using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;
using Ionic.Crc;

namespace TLSharp.Core.MTProto.Crypto {
    public class Crc32 : HashAlgorithm {
        public const UInt32 DefaultPolynomial = 0xedb88320u;
        public const UInt32 DefaultSeed = 0xffffffffu;

        private UInt32 hash;
        private UInt32 seed;
        private UInt32[] table;
        private static UInt32[] defaultTable;

        public Crc32() {
            table = InitializeTable(DefaultPolynomial);
            seed = DefaultSeed;
            hash = seed;
        }

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

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

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

        /// <summary>
        /// Возвращает хеш в BigEndian
        /// </summary>
        /// <returns></returns>
        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)
		    };
        }
    }
}