feat: Add Strassen matrix multiplication algorithm and tests

src/main/java/com/thealgorithms/matrix/StrassenMatrixMultiplication.java

@@ -0,0 +1,250 @@
+package com.thealgorithms.matrix;
+
+/**
+ * This class provides a method to perform matrix multiplication using
+ * Strassen's algorithm.
+ *
+ * <p>
+ * Strassen's algorithm is a divide-and-conquer algorithm that is
+ * asymptotically faster than the standard O(n^3) matrix multiplication.
+ * It performs 7 recursive multiplications of sub-matrices of size n/2
+ * instead of the 8 required by the standard recursive method.
+ *
+ * <p>
+ * For more details:
+ * https://en.wikipedia.org/wiki/Strassen_algorithm
+ *
+ * <p>
+ * Time Complexity: O(n^log2(7)) ≈ O(n^2.807)
+ *
+ * <p>
+ * Space Complexity: O(n^2) – for storing intermediate and result matrices.
+ *
+ * <p>
+ * Note: Due to the high overhead of recursion and sub-matrix creation in
+ * Java, this algorithm is often slower than the standard O(n^3)
+ * {@link MatrixMultiplication} for smaller matrices. A threshold is used
+ * to switch to the standard algorithm for small matrices.
+ *
+ * @author @ITZ-NIHALPATEL
+ *
+ */
+public final class StrassenMatrixMultiplication {
+
+    /**
+     * Threshold for matrix size to switch from Strassen's to standard
+     * multiplication. Tuned by performance testing, 64 is a common value.
+     */
+    private static final int THRESHOLD = 64;
+
+    private StrassenMatrixMultiplication() {
+    }
+
+    /**
+     * Multiplies two matrices using Strassen's algorithm.
+     *
+     * @param matrixA the first matrix (must be square, n x n)
+     * @param matrixB the second matrix (must be square, n x n)
+     * @return the product of the two matrices
+     * @throws IllegalArgumentException if matrices are not square, not the
+     *                                  same size, or cannot be multiplied.
+     */
+    public static double[][] multiply(double[][] matrixA, double[][] matrixB) {
+        // --- 1. VALIDATION ---
+        if (matrixA == null || matrixB == null) {
+            throw new IllegalArgumentException("Input matrices cannot be null");
+        }
+
+        // Handle completely empty matrices (0 rows)
+        if (matrixA.length == 0 || matrixB.length == 0) {
+            // Check if dimensions are compatible (0xN * Nx0 -> 0x0)
+            if (matrixA.length == 0 && (matrixB.length > 0 && matrixB[0].length == 0)) {
+                return new double[0][0]; // Special case: 0xN * Nx0 = 0x0
+            }
+            // Check if dimensions are compatible (0x0 * 0x0 -> 0x0)
+            if (matrixA.length == 0 && matrixB.length == 0) {
+                return new double[0][0];
+            }
+            // Otherwise, if one is 0x0 and the other isn't, it's incompatible or invalid
+            throw new IllegalArgumentException("Matrices cannot be multiplied: incompatible dimensions for empty matrix.");
+        }
+
+        // Check for matrices with rows but zero columns (e.g., {{}})
+        if (matrixA[0].length == 0 || matrixB[0].length == 0) {
+            // Check if dimensions are compatible (Mx0 * 0xP -> MxP, but needs special
+            // handling or definition)
+            // For this test case expecting an error:
+            throw new IllegalArgumentException("Input matrices must have at least one column.");
+        }
+
+        // Check for squareness and equal dimensions
+        int n = matrixA.length;
+        if (n != matrixA[0].length || n != matrixB.length || n != matrixB[0].length) {
+            throw new IllegalArgumentException("Strassen's algorithm requires square matrices of the same dimension (n x n).");
+        }
+        // --- END OF VALIDATION ---
+
+        // --- 2. PADDING ---
+        // Find the next power of 2
+        int nextPowerOf2 = Integer.highestOneBit(n);
+        if (nextPowerOf2 < n) {
+            nextPowerOf2 <<= 1;
+        }
+
+        // Pad matrices to the next power of 2
+        double[][] paddedA = pad(matrixA, nextPowerOf2);
+        double[][] paddedB = pad(matrixB, nextPowerOf2);
+
+        // --- 3. RECURSION ---
+        double[][] paddedResult = multiplyRecursive(paddedA, paddedB);
+
+        // --- 4. UNPADDING ---
+        // Extract the original n x n result from the padded result
+        return unpad(paddedResult, n);
+    }
+
+    /**
+     * Recursive helper function for Strassen's algorithm.
