Dot Net Perls

Problem

Develop an ROT13 algorithm and optimize it. You need to change the individual characters in a string based on logic. As an example, implement the ROT13 cipher to perform character transformations.

Solution: C#

ROT13 rotates each character in text by 13 places. The method is a primitive cipher similar to the one used by Julius Caeser, dictator of the Roman Republic, as the Caesar Cipher. Here's one implementation, but it is slow.

static string RotateA(string value)
{
    //
    // Version of Rotate that uses StringBuilder.
    //
    StringBuilder b = new StringBuilder();
    for (int i = 0; i < value.Length; i++)
    {
        int t = (int)value[i];
        if (t >= 65 && t < 91)
        {
            t += 13;
            if (t >= 91)
            {
                t -= 26;
            }
        }
        else if (t >= 97 && t < 123)
        {
            t += 13;
            if (t >= 123)
            {
                t -= 26;
            }
        }
        b.Append((char)t);
    }
    return b.ToString();
}

• It uses StringBuilder.
  StringBuilder contains a buffer and lots of logic to optimize appends. It is much slower than single-character modifications.
• It copies.
  The method iterates through the input string and copies it all to StringBuilder character-by-character.

Better ROT13 code

We can do much better than the method above and use the ToCharArray method. We convert the string into a char array and modify it in-place. This version is faster because it uses ToCharArray.

static string RotateB(string value)
{
    //
    // Modifies the characters in-place using ToCharArray.
    //
    char[] c = value.ToCharArray();
    for (int i = 0; i < c.Length; i++)
    {
        int t = (int)c[i];
        if (t >= 65 && t < 91)
        {
            t += 13;
            if (t >= 91)
            {
                t -= 26;
            }
        }
        else if (t >= 97 && t < 123)
        {
            t += 13;
            if (t >= 123)
            {
                t -= 26;
            }
        }
        c[i] = (char)t;
    }
    return new string(c);
}

• It uses ToCharArray.
  ToCharArray is an instance method on the string class. It returns a char[] array.
• It modifies in-place.
  You can modify the characters in a char[] array and leave them in that array, which is very good for performance and cuts memory usage.

How fast are the transformations?

They are both fast, but the version using ToCharArray is much faster. Here's a graph and a table comparing the two Rotate methods being run many times.

Summary

The method here highlights the benefits of ToCharArray when it is used properly. ToCharArray can help a lot with other kinds of character manipulation, which I show in an article about uppercasing first letters in strings.

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