Safe truncate string contains color tag

user_2714606 0
Viewed 76

I have a string which contains color tags.

var myString = "My name is <color=#FF00EE>ABCDE</color> and I love <color=#FFEE00>music</color>";

My string becomes "My name is ABCDE*(pink)* and I love music*(yellow)*"

I want to truncate if the string reaches max length but still keep color tag

var myTruncateString = "My name is <color=#FF00EE>ABCDE</color> and I love <color=#FFEE00>mu</color>";

My string becomes "My name is ABCDE*(pink)* and I love mu*(yellow)*"

Do you have any suggestion?

var stringWithoutFormat = String.Copy(myString);
stringWithoutFormat = Regex.Replace(stringWithoutFormat, "<color.*?>|</color>", "");

var maxLength = 20;
if (stringWithoutFormat.Length > maxLength)
{
    // What should I do next?
}
regex c#
2 Answers

Here's a relatively simply and NOT error-handling example of what I think you're trying to accomplish:

  • Don't count color tags when checking maximum length
  • Remove characters from the end, don't destroy color tags
  • If you end up with color tags with no text between them, remove those tags

Note: This code is not thoroughly tested. Feel free to use it for whatever you want, but I would write a lot of unit-tests here. In particular I'm scared about the existance of edge-cases that lead to an infinite loop.

public static string Shorten(string input, int requiredLength)
{
    var tokens = Tokenize(input).ToList();
    int current = tokens.Count - 1;
    
    // assumption: color tags doesn't contribute to *visible* length
    var totalLength = tokens.Where(t => t.Length == 1).Count();
    
    while (totalLength > requiredLength && current >= 0)
    {
        // infinite-loop detection
        if (lastCurrent == current && lastTotalLength == totalLength)
            throw new InvalidOperationException("Infinite loop detected");
        lastCurrent = current;
        lastTotalLength = totalLength;

        if (tokens[current].Length > 1)
        {
            if (current == 0)
                return "";
            
            if (tokens[current].StartsWith("</") && tokens[current - 1].StartsWith("<c"))
            {
                // Remove a <color></color> pair with no text between
                tokens.RemoveAt(current);
                tokens.RemoveAt(current - 1);
                current -= 2;
                
                // Since color tags doesn't contribute to length, don't adjust totalLength
                continue;
            }
            
            // Remove one character from inside the color tags
            tokens.RemoveAt(current - 1);
            current--;
            totalLength--;
        }
        else
        {
            // Remove last character from string
            tokens.RemoveAt(current);
            current--;
            totalLength--;
        }
    }

    // If we're now at the right length, but the last two tokens are <color></color>, remove them
    if (tokens.Count >= 2 && tokens.Last().StartsWith("</") && tokens[tokens.Count - 2].StartsWith("<c"))
    {
        tokens.RemoveAt(tokens.Count - 1);
        tokens.RemoveAt(tokens.Count - 1);
    }
    return string.Join("", tokens);
}

public static IEnumerable<string> Tokenize(string input)
{
    int index = 0;
    while (index < input.Length)
    {
        if (input[index] == '<')
        {
            int endIndex = index;
            while (endIndex < input.Length && input[endIndex] != '>')
                endIndex++;
            if (endIndex < input.Length)
                endIndex++;
            yield return input.Substring(index, endIndex - index);
            index = endIndex;
        }
        else
        {
            yield return input.Substring(index, 1);
            index++;
        }
    }
}

Example code:

var myString = "My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>";
for (int length = 1; length < 100; length++)
    Console.WriteLine($"{length}: {Shorten(myString, length)}");

Output:

1: M
2: My
3: My 
4: My n
5: My na
6: My nam
7: My name
8: My name 
9: My name i
10: My name is
11: My name is 
12: My name is <color=#ff00ee>A</color>
13: My name is <color=#ff00ee>AB</color>
14: My name is <color=#ff00ee>ABC</color>
15: My name is <color=#ff00ee>ABCD</color>
16: My name is <color=#ff00ee>ABCDE</color>
17: My name is <color=#ff00ee>ABCDE</color> 
18: My name is <color=#ff00ee>ABCDE</color> a
19: My name is <color=#ff00ee>ABCDE</color> an
20: My name is <color=#ff00ee>ABCDE</color> and
21: My name is <color=#ff00ee>ABCDE</color> and 
22: My name is <color=#ff00ee>ABCDE</color> and I
23: My name is <color=#ff00ee>ABCDE</color> and I 
24: My name is <color=#ff00ee>ABCDE</color> and I l
25: My name is <color=#ff00ee>ABCDE</color> and I lo
26: My name is <color=#ff00ee>ABCDE</color> and I lov
27: My name is <color=#ff00ee>ABCDE</color> and I love
28: My name is <color=#ff00ee>ABCDE</color> and I love 
29: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>m</color>
30: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>mu</color>
31: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>mus</color>
32: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>musi</color>
33: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>
34: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>
35: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>
36: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>
37: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>
38: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>
39: My name is <color=#ff00ee>ABCDE</color> and I love <color=#eeddff>music</color>
... and so on
user_2670

I generate 2 lists :

  • One that contains the indexes of the real text
  • One that contains the staring and ending indexes of tags

Then I take the text until the max length in the first array. Finally, I check if there was an opening tag and if so, I close it.

N.B. : My code doesn't handle nested tags. You'd have to change the closing tag part.

public static string Truncate(string text, int maxLength)
    {
        if (text.Length <= maxLength) return text;

        var tagIndexes = new List<int>();
        var realTextIndexes = new List<int>();
        bool isInTag = false;
        for (int i = 0; i < text.Length; i++)
        {
            if (text[i] == '<')
            {
                isInTag = true;
                tagIndexes.Add(i);
            }

            if (!isInTag)
            {
                realTextIndexes.Add(i);
            }

            if (text[i] == '>')
            {
                isInTag = false;
                tagIndexes.Add(i);
            }
        }

        if (realTextIndexes.Count <= maxLength) return text;

        string truncatedText = text.Substring(0, realTextIndexes[maxLength - 1] + 1);

        // Should we close a tag ?
        for (int i = 0; i < tagIndexes.Count; i++)
        {
            if (tagIndexes[i] > realTextIndexes[maxLength - 1])
            {
                if ((i % 4) == 2) // If the next tag is a closing tag
                {
                    truncatedText += text.Substring(tagIndexes[i], tagIndexes[i + 1] - tagIndexes[i] + 1);
                }

                break;
            }
        }

        return truncatedText;
    }
user_17851910
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