Quickest Method to Reverse in String in C#.net

Question

I\'m currently writing a quick solution for Euler Problem #4 where one must find the largest palindromic number from the product of two 3-digit numbers.

To identify if a

Answer 1

A you want to compare a number with its reverse it may be faster to reverse the number using division rather than converting it to a string. I still need to test the speed of it.

 private static int Reverse(int num) {
     int res = 0;
     while (num > 0) {
        int rm ;
        num = Math.DivRem(num, 10, out rm);
        res = res * 10 + rm;
     }
     return res;
  }

EDIT: DivRem was about 1% faster than division and module in my computer. A speed optimization is exit if the last digit is 0:

  private static int Reverse(int num) {
     int res = 0;
     int rm;
     num = Math.DivRem(num, 10, out rm);
     //Some magic value or return false, see below.
     if (rm == 0) return -1 ; 
     res = res * 10 + rm;
     while (num > 0) {
        num = Math.DivRem(num, 10, out rm);
        res = res * 10 + rm;
     }
     return res ;
  }

Making the method return a bool was slightly slower than comparing to a bool in a loop in my computer, but I don't understand why. Please test in your computer.

Multiplication and bit-shifing should be faster than division but probably are not precise enough. EDIT: using long seems be precise enough.

  private static int FastReverse(int num) {
     int res = 0;
     int q = (int)((214748365L * num) >> 31);
     int rm = num - 10 * q;
     num = q;
     if (rm == 0) return -1;
     res = res * 10 + rm;
     while (num > 0) {
        q = (int)((214748365L * num) >> 31);
        rm = num - 10 * q;
        num = q;
        res = res * 10 + rm;
     }
     return res;
  }

(214748365L * num) >> 31 is equal to i / 10 until 1,073,741,829 where 1 / 10 gives 107374182 and the multiplication + binary shifting gives 107374183.