Fix issue #94: GCD function doens't work and realize GCD function (#100)

* Fix issue #84: Permutation result is not right

* Fix issue #84: Permutation result is not right

* Fix issue #94: GCD function doens't work and realize GCD function

* Fix issue #95: LCM function doesn't work and realize LCM function based on GCD function

app/src/main/java/com/duy/ncalc/calculator/BasicCalculatorActivity.java

@@ -479,9 +479,64 @@ void setState(CalculatorState state) {
         mCurrentState = state;
     }
 
+    /**
+     * Parse out the two integers entered by the user and execute the lcm
+     * algorithm to solve the least common multiple of these two integers.
+     * @param nums
+     *        The string entered by the user, including parentheses and commas.
+     * @return The least common multiple of two integers
+     */
+
+    public String executeLCM(String nums){
+        int num1 = Integer.parseInt(nums.split(",")[0].trim());
+        int num2 = Integer.parseInt(nums.split(",")[1].trim());
+        int gcd = gcd(num1, num2);
+        //lcm(a,b) = |a*b| / gcd(a,b) (According to Wikipedia: https://en.wikipedia.org/wiki/Least_common_multiple)
+        int lcm = (num1 * num2) / gcd;
+        return lcm + "";
+    }
+
+    /**
+     * Parse out the two integers entered by the user and execute the gcd
+     * algorithm to solve the greatest common factor of these two integers.
+     * @param nums
+     *        The string entered by the user, including parentheses and commas.
+     * @return The greatest common divisor of two integers
+     */
+
+    public String executeGCD(String nums){
+        int num1 = Integer.parseInt(nums.split(",")[0].trim());
+        int num2 = Integer.parseInt(nums.split(",")[1].trim());
+        int res = gcd(num1, num2);
+        return res + "";
+    }
+
+    /**
+     * Recursively solve the greatest common factor of two integers.
+     * @param a The first integer entered by the user.
+     * @param b The second integer entered by the user.
+     * @return The greatest common divisor of two integers.
+     */
+
+    public static int gcd(int a, int b){
+        if (b == 0) {
+            return a;
+        }
+        return gcd(b, a % b);
+    }
+
     @Override
     public void onEvaluated(String expr, String result, int resultId) {
         if (resultId == LogicEvaluator.RESULT_OK) {
+            //Parse user input to get the function to be executed and operand.
+            String operator = expr.substring(0,3).toLowerCase();
+            String nums = expr.substring(4, expr.length() - 1);
+            if(operator.equals("gcd")){
+                result = "$$" + executeGCD(nums) + "$$";
+            }
+            if(operator.equals("lcm")){
+                result = "$$" + executeLCM(nums) + "$$";
+            }
             if (mCurrentState == CalculatorState.EVALUATE) {
                 onResult(result);
                 saveHistory(expr, result, true);