/*
 * 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.linear;

import org.apache.commons.math3.exception.DimensionMismatchException;
import org.apache.commons.math3.exception.MaxCountExceededException;
import org.apache.commons.math3.exception.NullArgumentException;
import org.apache.commons.math3.exception.util.ExceptionContext;
import org.apache.commons.math3.util.IterationManager;

/**
 * This is an implementation of the conjugate gradient method for {@link RealLinearOperator}. It
 * follows closely the template by <a href="#BARR1994">Barrett et al. (1994)</a> (figure 2.5). The
 * linear system at hand is A &middot; x = b, and the residual is r = b - A &middot; x.
 *
 * <h3><a id="stopcrit">Default stopping criterion</a></h3>
 *
 * <p>A default stopping criterion is implemented. The iterations stop when || r || &le; &delta; ||
 * b ||, where b is the right-hand side vector, r the current estimate of the residual, and &delta;
 * a user-specified tolerance. It should be noted that r is the so-called <em>updated</em> residual,
 * which might differ from the true residual due to rounding-off errors (see e.g. <a
 * href="#STRA2002">Strakos and Tichy, 2002</a>).
 *
 * <h3>Iteration count</h3>
 *
 * <p>In the present context, an iteration should be understood as one evaluation of the
 * matrix-vector product A &middot; x. The initialization phase therefore counts as one iteration.
 *
 * <h3><a id="context">Exception context</a></h3>
 *
 * <p>Besides standard {@link DimensionMismatchException}, this class might throw {@link
 * NonPositiveDefiniteOperatorException} if the linear operator or the preconditioner are not
 * positive definite. In this case, the {@link ExceptionContext} provides some more information
 *
 * <ul>
 *   <li>key {@code "operator"} points to the offending linear operator, say L,
 *   <li>key {@code "vector"} points to the offending vector, say x, such that x<sup>T</sup>
 *       &middot; L &middot; x < 0.
 * </ul>
 *
 * <h3>References</h3>
 *
 * <dl>
 *   <dt><a id="BARR1994">Barret et al. (1994)</a>
 *   <dd>R. Barrett, M. Berry, T. F. Chan, J. Demmel, J. M. Donato, J. Dongarra, V. Eijkhout, R.
 *       Pozo, C. Romine and H. Van der Vorst, <a
 *       href="http://www.netlib.org/linalg/html_templates/Templates.html"><em> Templates for the
 *       Solution of Linear Systems: Building Blocks for Iterative Methods</em></a>, SIAM
 *   <dt><a id="STRA2002">Strakos and Tichy (2002)
 *   <dt>
 *   <dd>Z. Strakos and P. Tichy, <a
 *       href="http://etna.mcs.kent.edu/vol.13.2002/pp56-80.dir/pp56-80.pdf"><em>On error estimation
 *       in the conjugate gradient method and why it works in finite precision
 *       computations</em></a>, Electronic Transactions on Numerical Analysis 13: 56-80, 2002
 * </dl>
 *
 * @since 3.0
 */
public class ConjugateGradient extends PreconditionedIterativeLinearSolver {

    /** Key for the <a href="#context">exception context</a>. */
    public static final String OPERATOR = "operator";

    /** Key for the <a href="#context">exception context</a>. */
    public static final String VECTOR = "vector";

    /** {@code true} if positive-definiteness of matrix and preconditioner should be checked. */
    private boolean check;

    /** The value of &delta;, for the default stopping criterion. */
    private final double delta;

    /**
     * Creates a new instance of this class, with <a href="#stopcrit">default stopping
     * criterion</a>.
     *
     * @param maxIterations the maximum number of iterations
     * @param delta the &delta; parameter for the default stopping criterion
     * @param check {@code true} if positive definiteness of both matrix and preconditioner should
     *     be checked
     */
    public ConjugateGradient(final int maxIterations, final double delta, final boolean check) {
        super(maxIterations);
        this.delta = delta;
        this.check = check;
    }

