001/**
002 * Copyright (c) 2011, The University of Southampton and the individual contributors.
003 * All rights reserved.
004 *
005 * Redistribution and use in source and binary forms, with or without modification,
006 * are permitted provided that the following conditions are met:
007 *
008 *   *  Redistributions of source code must retain the above copyright notice,
009 *      this list of conditions and the following disclaimer.
010 *
011 *   *  Redistributions in binary form must reproduce the above copyright notice,
012 *      this list of conditions and the following disclaimer in the documentation
013 *      and/or other materials provided with the distribution.
014 *
015 *   *  Neither the name of the University of Southampton nor the names of its
016 *      contributors may be used to endorse or promote products derived from this
017 *      software without specific prior written permission.
018 *
019 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
020 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
021 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
022 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
023 * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
024 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
025 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
026 * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
027 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
028 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
029 */
030package org.openimaj.lsh.composition;
031
032import java.util.List;
033
034import org.openimaj.util.hash.HashFunction;
035import org.openimaj.util.hash.HashFunctionFactory;
036import org.openimaj.util.hash.composition.HashComposition;
037
038import cern.jet.random.engine.MersenneTwister;
039
040/**
041 * Compose a set of hash functions by computing the dot product of the hashes
042 * they produce with a random vector.
043 *
044 * @author Jonathon Hare (jsh2@ecs.soton.ac.uk)
045 *
046 * @param <OBJECT>
047 *            Object being hashed
048 */
049public class RandomProjectionHashComposition<OBJECT> extends HashComposition<OBJECT> {
050        int[] projection;
051
052        /**
053         * Construct with the given functions.
054         *
055         * @param rng
056         *            the random number generator
057         * @param functions
058         *            the underlying hash functions.
059         */
060        public RandomProjectionHashComposition(MersenneTwister rng, List<HashFunction<OBJECT>> functions) {
061                super(functions);
062                createProjection(rng);
063        }
064
065        /**
066         * Construct with the given functions.
067         *
068         * @param rng
069         *            the random number generator
070         * @param first
071         *            the first function
072         * @param remainder
073         *            the remainder of the functions
074         */
075        @SafeVarargs
076        public RandomProjectionHashComposition(MersenneTwister rng, HashFunction<OBJECT> first,
077                        HashFunction<OBJECT>... remainder)
078        {
079                super(first, remainder);
080                createProjection(rng);
081        }
082
083        /**
084         * Construct with the factory which is used to produce the required number
085         * of functions.
086         *
087         * @param rng
088         *            the random number generator
089         * @param factory
090         *            the factory to use to produce the underlying hash functions.
091         * @param nFuncs
092         *            the number of functions to create for the composition
093         */
094        public RandomProjectionHashComposition(MersenneTwister rng, HashFunctionFactory<OBJECT> factory, int nFuncs) {
095                super(factory, nFuncs);
096                createProjection(rng);
097        }
098
099        private void createProjection(MersenneTwister rng) {
100                projection = new int[hashFunctions.size()];
101                for (int i = 0; i < hashFunctions.size(); i++)
102                        projection[i] = rng.nextInt();
103        }
104
105        @Override
106        public int computeHashCode(OBJECT object) {
107                int hash = 0;
108
109                for (int i = 0; i < projection.length; i++) {
110                        hash += projection[i] * hashFunctions.get(i).computeHashCode(object);
111                }
112
113                return hash;
114        }
115}