/*
    * 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.math4.legacy.ode;
  
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.Comparator;
  import java.util.Iterator;
  import java.util.List;
  import java.util.SortedSet;
  import java.util.TreeSet;
  
  import org.apache.commons.math4.legacy.analysis.solvers.BracketingNthOrderBrentSolver;
  import org.apache.commons.math4.legacy.analysis.solvers.UnivariateSolver;
  import org.apache.commons.math4.legacy.exception.DimensionMismatchException;
  import org.apache.commons.math4.legacy.exception.MaxCountExceededException;
  import org.apache.commons.math4.legacy.exception.NoBracketingException;
  import org.apache.commons.math4.legacy.exception.NumberIsTooSmallException;
  import org.apache.commons.math4.legacy.exception.util.LocalizedFormats;
  import org.apache.commons.math4.legacy.ode.events.EventHandler;
  import org.apache.commons.math4.legacy.ode.events.EventState;
  import org.apache.commons.math4.legacy.ode.sampling.AbstractStepInterpolator;
  import org.apache.commons.math4.legacy.ode.sampling.StepHandler;
  import org.apache.commons.math4.core.jdkmath.JdkMath;
  import org.apache.commons.math4.legacy.core.IntegerSequence;
  import org.apache.commons.numbers.core.Precision;
  
  /**
   * Base class managing common boilerplate for all integrators.
   * @since 2.0
   */
  public abstract class AbstractIntegrator implements FirstOrderIntegrator {
  
      /** Step handler. */
      protected Collection<StepHandler> stepHandlers;
  
      /** Current step start time. */
      protected double stepStart;
  
      /** Current stepsize. */
      protected double stepSize;
  
      /** Indicator for last step. */
      protected boolean isLastStep;
  
      /** Indicator that a state or derivative reset was triggered by some event. */
      protected boolean resetOccurred;
  
      /** Events states. */
      private Collection<EventState> eventsStates;
  
      /** Initialization indicator of events states. */
      private boolean statesInitialized;
  
      /** Name of the method. */
      private final String name;
  
      /** Counter for number of evaluations. */
      private IntegerSequence.Incrementor evaluations;
  
      /** Differential equations to integrate. */
      private transient ExpandableStatefulODE expandable;
  
      /** Build an instance.
       * @param name name of the method
       */
      public AbstractIntegrator(final String name) {
          this.name = name;
          stepHandlers = new ArrayList<>();
          stepStart = Double.NaN;
          stepSize  = Double.NaN;
          eventsStates = new ArrayList<>();
          statesInitialized = false;
          evaluations = IntegerSequence.Incrementor.create().withMaximalCount(Integer.MAX_VALUE);
      }
  
      /** Build an instance with a null name.
       */
      protected AbstractIntegrator() {
          this(null);
      }
  
      /** {@inheritDoc} */
     @Override
     public String getName() {
         return name;
     }
 
     /** {@inheritDoc} */
     @Override
     public void addStepHandler(final StepHandler handler) {
         stepHandlers.add(handler);
     }
 
     /** {@inheritDoc} */
     @Override
     public Collection<StepHandler> getStepHandlers() {
         return Collections.unmodifiableCollection(stepHandlers);
     }
 
     /** {@inheritDoc} */
     @Override
     public void clearStepHandlers() {
         stepHandlers.clear();
     }
 
     /** {@inheritDoc} */
     @Override
     public void addEventHandler(final EventHandler handler,
                                 final double maxCheckInterval,
                                 final double convergence,
                                 final int maxIterationCount) {
         addEventHandler(handler, maxCheckInterval, convergence,
                         maxIterationCount,
                         new BracketingNthOrderBrentSolver(convergence, 5));
     }
 
     /** {@inheritDoc} */
     @Override
     public void addEventHandler(final EventHandler handler,
                                 final double maxCheckInterval,
                                 final double convergence,
                                 final int maxIterationCount,
                                 final UnivariateSolver solver) {
         eventsStates.add(new EventState(handler, maxCheckInterval, convergence,
                                         maxIterationCount, solver));
     }
 
