feat(compression): Add Burrows-Wheeler Transform (BWT) and Move-to-Front (MTF)

Author: Microindole

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

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+package com.thealgorithms.compression;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Implementation of the Burrows-Wheeler Transform (BWT) and its inverse.
+ * <p>
+ * BWT is a reversible data transformation algorithm that rearranges a string into runs of
+ * similar characters. While not a compression algorithm itself, it significantly improves
+ * the compressibility of data for subsequent algorithms like Move-to-Front encoding and
+ * Run-Length Encoding.
+ * </p>
+ *
+ * <p>The transform works by:
+ * <ol>
+ *   <li>Generating all rotations of the input string</li>
+ *   <li>Sorting these rotations lexicographically</li>
+ *   <li>Taking the last column of the sorted matrix as output</li>
+ *   <li>Recording the index of the original string in the sorted matrix</li>
+ * </ol>
+ * </p>
+ *
+ * <p><b>Important:</b> The input string should end with a unique end-of-string marker
+ * (typically '$') that:
+ * <ul>
+ *   <li>Does not appear anywhere else in the text</li>
+ *   <li>Is lexicographically smaller than all other characters</li>
+ *   <li>Ensures unique rotations and enables correct inverse transformation</li>
+ * </ul>
+ * Without this marker, the inverse transform may not correctly reconstruct the original string.
+ * </p>
+ *
+ * <p><b>Time Complexity:</b>
+ * <ul>
+ *   <li>Forward transform: O(n² log n) where n is the string length</li>
+ *   <li>Inverse transform: O(n) using the LF-mapping technique</li>
+ * </ul>
+ * </p>
+ *
+ * <p><b>Example:</b></p>
+ * <pre>
+ * Input:  "banana$"
+ * Output: BWTResult("annb$aa", 4)
+ *         - "annb$aa" is the transformed string (groups similar characters)
+ *         - 4 is the index of the original string in the sorted rotations
+ * </pre>
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Burrows%E2%80%93Wheeler_transform">Burrows–Wheeler transform (Wikipedia)</a>
+ */
+public final class BurrowsWheelerTransform {
+
+    private BurrowsWheelerTransform() {
+    }
+
+    /**
+     * A container for the result of the forward BWT.
+     * <p>
+     * Contains the transformed string and the index of the original string
+     * in the sorted rotations matrix, both of which are required for the
+     * inverse transformation.
+     * </p>
+     */
+    public static class BWTResult {
+        /** The transformed string (last column of the sorted rotation matrix) */
+        public final String transformed;
+
+        /** The index of the original string in the sorted rotations matrix */
+        public final int originalIndex;
+
+        /**
+         * Constructs a BWTResult with the transformed string and original index.
+         *
+         * @param transformed the transformed string (L-column)
+         * @param originalIndex the index of the original string in sorted rotations
+         */
+        public BWTResult(String transformed, int originalIndex) {
+            this.transformed = transformed;
+            this.originalIndex = originalIndex;
+        }
+
+        @Override
+        public boolean equals(Object obj) {
+            if (this == obj) {
+                return true;
+            }
+            if (obj == null || getClass() != obj.getClass()) {
+                return false;
+            }
+            BWTResult bwtResult = (BWTResult) obj;
+            return originalIndex == bwtResult.originalIndex && transformed.equals(bwtResult.transformed);
+        }
+
+        @Override
+        public int hashCode() {
+            return 31 * transformed.hashCode() + originalIndex;
+        }
+
+        @Override
+        public String toString() {
+            return "BWTResult[transformed=" + transformed + ", originalIndex=" + originalIndex + "]";
+        }
+    }
+
+    /**
+     * Performs the forward Burrows-Wheeler Transform on the input string.
+     * <p>
+     * The algorithm generates all cyclic rotations of the input, sorts them
+     * lexicographically, and returns the last column of this sorted matrix
+     * along with the position of the original string.
+     * </p>
+     *
+     * <p><b>Note:</b> It is strongly recommended that the input string ends with
+     * a unique end-of-string marker (e.g., '$') that is lexicographically smaller
+     * than any other character in the string. This ensures correct inversion.</p>
+     *
