1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one 3 * or more contributor license agreements. See the NOTICE file 4 * distributed with this work for additional information 5 * regarding copyright ownership. The ASF licenses this file 6 * to you under the Apache License, Version 2.0 (the 7 * "License"); you may not use this file except in compliance 8 * with the License. You may obtain a copy of the License at 9 * 10 * https://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, 13 * software distributed under the License is distributed on an 14 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY 15 * KIND, either express or implied. See the License for the 16 * specific language governing permissions and limitations 17 * under the License. 18 */ 19 package org.apache.commons.compress.compressors.lz77support; 20 21 /** 22 * Parameters of the {@link LZ77Compressor compressor}. 23 */ 24 public final class Parameters { 25 26 /** 27 * Builder for {@link Parameters} instances. 28 */ 29 public static class Builder { 30 31 private final int windowSize; 32 private int minBackReferenceLength; 33 private int maxBackReferenceLength; 34 private int maxOffset; 35 private int maxLiteralLength; 36 private Integer niceBackReferenceLength; 37 private Integer maxCandidates; 38 private Integer lazyThreshold; 39 private Boolean lazyMatches; 40 41 private Builder(final int windowSize) { 42 if (windowSize < 2 || !isPowerOfTwo(windowSize)) { 43 throw new IllegalArgumentException("windowSize must be a power of two"); 44 } 45 this.windowSize = windowSize; 46 minBackReferenceLength = TRUE_MIN_BACK_REFERENCE_LENGTH; 47 maxBackReferenceLength = windowSize - 1; 48 maxOffset = windowSize - 1; 49 maxLiteralLength = windowSize; 50 } 51 52 /** 53 * Creates the {@link Parameters} instance. 54 * 55 * @return the configured {@link Parameters} instance. 56 */ 57 public Parameters build() { 58 // default settings tuned for a compromise of good compression and acceptable speed 59 final int niceLen = niceBackReferenceLength != null ? niceBackReferenceLength : Math.max(minBackReferenceLength, maxBackReferenceLength / 2); 60 final int candidates = maxCandidates != null ? maxCandidates : Math.max(256, windowSize / 128); 61 final boolean lazy = lazyMatches == null || lazyMatches; 62 final int threshold = lazy ? lazyThreshold != null ? lazyThreshold : niceLen : minBackReferenceLength; 63 64 return new Parameters(windowSize, minBackReferenceLength, maxBackReferenceLength, maxOffset, maxLiteralLength, niceLen, candidates, lazy, 65 threshold); 66 } 67 68 /** 69 * Changes the default setting for "nice back-reference length" and "maximum number of candidates" for improved compression ratio at the cost of 70 * compression speed. 71 * <p> 72 * Use this method after configuring "maximum back-reference length". 73 * </p> 74 * 75 * @return the builder 76 */ 77 public Builder tunedForCompressionRatio() { 78 niceBackReferenceLength = lazyThreshold = maxBackReferenceLength; 79 maxCandidates = Math.max(32, windowSize / 16); 80 lazyMatches = true; 81 return this; 82 } 83 84 /** 85 * Changes the default setting for "nice back-reference length" and "maximum number of candidates" for improved compression speed at the cost of 86 * compression ratio. 87 * <p> 88 * Use this method after configuring "maximum back-reference length". 89 * </p> 90 * 91 * @return the builder 92 */ 93 public Builder tunedForSpeed() { 94 niceBackReferenceLength = Math.max(minBackReferenceLength, maxBackReferenceLength / 8); 95 maxCandidates = Math.max(32, windowSize / 1024); 96 lazyMatches = false; 97 lazyThreshold = minBackReferenceLength; 98 return this; 99 } 100 101 /** 102 * Sets whether lazy matching should be performed. 103 * <p> 104 * 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 105 * position. 106 * </p> 107 * <p> 108 * Lazy matching is enabled by default and disabled when tuning for speed. 109 * </p> 110 * 111 * @param lazy whether lazy matching should be performed 112 * @return the builder 113 */ 114 public Builder withLazyMatching(final boolean lazy) { 115 lazyMatches = lazy; 116 return this; 117 } 118 119 /** 120 * Sets the threshold for lazy matching. 