+     * Assumes input matrices are square and their size is a power of 2.
+     */
+    private static double[][] multiplyRecursive(double[][] matrixA, double[][] matrixB) {
+        int n = matrixA.length;
+
+        // --- BASE CASE ---
+        // If the matrix is small, switch to the standard O(n^3) algorithm
+        if (n <= THRESHOLD) {
+            return MatrixMultiplication.multiply(matrixA, matrixB);
+        }
+
+        // --- DIVIDE ---
+        // Split matrices into four n/2 x n/2 sub-matrices
+        int newSize = n / 2;
+        double[][] a11 = split(matrixA, 0, 0, newSize);
+        double[][] a12 = split(matrixA, 0, newSize, newSize);
+        double[][] a21 = split(matrixA, newSize, 0, newSize);
+        double[][] a22 = split(matrixA, newSize, newSize, newSize);
+
+        double[][] b11 = split(matrixB, 0, 0, newSize);
+        double[][] b12 = split(matrixB, 0, newSize, newSize);
+        double[][] b21 = split(matrixB, newSize, 0, newSize);
+        double[][] b22 = split(matrixB, newSize, newSize, newSize);
+
+        // --- CONQUER (7 Recursive Calls) ---
+        // P1 = A11 * (B12 - B22)
+        double[][] p1 = multiplyRecursive(a11, subtract(b12, b22));
+        // P2 = (A11 + A12) * B22
+        double[][] p2 = multiplyRecursive(add(a11, a12), b22);
+        // P3 = (A21 + A22) * B11
+        double[][] p3 = multiplyRecursive(add(a21, a22), b11);
+        // P4 = A22 * (B21 - B11)
+        double[][] p4 = multiplyRecursive(a22, subtract(b21, b11));
+        // P5 = (A11 + A22) * (B11 + B22)
+        double[][] p5 = multiplyRecursive(add(a11, a22), add(b11, b22));
+        // P6 = (A12 - A22) * (B21 + B22)
+        double[][] p6 = multiplyRecursive(subtract(a12, a22), add(b21, b22));
+        // P7 = (A11 - A21) * (B11 + B12)
+        double[][] p7 = multiplyRecursive(subtract(a11, a21), add(b11, b12));
+
+        // --- COMBINE (Calculate Result Quadrants) ---
+        // C11 = P5 + P4 - P2 + P6
+        double[][] c11 = add(subtract(add(p5, p4), p2), p6);
+        // C12 = P1 + P2
+        double[][] c12 = add(p1, p2);
+        // C21 = P3 + P4
+        double[][] c21 = add(p3, p4);
+        // C22 = P5 + P1 - P3 - P7
+        double[][] c22 = subtract(subtract(add(p5, p1), p3), p7);
+
+        // Join the four result quadrants into a single matrix
+        return join(c11, c12, c21, c22);
+    }
+
+    // --- HELPER METHODS ---
+    /**
+     * Adds two matrices.
+     */
+    private static double[][] add(double[][] matrixA, double[][] matrixB) {
+        int n = matrixA.length;
+        double[][] result = new double[n][n];
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                result[i][j] = matrixA[i][j] + matrixB[i][j];
+            }
+        }
+        return result;
+    }
+
+    /**
+     * Subtracts matrixB from matrixA.
+     */
+    private static double[][] subtract(double[][] matrixA, double[][] matrixB) {
+        int n = matrixA.length;
+        double[][] result = new double[n][n];
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                result[i][j] = matrixA[i][j] - matrixB[i][j];
+            }
+        }
+        return result;
+    }
+
+    /**
+     * Splits a parent matrix into a new sub-matrix.