+     * @param text the input string to transform; must not be {@code null}
+     * @return a {@link BWTResult} object containing the transformed string (L-column)
+     *         and the index of the original string in the sorted rotations matrix;
+     *         returns {@code BWTResult("", -1)} for empty input
+     * @throws NullPointerException if {@code text} is {@code null}
+     */
+    public static BWTResult transform(String text) {
+        if (text == null || text.isEmpty()) {
+            return new BWTResult("", -1);
+        }
+
+        int n = text.length();
+
+        // Generate all rotations of the input string
+        String[] rotations = new String[n];
+        for (int i = 0; i < n; i++) {
+            rotations[i] = text.substring(i) + text.substring(0, i);
+        }
+
+        // Sort rotations lexicographically
+        Arrays.sort(rotations);
+
+        // Extract the last column and find the original string's position
+        StringBuilder lastColumn = new StringBuilder(n);
+        int originalIndex = -1;
+        for (int i = 0; i < n; i++) {
+            lastColumn.append(rotations[i].charAt(n - 1));
+            if (rotations[i].equals(text)) {
+                originalIndex = i;
+            }
+        }
+
+        return new BWTResult(lastColumn.toString(), originalIndex);
+    }
+
+    /**
+     * Performs the inverse Burrows-Wheeler Transform using the LF-mapping technique.
+     * <p>
+     * The LF-mapping (Last-First mapping) is an efficient method to reconstruct
+     * the original string from the BWT output without explicitly reconstructing
+     * the entire sorted rotations matrix.
+     * </p>
+     *
+     * <p>The algorithm works by:
+     * <ol>
+     *   <li>Creating the first column by sorting the BWT string</li>
+     *   <li>Building a mapping from first column indices to last column indices</li>
+     *   <li>Following this mapping starting from the original index to reconstruct the string</li>
+     * </ol>
+     * </p>
+     *
+     * @param bwtString the transformed string (L-column) from the forward transform; must not be {@code null}
+     * @param originalIndex the index of the original string row from the forward transform;
+     *                      use -1 for empty strings
+     * @return the original, untransformed string; returns empty string if input is empty or {@code originalIndex} is -1
+     * @throws NullPointerException if {@code bwtString} is {@code null}
+     * @throws IllegalArgumentException if {@code originalIndex} is out of valid range (except -1)
+     */
+    public static String inverseTransform(String bwtString, int originalIndex) {
+        if (bwtString == null || bwtString.isEmpty() || originalIndex == -1) {
+            return "";
+        }
+
+        int n = bwtString.length();
+        if (originalIndex < 0 || originalIndex >= n) {
+            throw new IllegalArgumentException("Original index must be between 0 and " + (n - 1) + ", got: " + originalIndex);
+        }
+
+        char[] lastColumn = bwtString.toCharArray();
+        char[] firstColumn = bwtString.toCharArray();
+        Arrays.sort(firstColumn);
+
+        // Create the "next" array for LF-mapping.
+        // next[i] stores the row index in the last column that corresponds to firstColumn[i]
+        int[] next = new int[n];
+
+        // Track the count of each character seen so far in the last column
+        Map<Character, Integer> countMap = new HashMap<>();
+
+        // Store the first occurrence index of each character in the first column
+        Map<Character, Integer> firstOccurrence = new HashMap<>();
+
+        for (int i = 0; i < n; i++) {
+            if (!firstOccurrence.containsKey(firstColumn[i])) {
+                firstOccurrence.put(firstColumn[i], i);
+            }
+        }
+
+        // Build the LF-mapping
+        for (int i = 0; i < n; i++) {
+            char c = lastColumn[i];
+            int count = countMap.getOrDefault(c, 0);
+            int firstIndex = firstOccurrence.get(c);
+            next[firstIndex + count] = i;
+            countMap.put(c, count + 1);
+        }
+
+        // Reconstruct the original string by following the LF-mapping
+        StringBuilder originalString = new StringBuilder(n);
+        int currentRow = originalIndex;
+        for (int i = 0; i < n; i++) {
+            originalString.append(firstColumn[currentRow]);
+            currentRow = next[currentRow];
+        }
+
+        return originalString.toString();
+    }
+}