121 * <p> 122 * 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 123 * value. 124 * </p> 125 * 126 * @param threshold the threshold for lazy matching 127 * @return the builder 128 */ 129 public Builder withLazyThreshold(final int threshold) { 130 lazyThreshold = threshold; 131 return this; 132 } 133 134 /** 135 * Sets the maximal length of a back-reference. 136 * <p> 137 * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like 138 * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}. 139 * </p> 140 * 141 * @param maxBackReferenceLength maximal length of a back-reference found. A value smaller than {@code minBackReferenceLength} is interpreted as 142 * {@code minBackReferenceLength}. {@code maxBackReferenceLength} is capped at {@code windowSize - 1}. 143 * @return the builder 144 */ 145 public Builder withMaxBackReferenceLength(final int maxBackReferenceLength) { 146 this.maxBackReferenceLength = maxBackReferenceLength < minBackReferenceLength ? minBackReferenceLength 147 : Math.min(maxBackReferenceLength, windowSize - 1); 148 return this; 149 } 150 151 /** 152 * Sets the maximal length of a literal block. 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 * <p> 170 * This settings can be used to tune the tradeoff between compression speed and compression ratio. 171 * </p> 172 * 173 * @param maxCandidates maximum number of back-reference candidates 174 * @return the builder 175 */ 176 public Builder withMaxNumberOfCandidates(final int maxCandidates) { 177 this.maxCandidates = maxCandidates; 178 return this; 179 } 180 181 /** 182 * Sets the maximal offset of a back-reference. 183 * <p> 184 * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like 185 * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}. 186 * </p> 187 * 188 * @param maxOffset maximal offset of a back-reference. A non-positive value as well as values bigger than {@code windowSize - 1} are interpreted as 189 * {@code windowSize * - 1}. 190 * @return the builder 191 */ 192 public Builder withMaxOffset(final int maxOffset) { 193 this.maxOffset = maxOffset < 1 ? windowSize - 1 : Math.min(maxOffset, windowSize - 1); 194 return this; 195 } 196 197 /** 198 * Sets the minimal length of a back-reference. 199 * <p> 200 * Ensures {@code maxBackReferenceLength} is not smaller than {@code minBackReferenceLength}. 201 * </p> 202 * <p> 203 * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like 204 * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}. 205 * </p> 206 * 207 * @param minBackReferenceLength the minimal length of a back-reference found. A true minimum of 3 is hard-coded inside of this implementation but 208 * bigger lengths can be configured. 209 * @throws IllegalArgumentException if {@code windowSize} is smaller than {@code minBackReferenceLength}. 210 * @return the builder 211 */ 212 public Builder withMinBackReferenceLength(final int minBackReferenceLength) { 213 this.minBackReferenceLength = Math.max(TRUE_MIN_BACK_REFERENCE_LENGTH, minBackReferenceLength); 214 if (windowSize < this.minBackReferenceLength) { 215 throw new IllegalArgumentException("minBackReferenceLength can't be bigger than windowSize"); 216 } 217 if (maxBackReferenceLength < this.minBackReferenceLength) { 218 maxBackReferenceLength = this.minBackReferenceLength; 219 } 220 return this; 221 } 222 223 /** 224 * Sets the "nice length" of a back-reference. 225 * <p> 226 * When a back-references if this size has been found, stop searching for longer back-references. 227 * </p> 228 * <p> 229 * This settings can be used to tune the tradeoff between compression speed and compression ratio. 230 * </p> 231 * 232 * @param niceLen the "nice length" of a back-reference 233 * @return the builder 234 */ 235 public Builder withNiceBackReferenceLength(final int niceLen) { 236 niceBackReferenceLength = niceLen; 237 return this; 238 } 239 } 240 241 /** 242 * The hard-coded absolute minimal length of a back-reference. 