Source code

001/*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 * http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing,
013 * software distributed under the License is distributed on an
014 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015 * KIND, either express or implied.  See the License for the
016 * specific language governing permissions and limitations
017 * under the License.
018 */
019package org.apache.commons.compress.compressors.lz77support;
020
021/**
022 * Parameters of the {@link LZ77Compressor compressor}.
023 */
024public final class Parameters {
025    /**
026     * Builder for {@link Parameters} instances.
027     */
028    public static class Builder {
029        private final int windowSize;
030        private int minBackReferenceLength, maxBackReferenceLength, maxOffset, maxLiteralLength;
031        private Integer niceBackReferenceLength, maxCandidates, lazyThreshold;
032        private Boolean lazyMatches;
033
034        private Builder(final int windowSize) {
035            if (windowSize < 2 || !isPowerOfTwo(windowSize)) {
036                throw new IllegalArgumentException("windowSize must be a power of two");
037            }
038            this.windowSize = windowSize;
039            minBackReferenceLength = TRUE_MIN_BACK_REFERENCE_LENGTH;
040            maxBackReferenceLength = windowSize - 1;
041            maxOffset = windowSize - 1;
042            maxLiteralLength = windowSize;
043        }
044
045        /**
046         * Creates the {@link Parameters} instance.
047         *
048         * @return the configured {@link Parameters} instance.
049         */
050        public Parameters build() {
051            // default settings tuned for a compromise of good compression and acceptable speed
052            final int niceLen = niceBackReferenceLength != null ? niceBackReferenceLength : Math.max(minBackReferenceLength, maxBackReferenceLength / 2);
053            final int candidates = maxCandidates != null ? maxCandidates : Math.max(256, windowSize / 128);
054            final boolean lazy = lazyMatches == null || lazyMatches;
055            final int threshold = lazy ? lazyThreshold != null ? lazyThreshold : niceLen : minBackReferenceLength;
056
057            return new Parameters(windowSize, minBackReferenceLength, maxBackReferenceLength, maxOffset, maxLiteralLength, niceLen, candidates, lazy,
058                    threshold);
059        }
060
061        /**
062         * Changes the default setting for "nice back-reference length" and "maximum number of candidates" for improved compression ratio at the cost of
063         * compression speed.
064         *
065         * <p>
066         * Use this method after configuring "maximum back-reference length".
067         * </p>
068         *
069         * @return the builder
070         */
071        public Builder tunedForCompressionRatio() {
072            niceBackReferenceLength = lazyThreshold = maxBackReferenceLength;
073            maxCandidates = Math.max(32, windowSize / 16);
074            lazyMatches = true;
075            return this;
076        }
077
078        /**
079         * Changes the default setting for "nice back-reference length" and "maximum number of candidates" for improved compression speed at the cost of
080         * compression ratio.
081         *
082         * <p>
083         * Use this method after configuring "maximum back-reference length".
084         * </p>
085         *
086         * @return the builder
087         */
088        public Builder tunedForSpeed() {
089            niceBackReferenceLength = Math.max(minBackReferenceLength, maxBackReferenceLength / 8);
090            maxCandidates = Math.max(32, windowSize / 1024);
091            lazyMatches = false;
092            lazyThreshold = minBackReferenceLength;
093            return this;
094        }
095
096        /**
097         * Sets whether lazy matching should be performed.
098         *
099         * <p>
100         * Lazy matching means that after a back-reference for a certain position has been found the compressor will try to find a longer match for the next
101         * position.
102         * </p>
103         *
104         * <p>
105         * Lazy matching is enabled by default and disabled when tuning for speed.
106         * </p>
107         *
108         * @param lazy whether lazy matching should be performed
109         * @return the builder
110         */
111        public Builder withLazyMatching(final boolean lazy) {
112            lazyMatches = lazy;
113            return this;
114        }
115
116        /**
117         * Sets the threshold for lazy matching.
118         *
119         * <p>
120         * Even if lazy matching is enabled it will not be performed if the length of the back-reference found for the current position is longer than this
121         * value.
