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The core of LensKit, providing basic implementations and algorithm support.

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/*
 * LensKit, an open source recommender systems toolkit.
 * Copyright 2010-2016 LensKit Contributors.  See CONTRIBUTORS.md.
 * Work on LensKit has been funded by the National Science Foundation under
 * grants IIS 05-34939, 08-08692, 08-12148, and 10-17697.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc., 51
 * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */
package org.lenskit.util;

import com.google.common.base.Function;

/**
 * A function for which an inverse is available.
 *
 * @param  The domain type.
 * @param  The co-domain type.
 */
public interface InvertibleFunction extends Function {
    /**
     * Returns the result of applying the inverse of this function to {@code input}.
     *
     * The following should generally hold:
     *
     * ```java
     * F object = value;
     * T result = function.apply(object);
     * F inverse = function.unapply(result);
     * assert inverse.equals(object);
     * ```
     *
     * @param input The input value.
     * @return The inverse of the input
     */
    F unapply(T input);
}