Wednesday, March 07, 2007

Compress and decompress strings in C#

by post@madskristensen.dk (Mads Kristensen) via .NET Slave on 3/7/2007 11:25:35 PM

A couple of days ago, I had to store some very big strings in an XML file. To keep the file size down I wanted to compress the strings using a GZipStream and then decompress them later when needed.

I modified the methods from Papenoo pa so they handled strings in stead of byte arrays.

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

publicstaticstring Compress(string text)
{
 byte[] buffer = Encoding.UTF8.GetBytes(text);
 MemoryStream ms =new MemoryStream();
 using (GZipStream zip =new GZipStream(ms, CompressionMode.Compress, true))
 {
  zip.Write(buffer, 0, buffer.Length);
 }

 ms.Position = 0;
 MemoryStream outStream =new MemoryStream();

 byte[] compressed =newbyte[ms.Length];
 ms.Read(compressed, 0, compressed.Length);

 byte[] gzBuffer =newbyte[compressed.Length + 4];
 System.Buffer.BlockCopy(compressed, 0, gzBuffer, 4, compressed.Length);
 System.Buffer.BlockCopy(BitConverter.GetBytes(buffer.Length), 0, gzBuffer, 0, 4);
 return Convert.ToBase64String (gzBuffer);
}

publicstaticstring Decompress(string compressedText)
{
 byte[] gzBuffer = Convert.FromBase64String(compressedText);
 using (MemoryStream ms =new MemoryStream())
 {
  int msgLength = BitConverter.ToInt32(gzBuffer, 0);
  ms.Write(gzBuffer, 4, gzBuffer.Length - 4);

  byte[] buffer =newbyte[msgLength];

  ms.Position = 0;
  using (GZipStream zip =new GZipStream(ms, CompressionMode.Decompress))
  {
   zip.Read(buffer, 0, buffer.Length);
  }

  return Encoding.UTF8.GetString(buffer);
 }
}

The strings need to be longer than 3-400 characters; otherwise the compression rate is not good enough.