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comparison: sources/huffman/bitwriter.cpp

sources/huffman/bitwriter.cpp

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1 /*
2 * skulltag::BitWriter class - Enables writing arbitrary bit lengths of data.
3 *
4 * Copyright 2009 Timothy Landers
5 * email: code.vortexcortex@gmail.com
6 *
7 * Permission is hereby granted, free of charge, to any person obtaining a copy
8 * of this software and associated documentation files (the "Software"), to deal
9 * in the Software without restriction, including without limitation the rights
10 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
11 * copies of the Software, and to permit persons to whom the Software is
12 * furnished to do so, subject to the following conditions:
13 *
14 * The above copyright notice and this permission notice shall be included in
15 * all copies or substantial portions of the Software.
16 *
17 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
18 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
19 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
20 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
21 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
22 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
23 * THE SOFTWARE.
24 */
25
26 #include "bitwriter.h"
27
28 /* The internal buffer of bits is an int which is initiallized to zero.
29 * The Bit Buffer: [00000000000000000000000000000000] - 0 bits stored.
30 *
31 * Bits stored in the bit buffer occupy the most significant bits available.
32 * When bits are stored their order is preserved.
33 * Storing the 5 bits 01011 into the bit buffer would result in:
34 * The Bit Buffer: [01011000000000000000000000000000] - 5 bits stored.
35 *
36 * Additionally storing 0x12f3 (13 bits) would result in:
37 * The Bit Buffer: [01011100101111001100000000000000] - 18 bits stored.
38 *
39 * Data is stored via any of the put(...) functions.
40 *
41 * To retrieve bytes (chars) of data from the bit buffer and empty the bit buffer: flush();
42 * Flushing the bit buffer takes groups of 8 bits (octets) and stores them in the
43 * output buffer at the current byte position. Calling flush() would cause output
44 * to receive 2 bytes (0x5c, 0xbc) == (01011100, 10111100) and the remaining bits would
45 * be moved to the most significant bit positions.
46 * The Bit Buffer: [11000000000000000000000000000000] - 2 bits stored.
47 * Note: The empty bits of the bit buffer must be zero to avoid additional masking operations.
48 * flush() is called automatically if the bit buffer is too full during a call to put(...)
49 *
50 * When the bit storage operation is complete call finish(...). Finish will flush as many full
51 * bytes of data into the output buffer as possible, the remaining bits will be padded out
52 * to a full octet with zeros (on their less significant side), then written to the output
53 * buffer. finish( bytesStored, padding ) passes by reference the number of bytes stored
54 * and number of padding bits added (0 to 7) to the last stored byte.
55 *
56 * Bits are not added one at a time, they are masked, shifted and bitwise OR'ed into
57 * the buffer in as large a group as possible. This allows the BitWriter to add multiple bits in a single
58 * operation instead of calling a function for each 1 bit added.
59 *
60 * Normal usage:
61 *
62 * char * dataArray = new char[ max_output_size ];
63 * BitWriter * buffer = new BitWriter( dataArray, max_output_size );
64 * ...
65 * // various calls to put(...);
66 * ...
67 * int numBytesOutput, paddedBits;
68 * buffer->finish( numBytesOutput, paddedBits );
69 * ...
70 * // do something with dataArray
71 * ...
72 * delete buffer;
73 * delete dataArray;
74 *
75 * */
76
77 /** Prevents naming convention problems via encapsulation. */
78 namespace skulltag {
79
80 // Static variable initially set to zero as a signal for init()
81 int BitWriter::intSize = 0;
82 int BitWriter::mask[32];
83
84 /** Initializes this BitWriter. */
85 void BitWriter::init(){
86
87 // initialize static variables if not initialized yet.
88 if ( intSize == 0 ){
89 intSize = sizeof maximumBytes;
90 mask[0] = 0;
91
92 // fill mask such that m = { 0, 1, 3, 7, 15, etc. }
93 for ( int i = 1; i < 32; i++ ) mask[i] = (mask[i-1] << 1) | 1;
94 }
95
96 // initialize member variables.
97 bitsAvailable = 0;
98 bytesAvailable = 0;
99 bufferBits = 0;
100 currentByte = 0;
101 maximumBytes = 0;
102 bitsLeft = intSize << 3;
103 }
104
105 /** Creates a new BitWriter. */
106 BitWriter::BitWriter(){
107 init();
108 }
109
110 /** Creates a new BitWriter.
111 * @param output Destination that bits will be written to.
112 * @param max Maximum number of chars to output. */
113 BitWriter::BitWriter( unsigned char * const output, int const &max ){
114 outputBuffer( output, max );
115 }
116
117 /** Sets the output buffer that bytes will be written to.
118 * @param output Destination that bits will be written to.
119 * @param max Maximum number of chars to output.
120 * @return true if successful, false otherwise. */
121 bool BitWriter::outputBuffer( unsigned char * const output, int const &max ){
