zxing/csharp/common/BitSource.cs
2013-01-18 20:14:03 +00:00

124 lines
3.5 KiB
C#
Executable file

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;
}
}
}