//	Spludlow Software
//	Copyright © Samuel P. Ludlow 2020 All Rights Reserved
//	Distributed under the terms of the GNU General Public License version 3
//	Distributed WITHOUT ANY WARRANTY; without implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE
//	https://www.spludlow.co.uk/LICENCE.TXT
//	The Spludlow logo is a registered trademark of Samuel P. Ludlow and may not be used without permission
//	v1.14

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.IO;

namespace Spludlow
{
	public class CRC32
	{
		private readonly uint[] ChecksumTable;
		private readonly uint Polynomial = 0xEDB88320;

		public CRC32()
		{
			ChecksumTable = new uint[0x100];

			for (uint index = 0; index < 0x100; ++index)
			{
				uint item = index;
				for (int bit = 0; bit < 8; ++bit)
					item = ((item & 1) != 0) ? (Polynomial ^ (item >> 1)) : (item >> 1);
				ChecksumTable[index] = item;
			}
		}

		public byte[] ComputeHash(Stream stream)
		{
			uint result = 0xFFFFFFFF;

			int current;
			while ((current = stream.ReadByte()) != -1)
				result = ChecksumTable[(result & 0xFF) ^ (byte)current] ^ (result >> 8);

			byte[] hash = BitConverter.GetBytes(~result);
			Array.Reverse(hash);
			return hash;
		}

		public byte[] ComputeHash(byte[] data)
		{
			using (MemoryStream stream = new MemoryStream(data))
				return ComputeHash(stream);
		}
	}
}