122 init(); // zero the vars.
123 currentByte = output;
124 if ( output == 0 ) return false;
125 if ( max < 1 ) return false;
126 bytesAvailable = max;
127 bitsAvailable = max << 3;
128 maximumBytes = max;
129 return true;
130 }
131
132 /** Appends a char worth of bits to the buffer.
133 * @param bits the bits to append.
134 * @return true if successful, false otherwise. */
135 bool BitWriter::put( unsigned char const &bits ){
136 return put( (int)bits, 8 );
137 }
138
139 /** Appends a short worth of bits to the buffer.
140 * @param bits the bits to append.
141 * @return true if successful, false otherwise. */
142 bool BitWriter::put( short const &bits ){
143 static int shortBitSize = (sizeof bits) << 3;
144 return put( (int)bits, shortBitSize );
145 }
146 /** Appends an int worth of bits to the buffer.
147 * @param bits the bits to append.
148 * @return true if successful, false otherwise. */
149 bool BitWriter::put( int const &bits ){
150 static int intBitSize = intSize << 3;
151 return put( bits, intBitSize);
152 }
153
154 /** Appends multiple chars from a buffer to this BitWriter.
155 * @param inputBuffer pointer to char data
156 * @param count number of chars to read
157 * @return true if successful, false otherwise. */
158 bool BitWriter::put( unsigned char const * const inputBuffer, int count ){
159 int i = 0;
160
161 // Read in 4 bytes at a time and send all at once to the bit buffer.
162 while ( (i + 3) < count ){
163 if ( !put(
164 ((int)inputBuffer[ i ] << 24) |
165 ((int)inputBuffer[i+1] << 16) |
166 ((int)inputBuffer[i+2] << 8) |
167 (int)inputBuffer[i+3], 32
168 ) ) return false;
169 i+=4;
170 }
171
172 // If any bytes remain, output them one at a time.
173 while ( i < count ){
174 if ( !put( (int)inputBuffer[ i ], 8 ) ) return false;
175 i++;
176 }
177
178 return true;
179 }
180
181 /** Appends a specified number of bits from an int to the buffer.
182 * @param bits the bits to append. <br>
183 * The bits should be stored in the least significant portion of the int.
184 * @param count the number of bits to append.
185 * @return true if successful, false otherwise. */
186 bool BitWriter::put( int const &bits, int count ){
187 if ( count > bitsAvailable ) return false;
188 if ( (bitsLeft < 1) && (!flush()) ) return false;
189 if ( count > bitsLeft ){
190 // not enough space in buffer, fill buffer with top end of input bits then flush.
191 bufferBits |= mask[bitsLeft] & (bits >> (count - bitsLeft));
192 count -= bitsLeft;
193 bitsAvailable -= bitsLeft;
194 bitsLeft = 0;
195
196 // Buffer's full, needs flushing.
197 if (!flush()) return false;
198 }
199
200 // if there are still bits of input...
201 if ( count > 0 ){
202
203 // shift the input bits up to the end of the bit buffer.
204 bufferBits |= (mask[count] & bits) << (bitsLeft - count);
205 bitsAvailable -= count;
206 bitsLeft -= count;
207 }
208 return true;
209 }
210
211 /** Writes any full chars of data stored in this BitWriter to the output char buffer.
212 * @return true if successful, false if an error occurs. */
213 bool BitWriter::flush(){
214 // static var to hold how many bits are in an int.
215 static int intBitSize = intSize << 3;
216 if ( currentByte == 0 ) return false;
217 int numBits = intBitSize - bitsLeft;
218
219 // while there's at least one octet of data in the buffer.
220 while ( numBits > 7 ){
221
222 // fail if no bytes can be written.
223 if ( bytesAvailable <= 0 ) return false;
224
225 // get a byte off the top end of the buffer.
226 *currentByte = (bufferBits >> (intBitSize - 8)) & mask[8];
227
228 // Set variables to reflect the change.
229 currentByte++;
230 bytesAvailable--;
231 bufferBits = bufferBits << 8;
232 bitsLeft += 8;
233 numBits -= 8;
234 }
235 return true;
236 }
237
238 /** Flushes this BitWriter then outputs any partial chars by padding them with zeros. <br>
239 * After calling finish() all other calls to update the BitWriter will fail until a buffer is set via outputBuffer().
240 * @param bytesWritten out: the number of bytes written to the output buffer.
241 * @param paddingBits out: the number of padding bits used in the final byte of output.
242 * @return true if successful, false if an error occurs. */
243 bool BitWriter::finish( int &bytesWritten, int &paddingBits ){
244 static int intBitSize = intSize << 3;
245 // set meaningful return values even if flush() fails.
246 bytesWritten = maximumBytes - bytesAvailable;
247 paddingBits = 0;
248 if ( flush() ){
249 // use a temp var to avoid setting paddingBits to invalid value on failure.
250 int pad = (8 - (intBitSize - bitsLeft)) & 7;
251 if ( pad > 0 ){
252 // all empty bits should be zero. Artificially extend by the number of bits needed.
253 bitsLeft -= pad;
254 if ( !flush() ){
255 // Prevent futher use even on failure.
256 init();
257 return false;
258 }
259 // return the temp bit padding value.
260 paddingBits = pad;
261 }
262 bytesWritten = maximumBytes - bytesAvailable;
263 init(); // set initial state -- no further writing can occur.
264 return true;
265 }
266 // Prevents futher use even on failure.
267 init();
268 return false;
269 }
270
271 }

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