122         * </p>
123         *
124         * @param threshold the threshold for lazy matching
125         * @return the builder
126         */
127        public Builder withLazyThreshold(final int threshold) {
128            lazyThreshold = threshold;
129            return this;
130        }
131
132        /**
133         * Sets the maximal length of a back-reference.
134         *
135         * <p>
136         * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like
137         * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}.
138         * </p>
139         *
140         * @param maxBackReferenceLength maximal length of a back-reference found. A value smaller than {@code minBackReferenceLength} is interpreted as
141         *                               {@code minBackReferenceLength}. {@code maxBackReferenceLength} is capped at {@code windowSize - 1}.
142         * @return the builder
143         */
144        public Builder withMaxBackReferenceLength(final int maxBackReferenceLength) {
145            this.maxBackReferenceLength = maxBackReferenceLength < minBackReferenceLength ? minBackReferenceLength
146                    : Math.min(maxBackReferenceLength, windowSize - 1);
147            return this;
148        }
149
150        /**
151         * Sets the maximal length of a literal block.
152         *
153         * <p>
154         * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like
155         * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}.
156         * </p>
157         *
158         * @param maxLiteralLength maximal length of a literal block. Negative numbers and 0 as well as values bigger than {@code windowSize} are interpreted as
159         *                         {@code windowSize}.
160         * @return the builder
161         */
162        public Builder withMaxLiteralLength(final int maxLiteralLength) {
163            this.maxLiteralLength = maxLiteralLength < 1 ? windowSize : Math.min(maxLiteralLength, windowSize);
164            return this;
165        }
166
167        /**
168         * Sets the maximum number of back-reference candidates that should be consulted.
169         *
170         * <p>
171         * This settings can be used to tune the tradeoff between compression speed and compression ratio.
172         * </p>
173         *
174         * @param maxCandidates maximum number of back-reference candidates
175         * @return the builder
176         */
177        public Builder withMaxNumberOfCandidates(final int maxCandidates) {
178            this.maxCandidates = maxCandidates;
179            return this;
180        }
181
182        /**
183         * Sets the maximal offset of a back-reference.
184         *
185         * <p>
186         * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like
187         * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}.
188         * </p>
189         *
190         * @param maxOffset maximal offset of a back-reference. A non-positive value as well as values bigger than {@code windowSize - 1} are interpreted as
191         *                  <code>windowSize
192         * - 1</code>    .
193         * @return the builder
194         */
195        public Builder withMaxOffset(final int maxOffset) {
196            this.maxOffset = maxOffset < 1 ? windowSize - 1 : Math.min(maxOffset, windowSize - 1);
197            return this;
198        }
199
200        /**
201         * Sets the minimal length of a back-reference.
202         *
203         * <p>
204         * Ensures {@code maxBackReferenceLength} is not smaller than {@code minBackReferenceLength}.
205         *
206         * <p>
207         * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like
208         * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}.
209         * </p>
210         *
211         * @param minBackReferenceLength the minimal length of a back-reference found. A true minimum of 3 is hard-coded inside of this implementation but
212         *                               bigger lengths can be configured.
213         * @throws IllegalArgumentException if {@code windowSize} is smaller than {@code minBackReferenceLength}.
214         * @return the builder
215         */
216        public Builder withMinBackReferenceLength(final int minBackReferenceLength) {
217            this.minBackReferenceLength = Math.max(TRUE_MIN_BACK_REFERENCE_LENGTH, minBackReferenceLength);
218            if (windowSize < this.minBackReferenceLength) {
219                throw new IllegalArgumentException("minBackReferenceLength can't be bigger than windowSize");
220            }
221            if (maxBackReferenceLength < this.minBackReferenceLength) {
222                maxBackReferenceLength = this.minBackReferenceLength;
223            }
224            return this;
225        }
226
227        /**
228         * Sets the "nice length" of a back-reference.
229         *
230         * <p>
231         * When a back-references if this size has been found, stop searching for longer back-references.
232         * </p>
233         *
234         * <p>
235         * This settings can be used to tune the tradeoff between compression speed and compression ratio.
