// LzInWindow.cs

using System;

namespace SevenZip.Compression.LZ
{
	class InWindow
	{
		public Byte[] _bufferBase = null; // pointer to buffer with data
		System.IO.Stream _stream;
		UInt32 _posLimit; // offset (from _buffer) of first byte when new block reading must be done
		bool _streamEndWasReached; // if (true) then _streamPos shows real end of stream

		UInt32 _pointerToLastSafePosition;

		public UInt32 _bufferOffset;

		public UInt32 _blockSize; // Size of Allocated memory block
		public UInt32 _pos; // offset (from _buffer) of curent byte
		UInt32 _keepSizeBefore; // how many BYTEs must be kept in buffer before _pos
		UInt32 _keepSizeAfter; // how many BYTEs must be kept buffer after _pos
		public UInt32 _streamPos; // offset (from _buffer) of first not read byte from Stream

		public void MoveBlock()
		{
			UInt32 offset = (UInt32)(_bufferOffset) + _pos - _keepSizeBefore;
			// we need one additional byte, since MovePos moves on 1 byte.
			if (offset > 0)
				offset--;
			
			UInt32 numBytes = (UInt32)(_bufferOffset) + _streamPos - offset;

			// check negative offset ????
			for (UInt32 i = 0; i < numBytes; i++)
				_bufferBase[i] = _bufferBase[offset + i];
			_bufferOffset -= offset;
		}

		public virtual void ReadBlock()
		{
			if (_streamEndWasReached)
				return;
			while (true)
			{
				int size = (int)((0 - _bufferOffset) + _blockSize - _streamPos);
				if (size == 0)
					return;
				int numReadBytes = _stream.Read(_bufferBase, (int)(_bufferOffset + _streamPos), size);
				if (numReadBytes == 0)
				{
					_posLimit = _streamPos;
					UInt32 pointerToPostion = _bufferOffset + _posLimit;
					if (pointerToPostion > _pointerToLastSafePosition)
						_posLimit = (UInt32)(_pointerToLastSafePosition - _bufferOffset);

					_streamEndWasReached = true;
					return;
				}
				_streamPos += (UInt32)numReadBytes;
				if (_streamPos >= _pos + _keepSizeAfter)
					_posLimit = _streamPos - _keepSizeAfter;
			}
		}

		void Free() { _bufferBase = null; }

		public void Create(UInt32 keepSizeBefore, UInt32 keepSizeAfter, UInt32 keepSizeReserv)
		{
			_keepSizeBefore = keepSizeBefore;
			_keepSizeAfter = keepSizeAfter;
			UInt32 blockSize = keepSizeBefore + keepSizeAfter + keepSizeReserv;
			if (_bufferBase == null || _blockSize != blockSize)
			{
				Free();
				_blockSize = blockSize;
				_bufferBase = new Byte[_blockSize];
			}
			_pointerToLastSafePosition = _blockSize - keepSizeAfter;
		}

		public void SetStream(System.IO.Stream stream) { _stream = stream; }
		public void ReleaseStream() { _stream = null; }

		public void Init()
		{
			_bufferOffset = 0;
			_pos = 0;
			_streamPos = 0;
			_streamEndWasReached = false;
			ReadBlock();
		}

		public void MovePos()
		{
			_pos++;
			if (_pos > _posLimit)
			{
				UInt32 pointerToPostion = _bufferOffset + _pos;
				if (pointerToPostion > _pointerToLastSafePosition)
					MoveBlock();
				ReadBlock();
			}
		}

		public Byte GetIndexByte(Int32 index) { return _bufferBase[_bufferOffset + _pos + index]; }

		// index + limit have not to exceed _keepSizeAfter;
		public UInt32 GetMatchLen(Int32 index, UInt32 distance, UInt32 limit)
		{
			if (_streamEndWasReached)
				if ((_pos + index) + limit > _streamPos)
					limit = _streamPos - (UInt32)(_pos + index);
			distance++;
			// Byte *pby = _buffer + (size_t)_pos + index;
			UInt32 pby = _bufferOffset + _pos + (UInt32)index;

			UInt32 i;
			for (i = 0; i < limit && _bufferBase[pby + i] == _bufferBase[pby + i - distance]; i++);
			return i;
		}

		public UInt32 GetNumAvailableBytes() { return _streamPos - _pos; }

		public void ReduceOffsets(Int32 subValue)
		{
			_bufferOffset += (UInt32)subValue;
			_posLimit -= (UInt32)subValue;
			_pos -= (UInt32)subValue;
			_streamPos -= (UInt32)subValue;
		}
	}
}
