import java.util.Scanner;
import java.util.Arrays;
import java.io.File;
import java.io.FileNotFoundException;

/* ArraySumRec.java
** Java application that includes a recursive method to compute the 
** sum of the elements in a segment of an array of int values.  
** Also includes a main() method, which is strictly for the purpose 
** of testing the method that sums.
**
** Authors: R. McCloskey and < ... >
** Known deficiences: 
*/
public class ArraySumRec {

   private static int depth;

   /* Returns the sum of the elements in the given array.
   */
   public static int sumOf(int[] b) { return sumOfAux(b, 0, b.length); }

   /* Returns the sum of the elements in the array segment described
   ** by the parameters (namely b[low..high)).  Employs recursion.
   */
   public static int sumOfAux(int[] b, int low, int high) { 
      depth = depth + 1;
      printMessage(String.format("Entering sumOfAux() to compute sum of b[%d..%d)\n",
                                 low, high));

      int result;


      // CODE MISSING HERE!!


      printMessage(String.format("Leaving sumOfAux(); sum of b[%d..%d) found to be %d\n", 
                                 low, high, result));
      depth--;

      return result;
   }


   /* Prints the specified number of spaces.
   */
   private static void printSpaces(int num) {
      for (int i=0; i != num; i++) { System.out.print(' '); }
   }

   /* Prints the given String, indented in accord with the current
   ** value of 'depth'.
   */
   private static void printMessage(String s) {
      printSpaces(3*depth);
      System.out.print(s);
   }


   // Remaining methods are strictly for the purpose 
   // of testing the sumOfAux() method above.
   // ----------------------------------------------------------


   /* Reads numbers from an input source, places them into an array,
   ** and invokes the sumOf() method above to compute their sum.
   ** If command line arguments are provided, they are taken to be the
   ** numbers to be summed.  Otherwise, the user is prompted to enter
   ** the name of a file containing input.  The first number in that
   ** file is taken to describe how many numbers follow that are to be 
   ** summed.
   */
   public static void main(String[] args) throws FileNotFoundException {
      int[] a;
      if (args.length != 0) { 
         // interpret command line args as being the elements of the
         // array to be summed
         a = strArrayToIntArray(args);
      }
      else {
         // In the absence of command line arguments, ask the user to
         // enter name of file in which is described an array of ints
         String fileName = getString("Enter name of input file: ");
         Scanner input = new Scanner(new File(fileName));
         a = readArray(input);
      }
      System.out.println("The array is as follows:");
      System.out.println(Arrays.toString(a));
      System.out.println();

      depth = -1;
      int sum = sumOf(a);
      System.out.printf("As computed by sumOf(), its elements sum to %d\n", sum); 
      int actualSum = sumOfUsingLoop(a);
      if (sum != actualSum) {
         System.out.println("***WRONG!! Actual sum is " + actualSum);
      }
   }

   /* Given an array of Strings, each of which is an integer numeral
   ** (e.g., "368", "-21"), returns an array of type int[] containing
   ** the corresponding numeric values.
   */
   private static int[] strArrayToIntArray(String[] ary) {
      int[] result = new int[ary.length];
      for (int i=0; i != ary.length; i++) {
         result[i] = Integer.parseInt(ary[i]);
      }
      return result;
   }

   /* Prints the given prompt and returns the response entered at the keyboard.
   */
   private static String getString(String prompt) {
      Scanner keyboard = new Scanner(System.in); 
      System.out.print(prompt);
      return keyboard.nextLine();
   }

   /* Returns a new array in which has been placed the numbers read by the
   ** given Scanner.  The first number, N, indicates the intended length
   ** of the array.  The N numbers that follow are placed into the array.
   */
   private static int[] readArray(Scanner input) {
      int[] result;
      int N = input.nextInt();   // read length of intended array
      result = new int[N];
      for (int i=0; i != N; i++) {  // read N lements into the array
         result[i] = input.nextInt();
      }
      return result;
   }

   /* Returns the sum of the elements in the given array
   ** using the obvious loop-based approach.
   */
   private static int sumOfUsingLoop(int[] a) {
      int sumSoFar = 0;
      for (int i=0; i != a.length; i++) {
         sumSoFar = sumSoFar + a[i];
      }
      return sumSoFar;
   }

}