zxing/csharp/common/BitSource.cs

/*
* Copyright 2008 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
{
    using System;
    using System.Text;

    /// <summary> A class which wraps a 2D array of bytes. The default usage is signed. If you want to use it as a
    /// unsigned container, it's up to you to do byteValue & 0xff at each location.
    /// *
    /// JAVAPORT: I'm not happy about the argument ordering throughout the file, as I always like to have
    /// the horizontal component first, but this is for compatibility with the C++ code. The original
    /// code was a 2D array of ints, but since it only ever gets assigned -1, 0, and 1, I'm going to use
    /// less memory and go with bytes.
    /// *
    /// </summary>
    /// <author>  dswitkin@google.com (Daniel Switkin)
    /// 
    /// </author>
    public sealed class BitSource
    { 
          private sbyte[] bytes;
          private int byteOffset;
          private int bitOffset;

          /**
           * @param 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.
           */
          public BitSource(sbyte[] bytes) {
            this.bytes = bytes;
          }

          /**
           * @param numBits number of bits to read
           * @return int representing the bits read. The bits will appear as the least-significant
           *         bits of the int
           * @throws IllegalArgumentException if numBits isn't in [1,32]
           */
          public int readBits(int numBits) {
            if (numBits < 1 || numBits > 32) {
              throw new Exception();
            }

            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;
          }

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