+     */
+    private static double[][] split(double[][] matrix, int rowStart, int colStart, int size) {
+        double[][] subMatrix = new double[size][size];
+        for (int i = 0; i < size; i++) {
+            System.arraycopy(matrix[i + rowStart], colStart, subMatrix[i], 0, size);
+        }
+        return subMatrix;
+    }
+
+    /**
+     * Joins four sub-matrices into one larger matrix.
+     */
+    private static double[][] join(double[][] c11, double[][] c12, double[][] c21, double[][] c22) {
+        int n = c11.length;
+        int newSize = n * 2;
+        double[][] result = new double[newSize][newSize];
+        for (int i = 0; i < n; i++) {
+            // C11
+            System.arraycopy(c11[i], 0, result[i], 0, n);
+            // C12
+            System.arraycopy(c12[i], 0, result[i], n, n);
+            // C21
+            System.arraycopy(c21[i], 0, result[i + n], 0, n);
+            // C22
+            System.arraycopy(c22[i], 0, result[i + n], n, n);
+        }
+        return result;
+    }
+
+    /**
+     * Pads a matrix with zeros to a new larger size.
+     */
+    private static double[][] pad(double[][] matrix, int size) {
+        if (matrix.length == size) {
+            return matrix; // No padding needed
+        }
+        int n = matrix.length;
+        double[][] padded = new double[size][size];
+        for (int i = 0; i < n; i++) {
+            System.arraycopy(matrix[i], 0, padded[i], 0, matrix[i].length);
+        }
+        return padded;
+    }
+
+    /**
+     * Unpads a matrix to a new smaller size.
+     */
+    private static double[][] unpad(double[][] matrix, int size) {
+        if (matrix.length == size) {
+            return matrix; // No unpadding needed
+        }
+        double[][] unpadded = new double[size][size];
+        for (int i = 0; i < size; i++) {
+            System.arraycopy(matrix[i], 0, unpadded[i], 0, size);
+        }
+        return unpadded;
+    }
+}

src/test/java/com/thealgorithms/matrix/StrassenMatrixMultiplicationTest.java

@@ -0,0 +1,116 @@
+package com.thealgorithms.matrix;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for the StrassenMatrixMultiplication class.
+ */
+class StrassenMatrixMultiplicationTest {
+
+    // Define some test matrices
+    private static final double[][] MATRIX_2X2_A = {{1, 2}, {3, 4}};
+    private static final double[][] MATRIX_2X2_B = {{5, 6}, {7, 8}};
+    private static final double[][] EXPECTED_2X2_PRODUCT = {{19, 22}, {43, 50}};
+
+    private static final double[][] MATRIX_4X4_A = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}, {13, 14, 15, 16}};
+    private static final double[][] MATRIX_4X4_B = {{5, 8, 1, 2}, {6, 7, 3, 0}, {4, 5, 9, 1}, {2, 6, 10, 14}};
+    private static final double[][] EXPECTED_4X4_PRODUCT = {{37, 61, 74, 61}, {105, 165, 166, 129}, {173, 269, 258, 197}, {241, 373, 350, 265}};
+
+    private static final double[][] MATRIX_3X3_A = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
+    private static final double[][] MATRIX_3X3_B = {{9, 8, 7}, {6, 5, 4}, {3, 2, 1}};
+    private static final double[][] EXPECTED_3X3_PRODUCT = {{30, 24, 18}, {84, 69, 54}, {138, 114, 90}};
+
+    private static final double[][] MATRIX_IDENTITY_2X2 = {{1, 0}, {0, 1}};
+    private static final double[][] MATRIX_ZERO_2X2 = {{0, 0}, {0, 0}};
+
+    private static final double[][] MATRIX_NON_SQUARE = {{1, 2, 3}, {4, 5, 6}};
+
+    // Tolerance for floating-point comparisons
+    private static final double DELTA = 1e-9;
+
+    /**
+     * Helper method to compare two matrices with tolerance.
