+            tempCodes.put(root.ch, "0");
+        }
+
+        this.huffmanCodes = Collections.unmodifiableMap(tempCodes);
+    }
+
+    /**
+     * Computes character frequencies and constructs the Huffman Tree using a min-heap.
+     * The optimal tree is built by repeatedly extracting the two lowest-frequency nodes
+     * and merging them until a single root node remains.
+     *
+     * @param text The input text to analyze.
+     */
+    private void buildTree(String text) {
+        Map<Character, Integer> freqMap = new HashMap<>();
+        for (char c : text.toCharArray()) {
+            freqMap.put(c, freqMap.getOrDefault(c, 0) + 1);
+        }
+
+        PriorityQueue<Node> pq = new PriorityQueue<>();
+        for (Map.Entry<Character, Integer> entry : freqMap.entrySet()) {
+            pq.add(new Node(entry.getKey(), entry.getValue()));
+        }
+
+        while (pq.size() > 1) {
+            Node left = pq.poll();
+            Node right = pq.poll();
+            pq.add(new Node(left.freq + right.freq, left, right));
+        }
+
+        root = pq.poll();
+    }
+
+    /**
+     * Recursively traverses the Huffman Tree to generate prefix-free binary codes.
+     * Left traversals append a '0' to the code, while right traversals append a '1'.
+     *
+     * @param node The current node in the traversal.
+     * @param code The accumulated binary string for the current path.
+     * @param map  The temporary dictionary to populate with the final character-to-code mappings.
+     */
+    private void generateCodes(Node node, String code, Map<Character, String> map) {
+        if (node == null) return;
+        if (node.isLeaf()) {
+            map.put(node.ch, code);
+            return;
+        }
+        generateCodes(node.left, code + "0", map);
+        generateCodes(node.right, code + "1", map);
+    }
+
+    /**
+     * Encodes the given plaintext string into a binary string using the generated Huffman dictionary.
+     *
+     * @param text The plaintext string to compress.
+     * @return A string of '0's and '1's representing the compressed data.
+     * Returns an empty string if the input is null or empty.
+     * @throws IllegalStateException    If attempting to encode when the Huffman tree is empty.
+     * @throws IllegalArgumentException If the input text contains a character not present
+     * in the original text used to build the tree.
+     */
+    public String encode(String text) {
+        if (text == null || text.isEmpty()) return "";
+        if (root == null) throw new IllegalStateException("Huffman tree is empty.");
+
+        StringBuilder sb = new StringBuilder();
+        for (char c : text.toCharArray()) {
+            if (!huffmanCodes.containsKey(c)) {
+                throw new IllegalArgumentException(String.format("Character '%c' (U+%04X) not found in Huffman dictionary.", c, (int) c));
+            }
+            sb.append(huffmanCodes.get(c));
+        }
+        return sb.toString();
+    }
+
+    /**
+     * Decodes the given binary string back into the original plaintext using the Huffman Tree.
+     * Validates the integrity of the binary payload during traversal.
+     *
+     * @param encodedText The binary string of '0's and '1's to decompress.
+     * @return The reconstructed plaintext string. Returns an empty string if the input is null or empty.
+     * @throws IllegalStateException    If attempting to decode when the Huffman tree is empty.
+     * @throws IllegalArgumentException If the binary string contains characters other than '0' or '1',
+     * or if the sequence ends abruptly without reaching a leaf node.
+     */
+    public String decode(String encodedText) {
+        if (encodedText == null || encodedText.isEmpty()) return "";
+        if (root == null) throw new IllegalStateException("Huffman tree is empty.");
+
+        StringBuilder sb = new StringBuilder();
+
+        if (root.isLeaf()) {
+            for (char bit : encodedText.toCharArray()) {
+                if (bit != '0') {
+                    throw new IllegalArgumentException("Invalid binary sequence for single-character tree.");
+                }
+                sb.append(root.ch);
+            }
+            return sb.toString();
+        }
+
+        Node current = root;
+        for (char bit : encodedText.toCharArray()) {
+            if (bit != '0' && bit != '1') {
+                throw new IllegalArgumentException("Encoded text contains invalid characters: " + bit);
+            }
+
+            current = (bit == '0') ? current.left : current.right;
+
+            if (current.isLeaf()) {
+                sb.append(current.ch);
+                current = root;
+            }
+        }
+
+        if (current != root) {
+            throw new IllegalArgumentException("Malformed encoded string: incomplete sequence ending.");
+        }
+
+        return sb.toString();
+    }
+
+    /**
+     * Retrieves the generated Huffman dictionary mapping characters to their binary codes.
