Plus
by Isai Damier, Android Engineer @ Google

/*******************************************************************
 * Author: Isai Damier
 * Title: LargeInteger.java
 * Project: geekviewpoint
 * Package: datastructure
 *
 * Description: Normally modern computers cannot handle very large
 *    numbers. So the task here is to create a datastructure that
 *    can handle numbers of perhaps infinite size. And to prove that
 *    the datastructure works, included are arithmetic operators.
 *
 *    Naturally the numbers are represented as Strings. And to
 *    perform addition and subtraction, stacks are used as the media.
 *    Finally, as any trusty calculator, this datastructure returns
 *    erroneous values for erroneous inputs and correct values for
 *    correct inputs. This is to say, if an input string does not
 *    represent a valid decimal integer, then the output string
 *    will be bogus.
 ******************************************************************/ 
 import java.util.Stack;

public class LargeInteger {

  private String number;

  public LargeInteger(String number) {
    this.number = number;
  }//constructor

  /***************************************************************
   * Function: plus
   * Inputs: @param addend
   *       The value to be added to this number
   * Output: String
   *       The sum of the addend and this number
   *
   * Description: Add the given value (i.e. addend) to this number.
   *
   * Technical Details: This is pretty much a brute force
   *    algorithm. The strings are changed to stacks of integers.
   *    Then the stacks are popped and the values are added and
   *    passed to the result.
   *
   *    The result is assembled in reverse just to keep things
   *    simple (Java has a nice append function). The result could
   *    have otherwise been assembled as result = sum + result.
   *
   **************************************************************/
  public String plus(String addend) {
    Stack<Integer> augendStack = toIntegerStack(this.number);
    Stack<Integer> addendStack = toIntegerStack(addend);

    StringBuffer result = new StringBuffer();
    int carry = 0, sum = 0, radix = 10;

    while (!augendStack.isEmpty() && !addendStack.isEmpty()) {
      sum = augendStack.pop() + addendStack.pop() + carry;
      result.append(sum % radix);
      carry = sum / radix;
    }

    while (!augendStack.isEmpty()) {
      sum = augendStack.pop() + carry;
      result.append(sum % radix);
      carry = sum / radix;
    }

    while (!addendStack.isEmpty()) {
      sum = addendStack.pop() + carry;
      result.append(sum % radix);
      carry = sum / radix;
    }

    if (carry > 0) {
      result.append(carry);
    }
    result.reverse();
    return result.toString();
  }
}

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

public class LargeIntegerTest {

  public LargeIntegerTest() {
  }

  /**
   * Test of plus method, of class LargeInteger.
   */
  @Test
  public void testPlus() {
    System.out.println("plus");
    LargeInteger bigInteger;

    String a = "8787284297498234897287345";
    String b = "7971813897918381831891879249579839712938";
    String p = "7971813897918390619176176747814737000283";
    bigInteger = new LargeInteger(a);
    assertEquals(p, bigInteger.plus(b));

    int max = 54321;
    for (int x = 0; x <= max; x += 31) {
      for (int y = 0; y <= max; y += 31) {
        bigInteger = new LargeInteger(x + "");
        assertEquals(x + y, Integer.parseInt(bigInteger.plus(y + "")));
        assertEquals(x + "", bigInteger.value());
        assertEquals((x + "").length(), bigInteger.size());
      }
    }
  }
}