Compress a string with zip

· December 15, 2008


We found a nasty performance bug in the code below. The DeCompress-method copies a string for each turn in the loop. That is a classic problem that creates a new copy of the string for each row. That became a major problem for a 3.8 Mb string…

I have now updated the code to use the System.Text.StringBuilder object instead. That took down the speed to about a tenth. Sorry that I didn’t catch that…


We had a quite special need the other day; we wanted to compress a part of our request, namely a XML-string that was sent to us.

Most of the examples I found on the net showed how to compress the content of a file. But here is the code that compress a string. The code uses ICSharpCode.SharpZipLib. Here you go:

/// <summary> /// Compress strings with ICSharpCode.SharpZipLib /// </summary> public class StringZipper { public static string Compress(string uncompressedString) { byte[] bytData = System.Text.Encoding.UTF8.GetBytes(uncompressedString); MemoryStream ms = new MemoryStream(); Stream s = new DeflaterOutputStream(ms); s.Write(bytData, 0, bytData.Length); s.Close(); byte[] compressedData = (byte[])ms.ToArray(); return System.Convert.ToBase64String(compressedData, 0, compressedData.Length); }

public static string DeCompress(string compressedString) { StringBuilder uncompressedString = new StringBuilder(); int totalLength = 0; byte[] bytInput = System.Convert.FromBase64String(compressedString); ; byte[] writeData = new byte[4096]; Stream s2 = new InflaterInputStream(new MemoryStream(bytInput)); while (true) { int size = s2.Read(writeData, 0, writeData.Length); if (size > 0) { totalLength += size; uncompressedString.Append(System.Text.Encoding.UTF8.GetString(writeData, 0, size)); } else { break; } } s2.Close(); return uncompressedString.ToString(); } }

And here is how to call it:

// Compress the string string orgString = “Hello compression with åäö.”; //orgString = File.ReadAllText(args[0]);

Stopwatch sw = new Stopwatch(); sw.Start(); string compressedString = StringZipper.Compress(orgString); sw.Stop(); string compressTime = sw.ElapsedMilliseconds.ToString();

// Decompress the string sw.Reset(); sw.Start(); string decompressedString = StringZipper.DeCompress(compressedString); sw.Stop(); string decompressTime = sw.ElapsedMilliseconds.ToString();

if (orgString == decompressedString) Console.WriteLine(“SAME”); else Console.WriteLine(“DIFFER”);

// Show some nice statistics decimal ratio = (decimal)compressedString.Length / (decimal)orgString.Length; Console.WriteLine(string.Format(“Orignal string was: {0} chars”, orgString.Length)); Console.WriteLine(string.Format(“Compressed string was: {0} chars”, compressedString.Length)); Console.WriteLine(string.Format(“DeCompressed string was: {0} chars “, decompressedString.Length)); Console.WriteLine(string.Format(“Compression ratio {0}”, ratio)); Console.WriteLine(string.Format(“The compression took {0} ms, DeCompression took {1} ms”, compressTime, decompressTime));

I should point out two things;

  • For short strings the compressed string might actually be longer than the original.
  • It’s not very good to store the result of the compression as a string (as done by System.Convert.ToBase64String), but it’s quite nice to have for serialization.

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