Nick's Blog

Microsoft .Net Framework is so powerful, and it provides so many common libraries to use. However, recently I am working on a legacy project to work out CRC32, and I can find no where. OK, I have to DIY. If you need it by chance, here are the source code.

/// 
/// Calculates a 32bit Cyclic Redundancy Checksum (CRC) using the
/// same polynomial used by Zip.
/// 
public class CRC32 : HashAlgorithm
{

    private static readonly UInt32[] crc32Table =
    {
	0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
	0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
	0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
	0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
	0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
	0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
	0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
	0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
	0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
	0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
	0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
	0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
	0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
	0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
	0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
	0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
	0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
	0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
	0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
	0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
	0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
	0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
	0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
	0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
	0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
	0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
	0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
	0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
	0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
	0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
	0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
	0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
	0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
	0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
	0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
	0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
	0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
	0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
	0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
	0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
	0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
	0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
	0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
	0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
	0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
	0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
	0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
	0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
	0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
	0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
	0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
	0x2d02ef8d
    };

    private UInt32 hash;
    private UInt32 seed;
    private const UInt32 DefaultSeed = 0xffffffff;

    /// 
    /// Construct an instance of the CRC32 class, pre-initialising the table
    /// for speed of lookup.
    /// 
    public CRC32()
    {
        this.seed = DefaultSeed;
        Initialize();
    }

    public override int HashSize
    {
        get
        {
            return 32;
        }
    }

    public CRC32(UInt32 seed)
    {
        this.seed = seed;
        Initialize();
    }

    protected override byte[] HashFinal()
    {
        hash = ~hash;
        this.HashValue = new byte[] {
						(byte)((hash>>24) & 0xff),
						(byte)((hash>>16) & 0xff),
						(byte)((hash>>8) & 0xff),
						(byte)(hash & 0xff)
						};
        return HashValue;
    }

    protected override void HashCore(byte[] buffer, int start, int length)
    {
        for (int i = start; i>8) 
                          ^ crc32Table[(buffer[i]) ^ ((hash) & 0x000000FF)];
            }
        }
    }

    public override void Initialize()
    {
        hash = seed;
    }
}

I am currently working on an ASP.Net web project, which allows client to download a well structured submission package. Due to security reason, web server can not create folder and write file on client side. Web server can generate a well structured zipped file and push to the client.

There could be two ways to achieve that;

Fist way:

1. Create folders and copy files into each folder
2. Use 3^rd party tool, eg winrar, 7zip to zip the whole folder to one zipped file
3. Send back download web address, and let client download the zipped file

If the zipped file is small and web server is powerful enough, client won’t feel any delay. However if the file is big, client side will get no any response until zip file is ready. We can put similar message, eg “Please wait… ”. But client side still don’t know how long would take. If zip file is not ready within time out period, server will get reset. This is a really awful user experience.

Second way:

1. Create folder and zip the files on the fly (on web server’s memory)
2. Send back zipped stream to client through Response
3. Client download  zipping file one by one

Client can clearly feel the progress of downloading.

Here is the code for 2^nd solution:

//prepare response header
Response.Clear();
//set Response Content Type
Response.ContentType = "application/x-zip-compressed";
Response.AddHeader("Content-Disposition", "attachment; filename=Download.zip");
ZipOutputStream zipOutput = new ZipOutputStream(Response.OutputStream);
try
{
            zipOutput.IsStreamOwner = false;
            //set compress level
            zipOutput.SetLevel(9);

            ArrayList fileList = functions.GetFileList();
            foreach (string fileName in fileList)
            {
                string folderName = "folder1" + @"";

                zipEntry = new ZipEntry(folderName + fileName);
                zipOutput.PutNextEntry(zipEntry);
                            
                byte[] file = functions.GetFile(fileName);
                //zip file
                zipOutput.Write(file, 0, file.Length);
                 //send to client for downloading
                Response.Flush();
            }
        }
        catch (Exception ex)
        {
            Response.Write(ex.ToString());
        }
        finally
        {
            zipOutput.Finish();
            zipOutput.Close();
            Response.End();
        }