compression string in c#

1
votes

I am using the following code to compress 2 strings, it works well in compression. However, when I try to decompress the second strings and if the first one is short (about 4 characters) I got Block length does not match with its complement exception. Here is the class I used for compression:

    using System;
    using System.IO;
    using System.IO.Compression;
    using System.Text;

    namespace CompressString {
    internal static class StringCompressor
{
    /// <summary>
    /// Compresses the string.
    /// </summary>
    /// <param name="text">The text.</param>
    /// <returns></returns>
    public static string CompressString(string value)
    {
        //Transform string into byte[] 
        byte[] byteArray = new byte[value.Length];
        int indexBA = 0;
        foreach (char item in value.ToCharArray())
        {
            byteArray[indexBA++] = (byte)item;
        }

        //Prepare for compress
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        System.IO.Compression.GZipStream sw = new System.IO.Compression.GZipStream(ms,
        System.IO.Compression.CompressionMode.Compress);

        //Compress
        sw.Write(byteArray, 0, byteArray.Length);
        //Close, DO NOT FLUSH cause bytes will go missing...
        sw.Close();

        //Transform byte[] zip data to string
        byteArray = ms.ToArray();
        System.Text.StringBuilder sB = new System.Text.StringBuilder(byteArray.Length);
        foreach (byte item in byteArray)
        {
            sB.Append((char)item);
        }
        ms.Close();
        sw.Dispose();
        ms.Dispose();

        return sB.ToString();
    }

    /// <summary>
    /// Decompresses the string.
    /// </summary>
    /// <param name="compressedText">The compressed text.</param>
    /// <returns></returns>
    public static string DecompressString(string sData)
    {
        byte[] byteArray = new byte[sData.Length];

        int indexBa = 0;
        foreach (char item in sData)
            byteArray[indexBa++] = (byte)item;

        MemoryStream memoryStream = new MemoryStream(byteArray);
        GZipStream gZipStream = new GZipStream(memoryStream, CompressionMode.Decompress);

        byteArray = new byte[1024];

        StringBuilder stringBuilder = new StringBuilder();

        int readBytes;
        while ((readBytes = gZipStream.Read(byteArray, 0, byteArray.Length)) != 0)
        {
            for (int i = 0; i < readBytes; i++) stringBuilder.Append((char)byteArray[i]);
        } gZipStream.Close(); memoryStream.Close(); gZipStream.Dispose(); memoryStream.Dispose(); return stringBuilder.ToString();
    }
}

}

I got the exception in DecompressString method in the line:

while ((readBytes = gZipStream.Read(byteArray, 0, byteArray.Length)) != 0)
3
c#compression
The built-in deflation algorithm of .NET is flaky and not recommended in any case - I'm not sure it's even been improves in more recent versions (can anyone clarify?) I'd say use SharpZipLib or something similar, just for starters. – Grant Thomas
What do you mean by that Grant? sorry but I am novice in compression algorithms.. Will SharpZipLib solve my problem? – user1477701
I've used the .NET built-in compression algorithms with great success. At the core, they are useful for compressing/decompressing data that you control directly. – dblood

3 Answers

2
votes

For others that are receiving the error message "Block length does not match with its complement", this can occur if you are trying to decompress a file that was not compressed.

1
votes

Your Compress and Decompress methods should accept and return byte arrays. If you then are required to produce string output you can convert the byte array to a base64 string.

For example:

// compress some string data
string stringData = GetSomeStringData();

// assumes the string data is UTF8 string - would work for ASCII encoding as well
byte[] uncompressedData = Encoding.UTF8.GetBytes(stringData);

byte[] compressedData = StringCompressor.Compress(uncompressedData);

// produce base64 encode string of compressed data
string compressedString = Convert.ToBase64String(compressedData);

// decompress base64 encoded string 
// first convert to byte array
byte[] dataToDecompress = Convert.FromBase64String(compressedString);

byte[] decompressedData = StringCompressor.Decompress(dataToDecompress);

// encode decompressed data as a UTF8 encoded string
string decompressedString = Encoding.UTF8.GetString(decompressedData);

Rename and Tweak your Compress() and Decompress() methods accordingly. If you really wanted to have a class that strictly worked with strings, embed the logic of the Base 64 encoding/decoding in your methods. However, if your class works directly with 'byte[]` your code will be much more flexible and reusable in other instances.

1
votes

I get the same "Block length does not match with its complement." System.IO.InvalidDataException.

I've found this information: http://yvanrodrigues.com/content/block-length-does-not-match-its-complement

It states: "Seek forward 2 bytes first. There is a two byte signature 0x789c at the beginning of the deflated file."

It should work if you skip these 2 bytes.

Further information: What does a zlib header look like? and .Net zlib inflate with .Net 4.5