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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:
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 * 	this list of conditions and the following disclaimer.
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 * 	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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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
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package org.openimaj.lsh.functions;

import org.openimaj.feature.DoubleFVComparison;

import cern.jet.random.Uniform;
import cern.jet.random.engine.MersenneTwister;

/**
 * A hash function factory for producing hash functions that use a Cauchy
 * distribution to approximate L1 distance.
 * 
 * @author Jonathon Hare ([email protected])
 */
public class DoubleCauchyFactory extends DoublePStableFactory {
	private class Function extends PStableFunction {
		Function(int ndims, MersenneTwister rng) {
			super(rng);

			final Uniform uniform = new Uniform(0, w, rng);
			b = (float) uniform.nextDouble();

			// random direction
			r = new double[ndims];
			for (int i = 0; i < ndims; i++) {
				r[i] = cauchy();
			}
		}

		private final double cauchy() {
			return Math.tan(Math.PI * (rng.nextDouble() - 0.5));
		}
	}

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

	@Override
	public Function create() {
		return new Function(ndims, rng);
	}

	@Override
	protected DoubleFVComparison fvDistanceFunction() {
		return DoubleFVComparison.EUCLIDEAN;
	}
}