+     *
+     * @return An unmodifiable map containing the character-to-binary-code mappings to prevent
+     * external mutation of the algorithm's state.
+     */
+    public Map<Character, String> getHuffmanCodes() {
+        return huffmanCodes;
+    }
+}
diff --git a/src/test/java/com/thealgorithms/compression/HuffmanCodingTest.java b/src/test/java/com/thealgorithms/compression/HuffmanCodingTest.java
@@ -0,0 +1,107 @@
+package com.thealgorithms.compression;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class HuffmanCodingTest {
+
+    @Test
+    void testStandardLifecycle() {
+        String input = "efficiency is key";
+        HuffmanCoding huffman = new HuffmanCoding(input);
+
+        String encoded = huffman.encode(input);
+        assertNotNull(encoded);
+        assertTrue(encoded.matches("[01]+"));
+        assertEquals(input, huffman.decode(encoded));
+    }
+
+    @Test
+    void testNullAndEmptyHandling() {
+        HuffmanCoding huffman = new HuffmanCoding("");
+        assertEquals("", huffman.encode(""));
+        assertEquals("", huffman.decode(""));
+
+        HuffmanCoding huffmanNull = new HuffmanCoding(null);
+        assertEquals("", huffmanNull.encode(null));
+        assertEquals("", huffmanNull.decode(null));
+    }
+
+    @Test
+    void testSingleCharacterEdgeCase() {
+        String input = "aaaaa";
+        HuffmanCoding huffman = new HuffmanCoding(input);
+
+        String encoded = huffman.encode(input);
+        assertEquals("00000", encoded);
+        assertEquals(input, huffman.decode(encoded));
+    }
+
+    @Test
+    void testUnicodeAndSpecialCharacters() {
+        // Tests spacing, symbols, non-latin alphabets, and surrogate pairs (emojis)
+        String input = "Hello, World! 🚀\nLine 2: こんにちは";
+        HuffmanCoding huffman = new HuffmanCoding(input);
+
+        String encoded = huffman.encode(input);
+        assertEquals(input, huffman.decode(encoded));
+    }
+
+    @Test
+    void testFailFastOnUnseenCharacter() {
+        HuffmanCoding huffman = new HuffmanCoding("abc");
+
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+            () -> huffman.encode("abcd") // 'd' was not in the original tree
+        );
+        assertTrue(exception.getMessage().contains("not found in Huffman dictionary"));
+    }
+
+    @Test
+    void testFailFastOnInvalidBinaryCharacter() {
+        HuffmanCoding huffman = new HuffmanCoding("abc");
+        String encoded = huffman.encode("abc");
+
+        // Inject a '2' into the binary stream
+        String corruptedEncoded = encoded + "2";
+
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> huffman.decode(corruptedEncoded));
+        assertTrue(exception.getMessage().contains("contains invalid characters"));
+    }
+
+    @Test
+    void testFailFastOnIncompleteSequence() {
+        HuffmanCoding huffman = new HuffmanCoding("abcd");
+        String encoded = huffman.encode("abc");
+
+        // Truncate the last bit to simulate an incomplete byte/sequence transfer
+        String truncatedEncoded = encoded.substring(0, encoded.length() - 1);
+
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> huffman.decode(truncatedEncoded));
+        assertTrue(exception.getMessage().contains("incomplete sequence"));
+    }
+
+    @Test
+    void testImmutabilityOfDictionary() {
+        HuffmanCoding huffman = new HuffmanCoding("abc");
+        var codes = huffman.getHuffmanCodes();
+
+        assertThrows(UnsupportedOperationException.class, () -> codes.put('z', "0101"));
+    }
+
+    @Test
+    void testStressVolume() {
+        StringBuilder sb = new StringBuilder();
+        // Generate a 100,000 character string
+        for (int i = 0; i < 100000; i++) {
+            sb.append((char) ('a' + (i % 26)));
+        }
+        String largeInput = sb.toString();
+
+        HuffmanCoding huffman = new HuffmanCoding(largeInput);
+        String encoded = huffman.encode(largeInput);
+
+        assertEquals(largeInput, huffman.decode(encoded));
+    }
+}
