shadowsocks-windows/shadowsocks-csharp/3rd/zxing/common/BitSource.cs

/*
* 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.
*/

using System;

namespace 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>
   /// 
   /// </summary>
   /// <author>  Sean Owen
   /// </author>
   /// <author>www.Redivivus.in (suraj.supekar@redivivus.in) - Ported from ZXING Java Source 
   /// </author>
   public sealed class BitSource
   {
      private readonly byte[] 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(byte[] bytes)
      {
         this.bytes = bytes;
      }

      /// <summary>
      /// index of next bit in current byte which would be read by the next call to {@link #readBits(int)}.
      /// </summary>
      public int BitOffset
      {
         get { return bitOffset; }
      }
  
      /// <summary>
      /// index of next byte in input byte array which would be read by the next call to {@link #readBits(int)}.
      /// </summary>
      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="ArgumentException">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 ArgumentException(numBits.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;
      }
   }
}