CODEWITHSUNDEEP
c#gzipgzipstreamcompression
Zachi Shtain
Zachi Shtain

Reputation: 836

Decompressing GZIP stream

I am trying to decompress a GZipped string which is part of response from a webservice. The string that I have is:

"[31,-117,8,0,0,0,0,0,0,0,109,-114,65,11,-62,48,12,-123,-1,75,-50,-61,-42,-127,30,122,21,111,-126,94,60,-119,-108,-72,102,44,-48,-75,-93,-21,100,56,-6,-33,-19,20,20,101,57,37,95,-14,94,-34,4,-63,-5,-72,-73,-44,-110,-117,-96,38,-88,26,-74,38,-112,3,117,-7,25,-82,5,24,-116,56,-97,-44,108,-23,28,24,-44,-85,83,34,-41,97,-88,24,-99,23,36,124,-120,94,99,-120,15,-42,-91,-108,91,45,-11,70,119,60,-110,21,-20,12,-115,-94,111,-80,-93,89,-41,-65,-127,-82,76,41,51,-19,52,90,-5,69,-85,76,-96,-128,64,22,35,-33,-23,-124,-79,-55,-1,-2,-10,-87,0,55,-76,55,10,-57,122,-9,73,42,-45,98,-44,5,-77,101,-3,58,-91,39,38,51,-15,121,21,1,0,0]"

I'm trying to decompress that string using the following method:

public static string UnZip(string value)
        {
            // Removing brackets from string
            value = value.TrimStart('[');
            value = value.TrimEnd(']');

            //Transform string into byte[]
            string[] strArray = value.Split(',');
            byte[] byteArray = new byte[strArray.Length];
            for (int i = 0; i < strArray.Length; i++)
            {
                if (strArray[i][0] != '-')
                    byteArray[i] = Convert.ToByte(strArray[i]);
                else
                {
                    int val = Convert.ToInt16(strArray[i]);
                    byteArray[i] = (byte)(val + 256);
                }
            }

            //Prepare for decompress
            System.IO.MemoryStream ms = new System.IO.MemoryStream(byteArray);
            System.IO.Compression.GZipStream sr = new System.IO.Compression.GZipStream(ms,
                System.IO.Compression.CompressionMode.Decompress);

            //Reset variable to collect uncompressed result
            byteArray = new byte[byteArray.Length];

            //Decompress
            int rByte = sr.Read(byteArray, 0, byteArray.Length);

            //Transform byte[] unzip data to string
            System.Text.StringBuilder sB = new System.Text.StringBuilder(rByte);
            //Read the number of bytes GZipStream red and do not a for each bytes in
            //resultByteArray;
            for (int i = 0; i < rByte; i++)
            {
                sB.Append((char)byteArray[i]);
            }
            sr.Close();
            ms.Close();
            sr.Dispose();
            ms.Dispose();
            return sB.ToString();
        }

The method is a modified version of the one in the following link: http://www.codeproject.com/Articles/27203/GZipStream-Compress-Decompress-a-string

Sadly, the result of that method is a corrupted string. More specifically, I know that the input string contains a compressed JSON object and the output string has only some of the expected string:

"{\"rootElement\":{\"children\":[{\"children\":[],\"data\":{\"fileUri\":\"file:////Luciano/e/orto_artzi_2006_0_5_pixel/index/shapefiles/index_cd20/shp_all/index_cd2.shp\",\"relativePath\":\"/i"

Any idea what could be the problem and how to solve it?

Upvotes: 0

Views: 8901

Answers (2)

Daniel Strickland
Daniel Strickland

Reputation: 1

Following on from Xanatos's answer in C# slightly modified to return a simple byte array. This takes a gzip compressed byte array and returns the inflated gunzipped array.

public static byte[] Decompress(byte[] compressed_data)
{
    var outputStream = new MemoryStream();
    using (var compressedStream = new MemoryStream(compressed_data))
    using (System.IO.Compression.GZipStream sr = new System.IO.Compression.GZipStream(
        compressedStream, System.IO.Compression.CompressionMode.Decompress))
    {
        sr.CopyTo(outputStream);
        outputStream.Position = 0;
        return outputStream.ToArray();
    }
}

Upvotes: 0

xanatos
xanatos

Reputation: 111810

Try

public static string UnZip(string value)
{
    // Removing brackets from string
    value = value.TrimStart('[');
    value = value.TrimEnd(']');

    //Transform string into byte[]
    string[] strArray = value.Split(',');
    byte[] byteArray = new byte[strArray.Length];
    for (int i = 0; i < strArray.Length; i++)
    {
        byteArray[i] = unchecked((byte)Convert.ToSByte(strArray[i]));
    }

    //Prepare for decompress
    using (System.IO.MemoryStream output = new System.IO.MemoryStream())
    {
        using (System.IO.MemoryStream ms = new System.IO.MemoryStream(byteArray))
        using (System.IO.Compression.GZipStream sr = new System.IO.Compression.GZipStream(ms, System.IO.Compression.CompressionMode.Decompress))
        {
            sr.CopyTo(output);
        }

        string str = Encoding.UTF8.GetString(output.GetBuffer(), 0, (int)output.Length);
        return str;
    }
}

The MemoryBuffer() doesn't "duplicate" the byteArray but is directly backed by it, so you can't reuse the byteArray.

I'll add that I find funny that they "compressed" a json of 277 characters to a stringized byte array of 620 characters.

As a sidenote, the memory occupation of this method is out-of-the-roof... The 620 character string (that in truth is a 277 byte array) to be decompressed causes the creation of strings/arrays for a total size of 4887 bytes (including the 620 initial character string) (disclaimer: the GC can reclaim part of this memory during the execution of the method). This is ok for byte arrays of 277 bytes... But for bigger ones the memory occupation will become quite big.

Upvotes: 6

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