mod with negative results

   public static final int A_CHAR = (int) 'a';
   public static final int Z_TO_A = (int) ('z' - 'a') + 1;
   // alphabet frequency
   static double[] table = { 8.2, 1.5, 2.8, 4.3, 12.7, 2.2, 2.0, 6.1, 7.0, 0.2,
         0.8, 4.0, 2.4, 6.7, 7.5, 1.9, 0.1, 6.0, 6.3, 9.1, 2.8, 1.0, 2.4, 0.2,
         2.0, 0.1 };

   // convert letter to number
   static int let2nat(char c) {
      return ((int) c) - A_CHAR;
   }

   // convert number to letter
   static char nat2let(int code) {
      return (char) (code + A_CHAR);
   }

   // shift a letter to another letter shftAmt spaces away
   static char shift(int shftAmt, char c) {
      if (let2nat(c) < 0 || let2nat(c) > Z_TO_A - 1) {
         return c;
      } else {
         int result = (let2nat(c) + shftAmt) % Z_TO_A;
         result += Z_TO_A;
         result %= Z_TO_A;
         return nat2let(result);
      }
   }