Greatest Common Divisor
by Isai Damier, Android Engineer @ Google

/**************************************************************************
 * Author: Isai Damier
 * Title: Greatest Common Divisor
 * Project: geekviewpoint
 * Package: algorithms
 *
 * Statement:
 *  Find the greatest common divisor of the given numbers
 *
 * Sample Input: 18, 27
 * Sample Output: 9
 *
 * Description: This function finds the greatest common
 *   divisor of the given numbers. The GCD of x and y is the greatest
 *   integer that divides both x and y evenly. For example the gcd of
 *   18 and 27 is 9 because 9*3 = 27 and 9*2= 18.
 *
 * Technical Details: Mathematicians have found numerous techniques
 *    for calculating the GCD of two numbers. The present
 *    implementation uses the mod operator. Other implementations
 *    may use subtraction, for example.
 *
 *************************************************************************/ 
 public int GCD(int a, int b) {
  if (0 == a || 0 == b) {
    return -1;
  }
  if (1 == a || 1 == b) {
    return 1;
  }

  while (0 < a && 0 < b) {
    if (a > b) {
      a %= b;
    } else {
      b %= a;
    }
  }
  return 0 == a ? b : a;
}

import org.junit.Test;
import static org.junit.Assert.*;

public class NumbersTest {

  /**
   * Test of GCD method, of class Numbers.
   */
  @Test
  public void testGCD() {
    System.out.println("GCD");
    Numbers greatestCommonDivisor = new Numbers();
    assertEquals(8, greatestCommonDivisor.GCD(24, 16));
    assertEquals(8, greatestCommonDivisor.GCD(16, 24));
    assertEquals(16, greatestCommonDivisor.GCD(16, 16));
    assertEquals(1, greatestCommonDivisor.GCD(13, 29));
  }
}