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

string test = "ABC";
string reversed = new String(test.ToCharArray().Reverse().ToArray());

Answer 2

The Stopwatch class needs reset after each run. the code below has been corrected

var d = s.ToCharArray();
Array.Reverse(d);
return s == new string(d);

---

using System;
using System.Diagnostics;

namespace longeststring_codegolf
{
    class Program
    {
        static void Main(string[] args)
        {
            int t = 0, v = 0;
            var sw = new Stopwatch();

            sw.Start();
            for (int i = 999; i > 99; i--)
                for (int j = 999; j > 99; j--)
                    if ((v = i * j) > t && IsPalindromicMine(v.ToString()))
                        t = v;
            sw.Stop();

            var elapsed = sw.Elapsed;
            var elapsedMilliseconds = sw.ElapsedMilliseconds;
            var elapsedTicks = sw.ElapsedTicks; 
            Console.WriteLine("Ticks: " + elapsedTicks.ToString());//~189000
            Console.WriteLine("Milliseconds: " + elapsedMilliseconds.ToString()); //~9

            sw = Stopwatch.StartNew();
            for (int i = 999; i > 99; i--)
                for (int j = 999; j > 99; j--)
                    if ((v = i * j) > t && IsPalindromic(v.ToString()))
                        t = v;
            sw.Stop();
            var elapsed2 = sw.Elapsed;
            var elapsedMilliseconds2 = sw.ElapsedMilliseconds;
            var elapsedTicks2 = sw.ElapsedTicks; 
            Console.WriteLine("Ticks: " + elapsedTicks2.ToString());//~388000
            Console.WriteLine("Milliseconds: " + elapsedMilliseconds2.ToString());//~20

        }

        static bool IsPalindromicMine(string s)
        {
            var d = s.ToCharArray();
            Array.Reverse(d);
            return s == new string(d);
        }

        static bool IsPalindromic(string s)
        {
            int len = s.Length;
            int half = len-- >> 1;
            for (int i = 0; i < half; i++)
                if (s[i] != s[len - i])
                    return false;
            return true;
        }

    }
}

Answer 3

public static String Reverse(string input) {
  var length = input.Length;
  var buffer = new char[length];
  for ( var i= 0; i < input.Length; i++ ) {
    buffer[i] = input[(length-i)-1];
  }
  return new String(buffer);
}

EDIT: Doh! Forgot to halve the length for perf :)

Answer 4

The fastest way I have found to reverse a string in C# is with the following code. It's faster reading in 32bits at a time instead of a char's length of 16bits. In debug mode, it is faster until you get to about 93 characters. Anything longer than that Array.Reverse() is faster. Using a release build and running outside of the IDE, this method will blow Array.Reverse() out of the water at any string length.

char[] MyCharArray = MyString.ToCharArray();
UIntStringReverse(ref MyCharArray);     //Code to reverse is below.
string ReversedString = new string(MyCharArray);


private static unsafe void UIntStringReverse(ref char[] arr)
{
    uint Temp;
    uint Temp2;

    fixed (char* arrPtr = &arr[0])
    {
        uint* p, q;
        p = (uint*)(arrPtr);
        q = (uint*)(arrPtr + arr.LongLength - 2);

        if (arr.LongLength == 2)
        {
            Temp = *p;
            *p = ((Temp & 0xFFFF0000) >> 16) | ((Temp & 0x0000FFFF) << 16); 
            return;
        }

        while (p < q)
        {
            Temp = *p;
            Temp2 = *q;

            *p = ((Temp2 & 0xFFFF0000) >> 16) | ((Temp2 & 0x0000FFFF) << 16); 
            *q = ((Temp & 0xFFFF0000) >> 16) | ((Temp & 0x0000FFFF) << 16);

            p++;
            q--;
        }
    }
}

Answer 5

Using ggf31416's FastReverse function, here is the solution to Project Euler's Problem #4 which completes on my computer in 47ms.

using System;
using System.Diagnostics;

namespace Euler_Problem_4
{
    class Program
    {
        static void Main(string[] args)
        {
            Stopwatch s = new Stopwatch();
            s.Start();

            int t = 0;

            for (int i = 999; i > 99; i--)
            {
                for (int j = i; j > 99; j--)
                {
                    if (i*j == FastReverse(i*j))
                    {
                        if (i * j > t)
                        {
                            t = i * j;
                        }
                    }
                }
            }

            Console.WriteLine(t);

            s.Stop();
            Console.WriteLine("{0}mins {1}secs {2}ms", s.Elapsed.Minutes, s.Elapsed.Seconds, s.Elapsed.Milliseconds);
            Console.ReadKey(true);

        }

        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;
        }
    }
}

Answer 6

Performance: Fastest string reversing algorithms... (final results)