CRC32.cs

/*
 * Created by SharpDevelop.
 * User: JT
 * Date: 13.3.2010
 * Time: 13:53
 * 
 * To change this template use Tools | Options | Coding | Edit Standard Headers.
 */

using System;
using System.Security.Cryptography;


namespace PDFv2
{
	/// <summary>
	/// Description of CRC32.
	/// </summary>
	public class Crc32 : HashAlgorithm
	{
	
    public const UInt32 DefaultPolynomial = 0xedb88320;
    public const UInt32 DefaultSeed = 0xffffffff;

    private UInt32 hash;
    private UInt32 seed;
    private UInt32[] table;
    private static UInt32[] defaultTable;
	
		
		public Crc32()
		{
			table = InitializeTable(DefaultPolynomial);
        	seed = DefaultSeed;
        	Initialize();
		}

    public Crc32(UInt32 polynomial, UInt32 seed) {
        table = InitializeTable(polynomial);
        this.seed = seed;
        Initialize();
    }

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

    protected override void HashCore(byte[] buffer, int start, int length) {
        hash = CalculateHash(table, hash, buffer, start, length);
    }

    protected override byte[] HashFinal() {
        byte[] hashBuffer = UInt32ToBigEndianBytes(~hash);
        this.HashValue = hashBuffer;
        return hashBuffer;
    }

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

    public static UInt32 Compute(byte[] buffer) {
        return ~CalculateHash(InitializeTable(DefaultPolynomial), DefaultSeed, buffer, 0, buffer.Length);
    }

    public static UInt32 Compute(UInt32 seed, byte[] buffer) {
        return ~CalculateHash(InitializeTable(DefaultPolynomial), seed, buffer, 0, buffer.Length);
    }

    public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer) {
        return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
    }

    private static UInt32[] InitializeTable(UInt32 polynomial) {
        if (polynomial == DefaultPolynomial && defaultTable != null)
            return defaultTable;

        UInt32[] createTable = new UInt32[256];
        for (int i = 0; i < 256; i++) {
            UInt32 entry = (UInt32)i;
            for (int j = 0; j < 8; j++)
                if ((entry & 1) == 1)
                    entry = (entry >> 1) ^ polynomial;
                else
                    entry = entry >> 1;
            createTable[i] = entry;
        }

        if (polynomial == DefaultPolynomial)
            defaultTable = createTable;

        return createTable;
    }

    private static UInt32 CalculateHash(UInt32[] table, UInt32 seed, byte[] buffer, int start, int size) {
        UInt32 crc = seed;
        for (int i = start; i < size; i++)
            unchecked {
                crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
            }
        return crc;
    }

    private byte[] UInt32ToBigEndianBytes(UInt32 x) {
        return new byte[] {
			(byte)((x >> 24) & 0xff),
			(byte)((x >> 16) & 0xff),
			(byte)((x >> 8) & 0xff),
			(byte)(x & 0xff)
		};
	}	
		
		
		
		
		
	}
}