236         * </p>
237         *
238         * @param niceLen the "nice length" of a back-reference
239         * @return the builder
240         */
241        public Builder withNiceBackReferenceLength(final int niceLen) {
242            niceBackReferenceLength = niceLen;
243            return this;
244        }
245    }
246
247    /**
248     * The hard-coded absolute minimal length of a back-reference.
249     */
250    public static final int TRUE_MIN_BACK_REFERENCE_LENGTH = LZ77Compressor.NUMBER_OF_BYTES_IN_HASH;
251
252    /**
253     * Initializes the builder for the compressor's parameters with a {@code minBackReferenceLength} of 3 and {@code max*Length} equal to
254     * {@code windowSize - 1}.
255     *
256     * <p>
257     * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like
258     * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}.
259     * </p>
260     *
261     * @param windowSize the size of the sliding window - this determines the maximum offset a back-reference can take. Must be a power of two.
262     * @throws IllegalArgumentException if windowSize is not a power of two.
263     * @return a builder configured for the given window size
264     */
265    public static Builder builder(final int windowSize) {
266        return new Builder(windowSize);
267    }
268
269    private static boolean isPowerOfTwo(final int x) {
270        // pre-condition: x > 0
271        return (x & x - 1) == 0;
272    }
273
274    private final int windowSize, minBackReferenceLength, maxBackReferenceLength, maxOffset, maxLiteralLength, niceBackReferenceLength, maxCandidates,
275            lazyThreshold;
276
277    private final boolean lazyMatching;
278
279    private Parameters(final int windowSize, final int minBackReferenceLength, final int maxBackReferenceLength, final int maxOffset,
280            final int maxLiteralLength, final int niceBackReferenceLength, final int maxCandidates, final boolean lazyMatching, final int lazyThreshold) {
281        this.windowSize = windowSize;
282        this.minBackReferenceLength = minBackReferenceLength;
283        this.maxBackReferenceLength = maxBackReferenceLength;
284        this.maxOffset = maxOffset;
285        this.maxLiteralLength = maxLiteralLength;
286        this.niceBackReferenceLength = niceBackReferenceLength;
287        this.maxCandidates = maxCandidates;
288        this.lazyMatching = lazyMatching;
289        this.lazyThreshold = lazyThreshold;
290    }
291
292    /**
293     * Gets whether to perform lazy matching.
294     *
295     * @return whether to perform lazy matching
296     */
297    public boolean getLazyMatching() {
298        return lazyMatching;
299    }
300
301    /**
302     * Gets the threshold for lazy matching.
303     *
304     * @return the threshold for lazy matching
305     */
306    public int getLazyMatchingThreshold() {
307        return lazyThreshold;
308    }
309
310    /**
311     * Gets the maximal length of a back-reference found.
312     *
313     * @return the maximal length of a back-reference found
314     */
315    public int getMaxBackReferenceLength() {
316        return maxBackReferenceLength;
317    }
318
319    /**
320     * Gets the maximum number of back-reference candidates to consider.
321     *
322     * @return the maximum number of back-reference candidates to consider
323     */
324    public int getMaxCandidates() {
325        return maxCandidates;
326    }
327
328    /**
329     * Gets the maximal length of a literal block.
330     *
331     * @return the maximal length of a literal block
332     */
333    public int getMaxLiteralLength() {
334        return maxLiteralLength;
335    }
336
337    /**
338     * Gets the maximal offset of a back-reference found.
339     *
340     * @return the maximal offset of a back-reference found
341     */
342    public int getMaxOffset() {
343        return maxOffset;
344    }
345
346    /**
347     * Gets the minimal length of a back-reference found.
348     *
349     * @return the minimal length of a back-reference found
350     */
351    public int getMinBackReferenceLength() {
352        return minBackReferenceLength;
353    }
354
355    /**
356     * Gets the length of a back-reference that is considered nice enough to stop searching for longer ones.
357     *
358     * @return the length of a back-reference that is considered nice enough to stop searching
359     */
360    public int getNiceBackReferenceLength() {
361        return niceBackReferenceLength;
362    }
363
364    /**
365     * Gets the size of the sliding window - this determines the maximum offset a back-reference can take.
366     *
367     * @return the size of the sliding window
368     */
369    public int getWindowSize() {
370        return windowSize;
371    }
372}