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A Java's Collaborative Filtering library to carry out experiments in research of Collaborative Filtering based Recommender Systems. The library has been designed from researchers to researchers.
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/*
 * 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.math3.util;


/**
 * Provides a standard interface for double arrays.  Allows different
 * array implementations to support various storage mechanisms
 * such as automatic expansion, contraction, and array "rolling".
 *
 */
public interface DoubleArray {

    /**
     * Returns the number of elements currently in the array.  Please note
     * that this may be different from the length of the internal storage array.
     *
     * @return number of elements
     */
    int getNumElements();

    /**
     * Returns the element at the specified index.  Note that if an
     * out of bounds index is supplied a ArrayIndexOutOfBoundsException
     * will be thrown.
     *
     * @param index index to fetch a value from
     * @return value stored at the specified index
     * @throws ArrayIndexOutOfBoundsException if index is less than
     *         zero or is greater than getNumElements() - 1.
     */
    double getElement(int index);

    /**
     * Sets the element at the specified index.  If the specified index is greater than
     * getNumElements() - 1, the numElements property
     * is increased to index +1 and additional storage is allocated
     * (if necessary) for the new element and all  (uninitialized) elements
     * between the new element and the previous end of the array).
     *
     * @param index index to store a value in
     * @param value value to store at the specified index
     * @throws ArrayIndexOutOfBoundsException if index is less than
     *         zero.
     */
    void setElement(int index, double value);

    /**
     * Adds an element to the end of this expandable array
     *
     * @param value to be added to end of array
     */
    void addElement(double value);

    /**
     * Adds elements to the end of this expandable array
     *
     * @param values to be added to end of array
     */
    void addElements(double[] values);

    /**
     * 
     * Adds an element to the end of the array and removes the first
     * element in the array.  Returns the discarded first element.
     * The effect is similar to a push operation in a FIFO queue.
     * 
     * 
     * Example: If the array contains the elements 1, 2, 3, 4 (in that order)
     * and addElementRolling(5) is invoked, the result is an array containing
     * the entries 2, 3, 4, 5 and the value returned is 1.
     * 
     *
     * @param value the value to be added to the array
     * @return the value which has been discarded or "pushed" out of the array
     *         by this rolling insert
     */
    double addElementRolling(double value);

    /**
     * Returns a double[] array containing the elements of this
     * DoubleArray.  If the underlying implementation is
     * array-based, this method should always return a copy, rather than a
     * reference to the underlying array so that changes made to the returned
     *  array have no effect on the DoubleArray.
     *
     * @return all elements added to the array
     */
    double[] getElements();

    /**
     * Clear the double array
     */
    void clear();

}