org.openimaj.lsh.functions.DoublePStableFactory Maven / Gradle / Ivy

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*/
/**
 * Copyright (c) 2011, The University of Southampton and the individual contributors.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 *   * 	Redistributions of source code must retain the above copyright notice,
 * 	this list of conditions and the following disclaimer.
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 *   *	Redistributions in binary form must reproduce the above copyright notice,
 * 	this list of conditions and the following disclaimer in the documentation
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 * 	contributors may be used to endorse or promote products derived from this
 * 	software without specific prior written permission.
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package org.openimaj.lsh.functions;

import org.openimaj.citation.annotation.Reference;
import org.openimaj.citation.annotation.ReferenceType;
import org.openimaj.util.array.SparseDoubleArray;
import org.openimaj.util.array.SparseDoubleArray.Entry;

import cern.jet.random.engine.MersenneTwister;

/**
 * Base class for hashing schemes based on P-Stable distributions. The hash
 * functions are of the form h(x) = floor((ax + b) / w).
 * 
 * @author Jonathon Hare ([email protected])
 */
 @Reference(
	type = ReferenceType.Inproceedings,
	author = { "Datar, Mayur", "Immorlica, Nicole", "Indyk, Piotr", "Mirrokni, Vahab S." },
	title = "Locality-sensitive hashing scheme based on p-stable distributions",
	year = "2004",
	booktitle = "Proceedings of the twentieth annual symposium on Computational geometry",
	pages = { "253", "", "262" },
	url = "http://doi.acm.org/10.1145/997817.997857",
	publisher = "ACM",
	series = "SCG '04"
)
public abstract class DoublePStableFactory extends DoubleHashFunctionFactory {
	protected abstract class PStableFunction extends DoubleHashFunction {
		protected double[] r;
		protected double b;

		PStableFunction(MersenneTwister rng) {
			super(rng);
		}

		@Override
		public final int computeHashCode(double[] point) {
			double val = 0;
			for (int i = 0; i < point.length; i++) {
				val += point[i] * r[i];
			}

			val = (val + b) / w;

			return (int) Math.floor(val);
		}
		
		@Override
		public int computeHashCode(SparseDoubleArray array) {
			double val = 0;
			
			for (Entry e : array.entries()) {
				val += e.value * r[e.index];
			}
			
			val = (val + b) / w;

			return (int) Math.floor(val);			
		}
	}

	double w;

	/**
	 * Construct with the given parameters.
	 * 
	 * @param ndims
	 *            number of dimensions of the data
	 * @param rng
	 *            the random number generator
	 * @param w
	 *            the width parameter
	 */
	public DoublePStableFactory(int ndims, MersenneTwister rng, double w) {
		super(ndims, rng);

		this.w = w;
	}
}