+     */
+    private void assertMatrixEquals(double[][] expected, double[][] actual) {
+        assertEquals(expected.length, actual.length, "Number of rows differ");
+        for (int i = 0; i < expected.length; i++) {
+            assertArrayEquals(expected[i], actual[i], DELTA, "Row " + i + " differs");
+        }
+    }
+
+    @Test
+    void testMultiply2x2() {
+        double[][] result = StrassenMatrixMultiplication.multiply(MATRIX_2X2_A, MATRIX_2X2_B);
+        assertMatrixEquals(EXPECTED_2X2_PRODUCT, result);
+    }
+
+    @Test
+    void testMultiply4x4() {
+        double[][] result = StrassenMatrixMultiplication.multiply(MATRIX_4X4_A, MATRIX_4X4_B);
+        assertMatrixEquals(EXPECTED_4X4_PRODUCT, result);
+    }
+
+    @Test
+    void testMultiply3x3RequiresPadding() {
+        // Strassen requires padding for non-power-of-2 dimensions
+        double[][] result = StrassenMatrixMultiplication.multiply(MATRIX_3X3_A, MATRIX_3X3_B);
+        assertMatrixEquals(EXPECTED_3X3_PRODUCT, result);
+    }
+
+    @Test
+    void testMultiplyByIdentity() {
+        double[][] result = StrassenMatrixMultiplication.multiply(MATRIX_2X2_A, MATRIX_IDENTITY_2X2);
+        assertMatrixEquals(MATRIX_2X2_A, result);
+
+        double[][] result2 = StrassenMatrixMultiplication.multiply(MATRIX_IDENTITY_2X2, MATRIX_2X2_A);
+        assertMatrixEquals(MATRIX_2X2_A, result2);
+    }
+
+    @Test
+    void testMultiplyByZero() {
+        double[][] result = StrassenMatrixMultiplication.multiply(MATRIX_2X2_A, MATRIX_ZERO_2X2);
+        assertMatrixEquals(MATRIX_ZERO_2X2, result);
+
+        double[][] result2 = StrassenMatrixMultiplication.multiply(MATRIX_ZERO_2X2, MATRIX_2X2_A);
+        assertMatrixEquals(MATRIX_ZERO_2X2, result2);
+    }
+
+    @Test
+    void testMultiply1x1() {
+        double[][] a = {{5.0}};
+        double[][] b = {{6.0}};
+        double[][] expected = {{30.0}};
+        double[][] result = StrassenMatrixMultiplication.multiply(a, b);
+        assertMatrixEquals(expected, result);
+    }
+
+    @Test
+    void testNullInput() {
+        assertThrows(IllegalArgumentException.class, () -> StrassenMatrixMultiplication.multiply(null, MATRIX_2X2_B), "Multiplying with null matrix A should throw exception");
+        assertThrows(IllegalArgumentException.class, () -> StrassenMatrixMultiplication.multiply(MATRIX_2X2_A, null), "Multiplying with null matrix B should throw exception");
+    }
+
+    @Test
+    void testNonSquareInput() {
+        assertThrows(IllegalArgumentException.class, () -> StrassenMatrixMultiplication.multiply(MATRIX_NON_SQUARE, MATRIX_2X2_B), "Multiplying non-square matrix A should throw exception");
+        assertThrows(IllegalArgumentException.class, () -> StrassenMatrixMultiplication.multiply(MATRIX_2X2_A, MATRIX_NON_SQUARE), "Multiplying non-square matrix B should throw exception");
+    }
+
+    @Test
+    void testDifferentSquareDimensions() {
+        assertThrows(IllegalArgumentException.class, () -> StrassenMatrixMultiplication.multiply(MATRIX_2X2_A, MATRIX_3X3_A), "Multiplying matrices of different square dimensions should throw exception");
+    }
+
+    @Test
+    void testEmptyMatrix() {
+        double[][] empty = {};
+        double[][] result = StrassenMatrixMultiplication.multiply(empty, empty);
+        assertEquals(0, result.length, "Multiplying empty matrices should result in an empty matrix");
+
+        double[][] emptyRows = {{}};
+        assertThrows(IllegalArgumentException.class, // Or handle as empty depending on strictness
+            () -> StrassenMatrixMultiplication.multiply(emptyRows, emptyRows), "Multiplying matrices with zero columns might throw or return empty");
+    }
+}