// -------------------------------------------------------
// StringMath
// -------------------------------------------------------

public class StringMath
{
   // -------------------------------------------------------
   // add - Return (as a string) the result of
   //       adding s1 and s2
   // -------------------------------------------------------

   public static String add(String s1, String s2)
   {
      while(s1.length() < s2.length())
         s1 = "0" + s1;

      while(s2.length() < s1.length())
         s2 = "0" + s2;

      int    index  = s1.length() - 1;
      String result = "";
      int    carry  = 0;

      while(index >= 0)
      {
         char c1 = s1.charAt(index);
         char c2 = s2.charAt(index);

         int sum = carry + (c1 - '0') + (c2 - '0');

         if(sum >= 10)
         {
            carry = 1;
            sum -= 10;
         }
         else
            carry = 0;

         result = (char)('0' + sum) + result;
         index--;
      }

      if(carry > 0)
         result = (char)('0' + carry) + result;

      return(result);
   }

   // -------------------------------------------------------
   // multiply - Return (as a string) the result of
   //            multiplying 's' by the supplied digit
   // -------------------------------------------------------

   public static String multiply(String s, char digit)
   {
      String result = "";
      int    index  = s.length() - 1;
      int    carry  = 0;

      while(index >= 0)
      {
         char c = s.charAt(index);

         int product = (digit - '0') * (c - '0') + carry;

         if(product >= 10)
         {
            carry = (product / 10);
            product %= 10;
         }
         else
            carry = 0;

         result = (char)('0' + product) + result;
         index--;
      }

      if(carry > 0)
         result = (char)('0' + carry) + result;
            
      return(result);
   }

   // -------------------------------------------------------
   // multiply - Return (as a string) the result of
   //            multiplying s1 and s2
   // -------------------------------------------------------

   public static String multiply(String s1, String s2)
   {
      String shorter = (s1.length() <= s2.length() ? s1 : s2);
      String longer  = (s1.length() <= s2.length() ? s2 : s1);

      String result = "";

      for(int i = shorter.length() - 1, j = 0; i >= 0; i--, j ++)
      {
         String product = multiply(longer, shorter.charAt(i));

         for(int k = 0; k < j; k ++)
            product += "0";

         result = add(result, product);
      }

      return(result);
   }

   // -------------------------------------------------------
   // testAddition
   // -------------------------------------------------------

   public static void testAddition()
   {
      final int MAX_POWER = 222;

      String value = "1";

      for(int i = 0; i <= MAX_POWER; i ++, value = add(value, value))
         System.out.printf("2^%d = %s\n", i, value);

      System.out.println();
   }

   // -------------------------------------------------------
   // testMultiplication
   // -------------------------------------------------------

   public static void testMultiplication()
   {
      final int NUMBER = 1000;
      String value     = "" + NUMBER;

      for(int i = NUMBER - 1; i > 1; i --)
         value = multiply(value, "" + i);

      System.out.printf("%d! = %s\n", NUMBER, value);
   }

   // -------------------------------------------------------
   // main
   // -------------------------------------------------------

   public static void main(String[] args)
   {
      testAddition();
      testMultiplication();
   }
}