243 */ 244 public static final int TRUE_MIN_BACK_REFERENCE_LENGTH = LZ77Compressor.NUMBER_OF_BYTES_IN_HASH; 245 246 /** 247 * Initializes the builder for the compressor's parameters with a {@code minBackReferenceLength} of 3 and {@code max*Length} equal to 248 * {@code windowSize - 1}. 249 * <p> 250 * It is recommended to not use this method directly but rather tune a pre-configured builder created by a format specific factory like 251 * {@link org.apache.commons.compress.compressors.snappy.SnappyCompressorOutputStream#createParameterBuilder}. 252 * </p> 253 * 254 * @param windowSize the size of the sliding window - this determines the maximum offset a back-reference can take. Must be a power of two. 255 * @throws IllegalArgumentException if windowSize is not a power of two. 256 * @return a builder configured for the given window size 257 */ 258 public static Builder builder(final int windowSize) { 259 return new Builder(windowSize); 260 } 261 262 private static boolean isPowerOfTwo(final int x) { 263 // pre-condition: x > 0 264 return (x & x - 1) == 0; 265 } 266 267 private final int windowSize; 268 private final int minBackReferenceLength; 269 private final int maxBackReferenceLength; 270 private final int maxOffset; 271 private final int maxLiteralLength; 272 private final int niceBackReferenceLength; 273 private final int maxCandidates; 274 private final int lazyThreshold; 275 276 private final boolean lazyMatching; 277 278 private Parameters(final int windowSize, final int minBackReferenceLength, final int maxBackReferenceLength, final int maxOffset, 279 final int maxLiteralLength, final int niceBackReferenceLength, final int maxCandidates, final boolean lazyMatching, final int lazyThreshold) { 280 this.windowSize = windowSize; 281 this.minBackReferenceLength = minBackReferenceLength; 282 this.maxBackReferenceLength = maxBackReferenceLength; 283 this.maxOffset = maxOffset; 284 this.maxLiteralLength = maxLiteralLength; 285 this.niceBackReferenceLength = niceBackReferenceLength; 286 this.maxCandidates = maxCandidates; 287 this.lazyMatching = lazyMatching; 288 this.lazyThreshold = lazyThreshold; 289 } 290 291 /** 292 * Gets whether to perform lazy matching. 293 * 294 * @return whether to perform lazy matching 295 */ 296 public boolean getLazyMatching() { 297 return lazyMatching; 298 } 299 300 /** 301 * Gets the threshold for lazy matching. 302 * 303 * @return the threshold for lazy matching 304 */ 305 public int getLazyMatchingThreshold() { 306 return lazyThreshold; 307 } 308 309 /** 310 * Gets the maximal length of a back-reference found. 311 * 312 * @return the maximal length of a back-reference found 313 */ 314 public int getMaxBackReferenceLength() { 315 return maxBackReferenceLength; 316 } 317 318 /** 319 * Gets the maximum number of back-reference candidates to consider. 320 * 321 * @return the maximum number of back-reference candidates to consider 322 */ 323 public int getMaxCandidates() { 324 return maxCandidates; 325 } 326 327 /** 328 * Gets the maximal length of a literal block. 329 * 330 * @return the maximal length of a literal block 331 */ 332 public int getMaxLiteralLength() { 333 return maxLiteralLength; 334 } 335 336 /** 337 * Gets the maximal offset of a back-reference found. 338 * 339 * @return the maximal offset of a back-reference found 340 */ 341 public int getMaxOffset() { 342 return maxOffset; 343 } 344 345 /** 346 * Gets the minimal length of a back-reference found. 347 * 348 * @return the minimal length of a back-reference found 349 */ 350 public int getMinBackReferenceLength() { 351 return minBackReferenceLength; 352 } 353 354 /** 355 * Gets the length of a back-reference that is considered nice enough to stop searching for longer ones. 356 * 357 * @return the length of a back-reference that is considered nice enough to stop searching 358 */ 359 public int getNiceBackReferenceLength() { 360 return niceBackReferenceLength; 361 } 362 363 /** 364 * Gets the size of the sliding window - this determines the maximum offset a back-reference can take. 365 * 366 * @return the size of the sliding window 367 */ 368 public int getWindowSize() { 369 return windowSize; 370 } 371 }