zxing/csharp/common/BitSource.cs

using System;

/*
 * Copyright 2007 ZXing authors
 *
 * 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 com.google.zxing.common
{

	/// <summary>
	/// <p>This provides an easy abstraction to read bits at a time from a sequence of bytes, where the
	/// number of bits read is not often a multiple of 8.</p>
	/// 
	/// <p>This class is thread-safe but not reentrant -- unless the caller modifies the bytes array
	/// it passed in, in which case all bets are off.</p>
	/// 
	/// @author Sean Owen
	/// </summary>
	public sealed class BitSource
	{

	  private readonly sbyte[] bytes;
	  private int byteOffset;
	  private int bitOffset;

	  /// <param name="bytes"> bytes from which this will read bits. Bits will be read from the first byte first.
	  /// Bits are read within a byte from most-significant to least-significant bit. </param>
	  public BitSource(sbyte[] bytes)
	  {
		this.bytes = bytes;
	  }

	  /// <returns> index of next bit in current byte which would be read by the next call to <seealso cref="#readBits(int)"/>. </returns>
	  public int BitOffset
	  {
		  get
		  {
			return bitOffset;
		  }
	  }

	  /// <returns> index of next byte in input byte array which would be read by the next call to <seealso cref="#readBits(int)"/>. </returns>
	  public int ByteOffset
	  {
		  get
		  {
			return byteOffset;
		  }
	  }

	  /// <param name="numBits"> number of bits to read </param>
	  /// <returns> int representing the bits read. The bits will appear as the least-significant
	  ///         bits of the int </returns>
	  /// <exception cref="IllegalArgumentException"> if numBits isn't in [1,32] or more than is available </exception>
	  public int readBits(int numBits)
	  {
		if (numBits < 1 || numBits > 32 || numBits > available())
		{
		  throw new System.ArgumentException(Convert.ToString(numBits));
		}

		int result = 0;

		// First, read remainder from current byte
		if (bitOffset > 0)
		{
		  int bitsLeft = 8 - bitOffset;
		  int toRead = numBits < bitsLeft ? numBits : bitsLeft;
		  int bitsToNotRead = bitsLeft - toRead;
		  int mask = (0xFF >> (8 - toRead)) << bitsToNotRead;
		  result = (bytes[byteOffset] & mask) >> bitsToNotRead;
		  numBits -= toRead;
		  bitOffset += toRead;
		  if (bitOffset == 8)
		  {
			bitOffset = 0;
			byteOffset++;
		  }
		}

		// Next read whole bytes
		if (numBits > 0)
		{
		  while (numBits >= 8)
		  {
			result = (result << 8) | (bytes[byteOffset] & 0xFF);
			byteOffset++;
			numBits -= 8;
		  }

		  // Finally read a partial byte
		  if (numBits > 0)
		  {
			int bitsToNotRead = 8 - numBits;
			int mask = (0xFF >> bitsToNotRead) << bitsToNotRead;
			result = (result << numBits) | ((bytes[byteOffset] & mask) >> bitsToNotRead);
			bitOffset += numBits;
		  }
		}

		return result;
	  }

	  /// <returns> number of bits that can be read successfully </returns>
	  public int available()
	  {
		return 8 * (bytes.Length - byteOffset) - bitOffset;
	  }

	}

}