001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017
018package org.apache.commons.math4.linear;
019
020import org.apache.commons.math4.exception.DimensionMismatchException;
021
022/**
023 * This class defines a linear operator operating on real ({@code double})
024 * vector spaces. No direct access to the coefficients of the underlying matrix
025 * is provided.
026 *
027 * The motivation for such an interface is well stated by
028 * <a href="#BARR1994">Barrett et al. (1994)</a>:
029 * <blockquote>
030 *  We restrict ourselves to iterative methods, which work by repeatedly
031 *  improving an approximate solution until it is accurate enough. These
032 *  methods access the coefficient matrix A of the linear system only via the
033 *  matrix-vector product y = A &middot; x
034 *  (and perhaps z = A<sup>T</sup> &middot; x). Thus the user need only
035 *  supply a subroutine for computing y (and perhaps z) given x, which permits
036 *  full exploitation of the sparsity or other special structure of A.
037 * </blockquote>
038 * <br>
039 *
040 * <dl>
041 *  <dt><a name="BARR1994">Barret et al. (1994)</a></dt>
042 *  <dd>
043 *   R. Barrett, M. Berry, T. F. Chan, J. Demmel, J. M. Donato, J. Dongarra,
044 *   V. Eijkhout, R. Pozo, C. Romine and H. Van der Vorst,
045 *   <em>Templates for the Solution of Linear Systems: Building Blocks for
046 *   Iterative Methods</em>, SIAM
047 *  </dd>
048 * </dl>
049 *
050 * @since 3.0
051 */
052public abstract class RealLinearOperator {
053    /**
054     * Returns the dimension of the codomain of this operator.
055     *
056     * @return the number of rows of the underlying matrix
057     */
058    public abstract int getRowDimension();
059
060    /**
061     * Returns the dimension of the domain of this operator.
062     *
063     * @return the number of columns of the underlying matrix
064     */
065    public abstract int getColumnDimension();
066
067    /**
068     * Returns the result of multiplying {@code this} by the vector {@code x}.
069     *
070     * @param x the vector to operate on
071     * @return the product of {@code this} instance with {@code x}
072     * @throws DimensionMismatchException if the column dimension does not match
073     * the size of {@code x}
074     */
075    public abstract RealVector operate(final RealVector x)
076        throws DimensionMismatchException;
077
078    /**
079     * Returns the result of multiplying the transpose of {@code this} operator
080     * by the vector {@code x} (optional operation). The default implementation
081     * throws an {@link UnsupportedOperationException}. Users overriding this
082     * method must also override {@link #isTransposable()}.
083     *
084     * @param x the vector to operate on
085     * @return the product of the transpose of {@code this} instance with
086     * {@code x}
087     * @throws org.apache.commons.math4.exception.DimensionMismatchException
088     * if the row dimension does not match the size of {@code x}
089     * @throws UnsupportedOperationException if this operation is not supported
090     * by {@code this} operator
091     */
092    public RealVector operateTranspose(final RealVector x)
093        throws DimensionMismatchException, UnsupportedOperationException {
094        throw new UnsupportedOperationException();
095    }
096
097    /**
098     * Returns {@code true} if this operator supports
099     * {@link #operateTranspose(RealVector)}. If {@code true} is returned,
100     * {@link #operateTranspose(RealVector)} should not throw
101     * {@code UnsupportedOperationException}. The default implementation returns
102     * {@code false}.
103     *
104     * @return {@code false}
105     */
106    public boolean isTransposable() {
107        return false;
108    }
109}