Building a smart string trimming function in C#

Question

I am attempting to build a string extension method to trim a string to a certain length but with not breaking a word. I wanted to check to see if there was anything built into the framework or a more clever method than mine. Here's mine so far (not thoroughly tested):

public static string SmartTrim(this string s, int length)
        {
            StringBuilder result = new StringBuilder();

            if (length >= 0)
            {
                if (s.IndexOf(' ') > 0)
                {
                    string[] words = s.Split(' ');
                    int index = 0;

                    while (index < words.Length - 1 && result.Length + words[index + 1].Length <= length)
                    {
                        result.Append(words[index]);
                        result.Append(" ");
                        index++;
                    }

                    if (result.Length > 0)
                    {
                        result.Remove(result.Length - 1, 1);
                    }
                }
                else
                {
                    result.Append(s.Substring(0, length));
                }
            }
            else
            {
                throw new ArgumentOutOfRangeException("length", "Value cannot be negative.");
            }

            return result.ToString();
        }

Answer 1

I'd use string.LastIndexOf - at least if we only care about spaces. Then there's no need to create any intermediate strings...

As yet untested:

public static string SmartTrim(this string text, int length)
{
    if (text == null)
    {
        throw new ArgumentNullException("text");
    }
    if (length < 0)
    {
        throw new ArgumentOutOfRangeException();
    }
    if (text.Length <= length)
    {
        return text;
    }
    int lastSpaceBeforeMax = text.LastIndexOf(' ', length);
    if (lastSpaceBeforeMax == -1)
    {
        // Perhaps define a strategy here? Could return empty string,
        // or the original
        throw new ArgumentException("Unable to trim word");
    }
    return text.Substring(0, lastSpaceBeforeMax);        
}

Test code:

public class Test
{
    static void Main()
    {
        Console.WriteLine("'{0}'", "foo bar baz".SmartTrim(20));
        Console.WriteLine("'{0}'", "foo bar baz".SmartTrim(3));
        Console.WriteLine("'{0}'", "foo bar baz".SmartTrim(4));
        Console.WriteLine("'{0}'", "foo bar baz".SmartTrim(5));
        Console.WriteLine("'{0}'", "foo bar baz".SmartTrim(7));
    }
}

Results:

'foo bar baz'
'foo'
'foo'
'foo'
'foo bar'

Answer 2

How about a Regex based solution ? You will probably want to test some more, and do some bounds checking; but this is what spring to my mind:

using System;
using System.Text.RegularExpressions;

namespace Stackoverflow.Test
{
    static class Test
    {
        private static readonly Regex regWords = new Regex("\\w+", RegexOptions.Compiled);

        static void Main()
        {
            Console.WriteLine("The quick brown fox jumped over the lazy dog".SmartTrim(8));
            Console.WriteLine("The quick brown fox jumped over the lazy dog".SmartTrim(20));
            Console.WriteLine("Hello, I am attempting to build a string extension method to trim a string to a certain length but with not breaking a word. I wanted to check to see if there was anything built into the framework or a more clever method than mine".SmartTrim(100));
        }

        public static string SmartTrim(this string s, int length)
        {
            var matches = regWords.Matches(s);
            foreach (Match match in matches)
            {
                if (match.Index + match.Length > length)
                {
                    int ln = match.Index + match.Length > s.Length ? s.Length : match.Index + match.Length;
                    return s.Substring(0, ln);
                }
            }
            return s;
        }
    }
}