programming-examples/java/Data_Structures/Matrix.java

public class Matrix {
	public static int[][] matrixAdd(int[][] a, int[][] b) {
		int n = a.length;
		int m = a[0].length;
		int[][] res = new int[n][m];
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < m; j++) {
				res[i][j] = a[i][j] + b[i][j];
			}
		}
		return res;
	}

	public static int[][] matrixMul(int[][] a, int[][] b) {
		int n = a.length;
		int m = a[0].length;
		int k = b[0].length;
		int[][] res = new int[n][k];
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < k; j++) {
				for (int p = 0; p < m; p++) {
					res[i][j] = res[i][j] + a[i][p] * b[p][j];
				}
			}
		}
		return res;
	}

	public static int[][] matrixPow(int[][] a, int p) {
		if (p == 0) {
			return matrixUnit(a.length);
		} else if (p % 2 == 0) {
			return matrixPow(matrixMul(a, a), p / 2);
		} else {
			return matrixMul(a, matrixPow(a, p - 1));
		}
	}

	public static int[][] matrixPowSum(int[][] a, int p) {
		int n = a.length;
		if (p == 0) {
			return new int[n][n];
		}
		if (p % 2 == 0) {
			return matrixMul(matrixPowSum(a, p / 2), matrixAdd(matrixUnit(n), matrixPow(a, p / 2)));
		} else {
			return matrixAdd(a, matrixMul(matrixPowSum(a, p - 1), a));
		}
	}

	public static int[][] matrixUnit(int n) {
		int[][] res = new int[n][n];
		for (int i = 0; i < n; ++i) {
			res[i][i] = 1;
		}
		return res;
	}

	// Usage example
	public static void main(String[] args) {
		int[][] a = { { 1, 2 }, { 3, 4 } };
		int[][] b = matrixUnit(2);
		int[][] c = matrixMul(a, b);
	}
}