Claudenw commented on a change in pull request #258: URL: https://github.com/apache/commons-collections/pull/258#discussion_r802919483
########## File path: src/main/java/org/apache/commons/collections4/bloomfilter/hasher/SimpleHasher.java ########## @@ -0,0 +1,217 @@ +/* + * Licensed to the Apache Software Foundation (ASF) under one or more + * contributor license agreements. See the NOTICE file distributed with + * this work for additional information regarding copyright ownership. + * The ASF licenses this file to You 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. + */ +package org.apache.commons.collections4.bloomfilter.hasher; + +import java.util.Objects; +import java.util.function.IntPredicate; + +import org.apache.commons.collections4.bloomfilter.IndexProducer; +import org.apache.commons.collections4.bloomfilter.Shape; +import org.apache.commons.collections4.bloomfilter.hasher.filter.Filter; + +/** + * A Hasher that implements combinatorial hashing as as described by + * <a href="https://www.eecs.harvard.edu/~michaelm/postscripts/tr-02-05.pdf";>Krisch amd Mitzenmacher</a>. + * <p> + * Common use for this hasher is to generate a byte array as the output of a hashing + * or MessageDigest algorithm.</p> + * + * @since 4.5 + */ +public final class SimpleHasher implements Hasher { + /** + * A default increment used when the requested increment is zero. This is the same + * default increment used in Java's SplittableRandom random number generator. It is the + * fractional representation of the golden ratio (0.618...) with a base of 2^64. + */ + public static final long DEFAULT_INCREMENT = 0x9e3779b97f4a7c15L; + + /** + * This mask is used to obtain the value of an int as if it were unsigned. + */ + private static final long LONG_MASK = 0xffffffffL; + + /** + * The initial hash value. + */ + private final long initial; + + /** + * The value to increment the hash value by. + */ + private final long increment; + + /** + * Convert bytes to long. + * @param byteArray the byte array to extract the values from. + * @param offset the offset to start extraction from. + * @param len the length of the extraction, may be longer than 8. + * @return + */ + private static long toLong(byte[] byteArray, int offset, int len) { + long val = 0; + len = Math.min(len, Long.BYTES); + for (int i = 0; i < len; i++) { + val <<= 8; + val |= (byteArray[offset + i] & 0x00FF); + } + return val; + } + + /** + * Constructs the SimpleHasher from a byte array. + * <p>The byte array is split in 2 and each half is interpreted as a long value. + * Excess bytes are ignored. This simplifies the conversion from a Digest or hasher algorithm output + * to the two values used by the SimpleHasher.</p> + * <p><em>If the second long is zero the DEFAULT_INCREMENT is used instead.</em></p> + * @param buffer the buffer to extract the longs from. + * @throws IllegalArgumentException is buffer length is zero. + */ + public SimpleHasher(byte[] buffer) { + if (buffer.length == 0) { + throw new IllegalArgumentException("buffer length must be greater than 0"); + } + int segment = buffer.length / 2; + this.initial = toLong(buffer, 0, segment); + long possibleIncrement = toLong(buffer, segment, buffer.length - segment); + this.increment = possibleIncrement == 0 ? DEFAULT_INCREMENT : possibleIncrement; + } + + /** + * Constructs the SimpleHasher from 2 longs. The long values will be interpreted as unsigned values. + * <p><em>If the increment is zero the DEFAULT_INCREMENT is used instead.</em></p> + * @param initial The initial value for the hasher. + * @param increment The value to increment the hash by on each iteration. + */ + public SimpleHasher(long initial, long increment) { + this.initial = initial; + this.increment = increment == 0 ? DEFAULT_INCREMENT : increment; + } + + /** + * This method divides a long < 0 as an unsigned long and returns the remainder. + * + * @param dividend the number to divide. + * @param divisor the divisor + * @return the remainder + */ + private static int remainder(long dividend, int divisor) { + long longDivisor = divisor & LONG_MASK; + long remainder; + long quotient; + if (divisor == 1) { + return 0; + } + + // Approximate the quotient and remainder + quotient = (dividend >>> 1) / (longDivisor >>> 1); + remainder = dividend - quotient * longDivisor; + + // Correct the approximation + while (remainder < 0) { + remainder += longDivisor; + quotient--; + } + while (remainder >= longDivisor) { + remainder -= longDivisor; + quotient++; + } + return (int) remainder; + } + + /** + * Calculates the modulus of an unsigned long value and an integer divisor. + * @param dividend the unsigned long value to divide. + * @param divisor the divisor + * @return the remainder. + */ + static int mod(long dividend, int divisor) { Review comment: The goal here is to implement a Hasher based on Krisch and Mitzenmacher's [Building a Better Bloom Filter](https://www.eecs.harvard.edu/~michaelm/postscripts/tr-02-05.pdf). They do not discuss signed -vs- unsigned calculations so we could switch to signed longs. In this case the `mod(long,int)` function would be: ``` int mod(long dividend, int divisor) { int result = dividend % divisor; return result<0? result+divisor : result; } ``` Since the divisor is known to be positive this should work. Is it fast enough? for filtering I would like to add a method `uniqueIndices( Shape shape )` that applies a filter to the IndexProducer. This will allow proper filtering to be applied in the HashingCollection. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
