sources/huffman/bitreader.cpp

/*
 * skulltag::BitReader class - Allows reading arbitrary bit lengths of data.
 * Version 1 - Revsion 0
 *
 * Copyright 2009 Timothy Landers
 * email: code.vortexcortex@gmail.com
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include "bitreader.h"
/** Prevents naming convention problems via encapsulation. */
namespace skulltag {
	// BitReader class implementation
	
	int BitReader::intBitSize = 0;
	int BitReader::intSize = 0;
	int BitReader::mask[32] = {0};

	/** Creates a new BitReader. */
	BitReader::BitReader(){
		init();
	}
	
	/** Creates a new BitReader.
	 * @param input		Source of data that bits will be read from.
	 * @param max		Maximum number of chars to read. */
	BitReader::BitReader( unsigned char const * input, int const &max ){
		inputBuffer( input, max );
	}
		
	/** Sets the input buffer that bytes will be read from.
	 * @param input		Source of data that bits will be read from.
	 * @param max		Maximum number of chars to input.
	 * @return true if successful, false if an error occurs. */
	bool BitReader::inputBuffer( unsigned char const * input, int const &max ){
		init(); // zero the vars.
		currentByte = input;
		if ( input == 0 ) return false;
		if ( max < 1 ) return false;
		bytesAvailable = max;
		bitsAvailable = max << 3;
		maximumBytes = max;
		return true;
	}

	/** Initializes this BitReader */
	void BitReader::init(){
		// initialize static variables if not initialized yet.
		if ( intSize == 0 ){
			intSize = sizeof maximumBytes;
			mask[0] = 0;

			// fill mask such that m = { 0, 1, 3, 7, 15, etc. }
			for ( int i = 1; i < 32; i++ ) mask[i] = (mask[i-1] << 1) | 1;
			
			intBitSize = intSize << 3;
		}

		// initialize member variables.
		bitsAvailable =		0;
		bytesAvailable =	0;
		bufferBits =		0;
		currentByte =		0;
		maximumBytes =		0;
		bitsUsed =			0;
	}
	
	/** Fills the internal bit buffer.
	 * @return true if successful, false if an error occurs. */
	bool BitReader::fill(){
		if ( (currentByte == 0) || (bytesAvailable <= 0) ) return false;

		// while there's at least one octet free in the buffer, and one byte to read.
		while ( (bitsUsed < (intBitSize - 8)) && (bytesAvailable > 0) ){

			// put a byte into the bottom end of the buffer.
			bufferBits |= (*currentByte & mask[8]) << (intBitSize - bitsUsed - 8);

			// Set variables to reflect the change.
			currentByte++;
			bytesAvailable--;
			bitsUsed += 8;
		}
		return true;
	}
	
	/** Fetches a specified number of bits and stores them in an int.
	 * @param bits 		destination of the retrieved bits. <br>
	 *		The bits will be stored in the least significant portion of the int.
	 * @param count		the number of bits to fetch.
	 * @return the number of bits read -- may not be equal to the amount requested. */
	int BitReader::get( int &bits, int const &count ){
		bits = 0;
		// Requesting more bits than are available.
		if ( count > bitsAvailable ) return 0;
		if ( (count > bitsUsed) && (!fill()) ) return 0;
		bits = (bufferBits >> (intBitSize - count)) & mask[count];
		// lesser of bits in buffer or requested bits.
		int got = (bitsUsed < count) ? bitsUsed : count;
		// get as many bits from the buffer as we can.
		if ( got > 0 ){
			bufferBits <<= got;
			bitsUsed -= got;
			bitsAvailable -= got;
		}
		// if more bits are requested.
		if ( count > got ){
			if (!fill()) {
				bits = (bits >> (count - got)) & mask[count - got];
				return got;
			}
			got = count - got;
			// avoid reading more bits than available.
			if ( got <= bitsAvailable ){
				bits |= (bufferBits >> (intBitSize - got)) & mask[got];
				bufferBits <<= got;
				bitsUsed -= got;
				bitsAvailable -= got;
			}
		}
		return count;
	}
	
	/** @return Amount of bits that can be read from this BitReader. */
	int BitReader::availableBits(){ return bitsAvailable; }
	
} // end namespace skulltag