using System;

namespace ICSharpCode.SharpZipLib.Zip.Compression.Streams
{
	/// <summary>
	/// This class allows us to retrieve a specified number of bits from
	/// the input buffer, as well as copy big byte blocks.
	///
	/// It uses an int buffer to store up to 31 bits for direct
	/// manipulation.  This guarantees that we can get at least 16 bits,
	/// but we only need at most 15, so this is all safe.
	///
	/// There are some optimizations in this class, for example, you must
	/// never peek more than 8 bits more than needed, and you must first
	/// peek bits before you may drop them.  This is not a general purpose
	/// class but optimized for the behaviour of the Inflater.
	///
	/// authors of the original java version : John Leuner, Jochen Hoenicke
	/// </summary>
	public class StreamManipulator
	{
		/// <summary>
		/// Get the next sequence of bits but don't increase input pointer.  bitCount must be
		/// less or equal 16 and if this call succeeds, you must drop
		/// at least n - 8 bits in the next call.
		/// </summary>
		/// <param name="bitCount">The number of bits to peek.</param>
		/// <returns>
		/// the value of the bits, or -1 if not enough bits available.  */
		/// </returns>
		public int PeekBits(int bitCount)
		{
			if (bitsInBuffer_ < bitCount) {
				if (windowStart_ == windowEnd_) {
					return -1; // ok
				}
				buffer_ |= (uint)((window_[windowStart_++] & 0xff |
								 (window_[windowStart_++] & 0xff) << 8) << bitsInBuffer_);
				bitsInBuffer_ += 16;
			}
			return (int)(buffer_ & ((1 << bitCount) - 1));
		}

		/// <summary>
		/// Drops the next n bits from the input.  You should have called PeekBits
		/// with a bigger or equal n before, to make sure that enough bits are in
		/// the bit buffer.
		/// </summary>
		/// <param name="bitCount">The number of bits to drop.</param>
		public void DropBits(int bitCount)
		{
			buffer_ >>= bitCount;
			bitsInBuffer_ -= bitCount;
		}

		/// <summary>
		/// Gets the next n bits and increases input pointer.  This is equivalent
		/// to <see cref="PeekBits"/> followed by <see cref="DropBits"/>, except for correct error handling.
		/// </summary>
		/// <param name="bitCount">The number of bits to retrieve.</param>
		/// <returns>
		/// the value of the bits, or -1 if not enough bits available.
		/// </returns>
		public int GetBits(int bitCount)
		{
			int bits = PeekBits(bitCount);
			if (bits >= 0) {
				DropBits(bitCount);
			}
			return bits;
		}

		/// <summary>
		/// Gets the number of bits available in the bit buffer.  This must be
		/// only called when a previous PeekBits() returned -1.
		/// </summary>
		/// <returns>
		/// the number of bits available.
		/// </returns>
		public int AvailableBits {
			get {
				return bitsInBuffer_;
			}
		}

		/// <summary>
		/// Gets the number of bytes available.
		/// </summary>
		/// <returns>
		/// The number of bytes available.
		/// </returns>
		public int AvailableBytes {
			get {
				return windowEnd_ - windowStart_ + (bitsInBuffer_ >> 3);
			}
		}

		/// <summary>
		/// Skips to the next byte boundary.
		/// </summary>
		public void SkipToByteBoundary()
		{
			buffer_ >>= (bitsInBuffer_ & 7);
			bitsInBuffer_ &= ~7;
		}

		/// <summary>
		/// Returns true when SetInput can be called
		/// </summary>
		public bool IsNeedingInput {
			get {
				return windowStart_ == windowEnd_;
			}
		}

		/// <summary>
		/// Copies bytes from input buffer to output buffer starting
		/// at output[offset].  You have to make sure, that the buffer is
		/// byte aligned.  If not enough bytes are available, copies fewer
		/// bytes.
		/// </summary>
		/// <param name="output">
		/// The buffer to copy bytes to.
		/// </param>
		/// <param name="offset">
		/// The offset in the buffer at which copying starts
		/// </param>
		/// <param name="length">
		/// The length to copy, 0 is allowed.
		/// </param>
		/// <returns>
		/// The number of bytes copied, 0 if no bytes were available.
		/// </returns>
		/// <exception cref="ArgumentOutOfRangeException">
		/// Length is less than zero
		/// </exception>
		/// <exception cref="InvalidOperationException">
		/// Bit buffer isnt byte aligned
		/// </exception>
		public int CopyBytes(byte[] output, int offset, int length)
		{
			if (length < 0) {
				throw new ArgumentOutOfRangeException("nameof(length)");
			}

			if ((bitsInBuffer_ & 7) != 0) {
				// bits_in_buffer may only be 0 or a multiple of 8
				throw new InvalidOperationException("Bit buffer is not byte aligned!");
			}

			int count = 0;
			while ((bitsInBuffer_ > 0) && (length > 0)) {
				output[offset++] = (byte)buffer_;
				buffer_ >>= 8;
				bitsInBuffer_ -= 8;
				length--;
				count++;
			}

			if (length == 0) {
				return count;
			}

			int avail = windowEnd_ - windowStart_;
			if (length > avail) {
				length = avail;
			}
			System.Array.Copy(window_, windowStart_, output, offset, length);
			windowStart_ += length;

			if (((windowStart_ - windowEnd_) & 1) != 0) {
				// We always want an even number of bytes in input, see peekBits
				buffer_ = (uint)(window_[windowStart_++] & 0xff);
				bitsInBuffer_ = 8;
			}
			return count + length;
		}

		/// <summary>
		/// Resets state and empties internal buffers
		/// </summary>
		public void Reset()
		{
			buffer_ = 0;
			windowStart_ = windowEnd_ = bitsInBuffer_ = 0;
		}

		/// <summary>
		/// Add more input for consumption.
		/// Only call when IsNeedingInput returns true
		/// </summary>
		/// <param name="buffer">data to be input</param>
		/// <param name="offset">offset of first byte of input</param>
		/// <param name="count">number of bytes of input to add.</param>
		public void SetInput(byte[] buffer, int offset, int count)
		{
			if (buffer == null) {
				throw new ArgumentNullException("nameof(buffer)");
			}

			if (offset < 0) {
				throw new ArgumentOutOfRangeException("nameof(offset)", "Cannot be negative");
			}

			if (count < 0) {
				throw new ArgumentOutOfRangeException("nameof(count)", "Cannot be negative");
			}

			if (windowStart_ < windowEnd_) {
				throw new InvalidOperationException("Old input was not completely processed");
			}

			int end = offset + count;

			// We want to throw an ArrayIndexOutOfBoundsException early.
			// Note the check also handles integer wrap around.
			if ((offset > end) || (end > buffer.Length)) {
				throw new ArgumentOutOfRangeException("nameof(count)");
			}

			if ((count & 1) != 0) {
				// We always want an even number of bytes in input, see PeekBits
				buffer_ |= (uint)((buffer[offset++] & 0xff) << bitsInBuffer_);
				bitsInBuffer_ += 8;
			}

			window_ = buffer;
			windowStart_ = offset;
			windowEnd_ = end;
		}

		#region Instance Fields
		private byte[] window_;
		private int windowStart_;
		private int windowEnd_;

		private uint buffer_;
		private int bitsInBuffer_;
		#endregion
	}
}