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

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+package com.thealgorithms.compression;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * Implementation of the Move-to-Front (MTF) transform and its inverse.
+ * <p>
+ * MTF is a data transformation algorithm that encodes each symbol in the input
+ * as its current position in a dynamically-maintained list, then moves that symbol
+ * to the front of the list. This transformation is particularly effective when used
+ * after the Burrows-Wheeler Transform (BWT), as BWT groups similar characters together.
+ * </p>
+ *
+ * <p>The transform converts runs of repeated characters into sequences of small integers
+ * (often zeros), which are highly compressible by subsequent entropy encoding algorithms
+ * like Run-Length Encoding (RLE) or Huffman coding. This technique is used in the
+ * bzip2 compression algorithm.
+ * </p>
+ *
+ * <p><b>How it works:</b>
+ * <ol>
+ *   <li>Maintain a list of symbols (the alphabet), initially in a fixed order</li>
+ *   <li>For each input symbol:
+ *     <ul>
+ *       <li>Output its current index in the list</li>
+ *       <li>Move that symbol to the front of the list</li>
+ *     </ul>
+ *   </li>
+ * </ol>
+ * This means frequently occurring symbols quickly move to the front and are encoded
+ * with small indices (often 0), while rare symbols remain near the back.
+ * </p>
+ *
+ * <p><b>Time Complexity:</b>
+ * <ul>
+ *   <li>Forward transform: O(n × m) where n is input length and m is alphabet size</li>
+ *   <li>Inverse transform: O(n × m)</li>
+ * </ul>
+ * Note: Using {@link LinkedList} for O(1) insertions and O(m) search operations.
+ * </p>
+ *
+ * <p><b>Example:</b></p>
+ * <pre>
+ * Input:    "annb$aa"
+ * Alphabet: "$abn" (initial order)
+ * Output:   [1, 3, 0, 3, 3, 3, 0]
+ *
+ * Step-by-step:
+ * - 'a': index 1 in [$,a,b,n] → output 1, list becomes [a,$,b,n]
+ * - 'n': index 3 in [a,$,b,n] → output 3, list becomes [n,a,$,b]
+ * - 'n': index 0 in [n,a,$,b] → output 0, list stays [n,a,$,b]
+ * - 'b': index 3 in [n,a,$,b] → output 3, list becomes [b,n,a,$]
+ * - etc.
+ *
+ * Notice how repeated 'n' characters produce zeros after the first occurrence!
+ * </pre>
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Move-to-front_transform">Move-to-front transform (Wikipedia)</a>
+ */
+public final class MoveToFront {
+
+    private MoveToFront() {
+    }
+
+    /**
+     * Performs the forward Move-to-Front transform.
+     * <p>
+     * Converts the input string into a list of integers, where each integer represents
+     * the position of the corresponding character in a dynamically-maintained alphabet list.
+     * </p>
+     *
+     * <p><b>Note:</b> All characters in the input text must exist in the provided alphabet,
+     * otherwise an {@link IllegalArgumentException} is thrown. The alphabet should contain
+     * all unique characters that may appear in the input.</p>
+     *
+     * @param text the input string to transform; if empty, returns an empty list
+     * @param initialAlphabet a string containing the initial ordered set of symbols
+     *                        (e.g., "$abn" or the full ASCII set); must not be empty
+     *                        when {@code text} is non-empty
+     * @return a list of integers representing the transformed data, where each integer
+     *         is the index of the corresponding input character in the current alphabet state
+     * @throws IllegalArgumentException if {@code text} is non-empty and {@code initialAlphabet}
+     *                                  is {@code null} or empty
+     * @throws IllegalArgumentException if any character in {@code text} is not found in
+     *                                  {@code initialAlphabet}
+     */
+    public static List<Integer> transform(String text, String initialAlphabet) {
+        if (text == null || text.isEmpty()) {
+            return new ArrayList<>();
+        }
+        if (initialAlphabet == null || initialAlphabet.isEmpty()) {
+            throw new IllegalArgumentException("Alphabet cannot be null or empty when text is not empty.");
+        }
+
+        List<Integer> output = new ArrayList<>(text.length());
+
+        // Use LinkedList for O(1) add-to-front and O(n) remove operations
+        // This is more efficient than ArrayList for the move-to-front pattern
+        List<Character> alphabet = initialAlphabet.chars().mapToObj(c -> (char) c).collect(Collectors.toCollection(LinkedList::new));
+
+        for (char c : text.toCharArray()) {
+            int index = alphabet.indexOf(c);
+            if (index == -1) {
+                throw new IllegalArgumentException("Symbol '" + c + "' not found in the initial alphabet.");
+            }
+
+            output.add(index);
+
+            // Move the character to the front
+            Character symbol = alphabet.remove(index);
+            alphabet.addFirst(symbol);
+        }
+        return output;
+    }
+
+    /**
+     * Performs the inverse Move-to-Front transform.
+     * <p>
+     * Reconstructs the original string from the list of indices produced by the
+     * forward transform. This requires the exact same initial alphabet that was
+     * used in the forward transform.
+     * </p>
+     *
+     * <p><b>Important:</b> The {@code initialAlphabet} parameter must be identical
+     * to the one used in the forward transform, including character order, or the
+     * output will be incorrect.</p>
+     *
+     * @param indices the list of integers from the forward transform; if empty or {@code null},
+     *                returns an empty string
+     * @param initialAlphabet the exact same initial alphabet string used for the forward transform;
+     *                        if {@code null} or empty, returns an empty string
+     * @return the original, untransformed string
+     * @throws IllegalArgumentException if any index in {@code indices} is negative or
+     *                                  exceeds the current alphabet size
+     */
+    public static String inverseTransform(List<Integer> indices, String initialAlphabet) {
+        if (indices == null || indices.isEmpty() || initialAlphabet == null || initialAlphabet.isEmpty()) {
+            return "";
+        }
+
+        StringBuilder output = new StringBuilder(indices.size());
+
+        // Use LinkedList for O(1) add-to-front and O(n) remove operations
+        List<Character> alphabet = initialAlphabet.chars().mapToObj(c -> (char) c).collect(Collectors.toCollection(LinkedList::new));
+
+        for (int index : indices) {
+            if (index < 0 || index >= alphabet.size()) {
+                throw new IllegalArgumentException("Index " + index + " is out of bounds for the current alphabet of size " + alphabet.size() + ".");
+            }
+
+            // Get the symbol at the index
+            char symbol = alphabet.get(index);
+            output.append(symbol);
+
+            // Move the symbol to the front (mirroring the forward transform)
+            alphabet.remove(index);
+            alphabet.addFirst(symbol);
+        }
+        return output.toString();
+    }
+}