﻿/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace DotPulsar.Internal
{
    using System.Buffers;

    public static class Crc32C
    {
        private const uint _generator = 0x82F63B78u;

        private static readonly uint[] _lookup;

        static Crc32C()
        {
            _lookup = new uint[16 * 256];

            for (uint i = 0; i < 256; i++)
            {
                var entry = i;

                for (var j = 0; j < 16; j++)
                {
                    for (var k = 0; k < 8; k++)
                        entry = (entry & 1) == 1 ? _generator ^ (entry >> 1) : entry >> 1;

                    _lookup[j * 256 + i] = entry;
                }
            }
        }

        public static uint Calculate(ReadOnlySequence<byte> sequence)
        {
            var block = new uint[16];
            var checksum = uint.MaxValue;
            var remaningBytes = sequence.Length;
            var readingBlock = remaningBytes >= 16;
            var offset = 15;

            foreach (var memory in sequence)
            {
                var span = memory.Span;

                for (var i = 0; i < span.Length; ++i)
                {
                    var currentByte = span[i];

                    if (!readingBlock)
                    {
                        checksum = _lookup[(byte) (checksum ^ currentByte)] ^ (checksum >> 8);
                        continue;
                    }

                    var offSetBase = offset * 256;

                    if (offset > 11)
                        block[offset] = _lookup[offSetBase + ((byte) (checksum >> (8 * (15 - offset))) ^ currentByte)];
                    else
                        block[offset] = _lookup[offSetBase + currentByte];

                    --remaningBytes;

                    if (offset == 0)
                    {
                        offset = 15;
                        readingBlock = remaningBytes >= 16;
                        checksum = 0;

                        for (var j = 0; j < block.Length; ++j)
                            checksum ^= block[j];
                    }
                    else
                    {
                        --offset;
                    }
                }
            }

            return checksum ^ uint.MaxValue;
        }
    }
}