Claudenw commented on code in PR #258: URL: https://github.com/apache/commons-collections/pull/258#discussion_r897710096
########## src/main/java/org/apache/commons/collections4/bloomfilter/SimpleHasher.java: ########## @@ -0,0 +1,196 @@ +/* + * 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; + +import java.util.Objects; +import java.util.function.IntPredicate; + +/** + * A Hasher that implements combinatorial hashing as as described by + * <a href="https://www.eecs.harvard.edu/~michaelm/postscripts/tr-02-05.pdf";>Krisch and 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 class SimpleHasher implements Hasher { + + /** + * 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. + * @see #getDefaultIncrement() + */ + 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 ? getDefaultIncrement() : 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. + * @see #getDefaultIncrement() + */ + public SimpleHasher(long initial, long increment) { + this.initial = initial; + this.increment = increment == 0 ? getDefaultIncrement() : increment; + } + + /** + * Get the default increment used when the requested increment is zero. + * <p> + * By default 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. + * </p><p> + * Implementations may want to override this value to match defaults in legacy implementations. + * </p> + * @return The default increment to use when the requested increment is zero. + */ + public long getDefaultIncrement() { + return 0x9e3779b97f4a7c15L; + } + + /** + * Performs a modulus calculation on an unsigned long and an integer divisor. + * @param dividend a unsigned long value to calculate the modulus of. + * @param divisor the divisor for the modulus calculation. + * @return the remainder or modulus value. + */ + static int mod(long dividend, int divisor) { + // See Hacker's Delight (2nd ed), section 9.3. + // Assume divisor is positive. + // Divide half the unsigned number and then double the quotient result. + final long quotient = ((dividend >>> 1) / divisor) << 1; + final long remainder = dividend - quotient * divisor; + // remainder in [0, 2 * divisor) + return (int) (remainder >= divisor ? remainder - divisor : remainder); + } + + @Override + public IndexProducer indices(final Shape shape) { + Objects.requireNonNull(shape, "shape"); + + return new IndexProducer() { + + @Override + public boolean forEachIndex(IntPredicate consumer) { + Objects.requireNonNull(consumer, "consumer"); + int bits = shape.getNumberOfBits(); + /* + * Essentially this is computing a wrapped modulus from a start point and an + * increment. So actually you only need two modulus operations before the loop. + * This avoids any modulus operation inside the while loop. It uses a long index + * to avoid overflow. + */ + long index = mod(initial, bits); + int inc = mod(increment, bits); Review Comment: added to https://issues.apache.org/jira/browse/COLLECTIONS-824 -- 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]