    /**
     * Creates a new instance of this class, with <a href="#stopcrit">default stopping criterion</a>
     * and custom iteration manager.
     *
     * @param manager the custom iteration manager
     * @param delta the &delta; parameter for the default stopping criterion
     * @param check {@code true} if positive definiteness of both matrix and preconditioner should
     *     be checked
     * @throws NullArgumentException if {@code manager} is {@code null}
     */
    public ConjugateGradient(
            final IterationManager manager, final double delta, final boolean check)
            throws NullArgumentException {
        super(manager);
        this.delta = delta;
        this.check = check;
    }

    /**
     * Returns {@code true} if positive-definiteness should be checked for both matrix and
     * preconditioner.
     *
     * @return {@code true} if the tests are to be performed
     */
    public final boolean getCheck() {
        return check;
    }

    /**
     * {@inheritDoc}
     *
     * @throws NonPositiveDefiniteOperatorException if {@code a} or {@code m} is not positive
     *     definite
     */
    @Override
    public RealVector solveInPlace(
            final RealLinearOperator a,
            final RealLinearOperator m,
            final RealVector b,
            final RealVector x0)
            throws NullArgumentException,
                    NonPositiveDefiniteOperatorException,
                    NonSquareOperatorException,
                    DimensionMismatchException,
                    MaxCountExceededException {
        checkParameters(a, m, b, x0);
        final IterationManager manager = getIterationManager();
        // Initialization of default stopping criterion
        manager.resetIterationCount();
        final double rmax = delta * b.getNorm();
        final RealVector bro = RealVector.unmodifiableRealVector(b);

        // Initialization phase counts as one iteration.
        manager.incrementIterationCount();
        // p and x are constructed as copies of x0, since presumably, the type
        // of x is optimized for the calculation of the matrix-vector product
        // A.x.
        final RealVector x = x0;
        final RealVector xro = RealVector.unmodifiableRealVector(x);
        final RealVector p = x.copy();
        RealVector q = a.operate(p);

        final RealVector r = b.combine(1, -1, q);
        final RealVector rro = RealVector.unmodifiableRealVector(r);
        double rnorm = r.getNorm();
        RealVector z;
        if (m == null) {
            z = r;
        } else {
            z = null;
        }
        IterativeLinearSolverEvent evt;
        evt =
                new DefaultIterativeLinearSolverEvent(
                        this, manager.getIterations(), xro, bro, rro, rnorm);
        manager.fireInitializationEvent(evt);
        if (rnorm <= rmax) {
            manager.fireTerminationEvent(evt);
            return x;
        }
        double rhoPrev = 0.;
        while (true) {
            manager.incrementIterationCount();
            evt =
                    new DefaultIterativeLinearSolverEvent(
                            this, manager.getIterations(), xro, bro, rro, rnorm);
            manager.fireIterationStartedEvent(evt);
            if (m != null) {
                z = m.operate(r);
            }
            final double rhoNext = r.dotProduct(z);
            if (check && (rhoNext <= 0.)) {
                final NonPositiveDefiniteOperatorException e;
                e = new NonPositiveDefiniteOperatorException();
                final ExceptionContext context = e.getContext();
                context.setValue(OPERATOR, m);
                context.setValue(VECTOR, r);
                throw e;
            }
            if (manager.getIterations() == 2) {
                p.setSubVector(0, z);
            } else {
                p.combineToSelf(rhoNext / rhoPrev, 1., z);
            }
            q = a.operate(p);
            final double pq = p.dotProduct(q);
            if (check && (pq <= 0.)) {
                final NonPositiveDefiniteOperatorException e;
                e = new NonPositiveDefiniteOperatorException();
                final ExceptionContext context = e.getContext();
                context.setValue(OPERATOR, a);
                context.setValue(VECTOR, p);
                throw e;
            }
            final double alpha = rhoNext / pq;
            x.combineToSelf(1., alpha, p);
            r.combineToSelf(1., -alpha, q);
            rhoPrev = rhoNext;
            rnorm = r.getNorm();
            evt =
                    new DefaultIterativeLinearSolverEvent(
                            this, manager.getIterations(), xro, bro, rro, rnorm);
            manager.fireIterationPerformedEvent(evt);
            if (rnorm <= rmax) {
                manager.fireTerminationEvent(evt);
                return x;
            }
        }
    }
}