     /** {@inheritDoc} */
     @Override
     public Collection<EventHandler> getEventHandlers() {
         final List<EventHandler> list = new ArrayList<>(eventsStates.size());
         for (EventState state : eventsStates) {
             list.add(state.getEventHandler());
         }
         return Collections.unmodifiableCollection(list);
     }
 
     /** {@inheritDoc} */
     @Override
     public void clearEventHandlers() {
         eventsStates.clear();
     }
 
     /** {@inheritDoc} */
     @Override
     public double getCurrentStepStart() {
         return stepStart;
     }
 
     /** {@inheritDoc} */
     @Override
     public double getCurrentSignedStepsize() {
         return stepSize;
     }
 
     /** {@inheritDoc} */
     @Override
     public void setMaxEvaluations(int maxEvaluations) {
         evaluations = evaluations.withMaximalCount((maxEvaluations < 0) ? Integer.MAX_VALUE : maxEvaluations);
     }
 
     /** {@inheritDoc} */
     @Override
     public int getMaxEvaluations() {
         return evaluations.getMaximalCount();
     }
 
     /** {@inheritDoc} */
     @Override
     public int getEvaluations() {
         return evaluations.getCount();
     }
 
     /** Prepare the start of an integration.
      * @param t0 start value of the independent <i>time</i> variable
      * @param y0 array containing the start value of the state vector
      * @param t target time for the integration
      */
     protected void initIntegration(final double t0, final double[] y0, final double t) {
 
         evaluations = evaluations.withStart(0);
 
         for (final EventState state : eventsStates) {
             state.setExpandable(expandable);
             state.getEventHandler().init(t0, y0, t);
         }
 
         for (StepHandler handler : stepHandlers) {
             handler.init(t0, y0, t);
         }
 
         setStateInitialized(false);
     }
 
     /** Set the equations.
      * @param equations equations to set
      */
     protected void setEquations(final ExpandableStatefulODE equations) {
         this.expandable = equations;
     }
 
     /** Get the differential equations to integrate.
      * @return differential equations to integrate
      * @since 3.2
      */
     protected ExpandableStatefulODE getExpandable() {
         return expandable;
     }
 
     /** Get the evaluations counter.
      * @return evaluations counter
      * @since 3.6
      */
     protected IntegerSequence.Incrementor getCounter() {
         return evaluations;
     }
 
     /** {@inheritDoc} */
     @Override
     public double integrate(final FirstOrderDifferentialEquations equations,
                             final double t0, final double[] y0, final double t, final double[] y)
         throws DimensionMismatchException, NumberIsTooSmallException,
                MaxCountExceededException, NoBracketingException {
 
         if (y0.length != equations.getDimension()) {
             throw new DimensionMismatchException(y0.length, equations.getDimension());
         }
         if (y.length != equations.getDimension()) {
             throw new DimensionMismatchException(y.length, equations.getDimension());
         }
 
         // prepare expandable stateful equations
         final ExpandableStatefulODE expandableODE = new ExpandableStatefulODE(equations);
         expandableODE.setTime(t0);
         expandableODE.setPrimaryState(y0);
 
         // perform integration
         integrate(expandableODE, t);
 
         // extract results back from the stateful equations
         System.arraycopy(expandableODE.getPrimaryState(), 0, y, 0, y.length);
         return expandableODE.getTime();
     }
 
     /** Integrate a set of differential equations up to the given time.
      * <p>This method solves an Initial Value Problem (IVP).</p>
      * <p>The set of differential equations is composed of a main set, which
      * can be extended by some sets of secondary equations. The set of
      * equations must be already set up with initial time and partial states.
      * At integration completion, the final time and partial states will be
      * available in the same object.</p>
      * <p>Since this method stores some internal state variables made
      * available in its public interface during integration ({@link
      * #getCurrentSignedStepsize()}), it is <em>not</em> thread-safe.</p>
      * @param equations complete set of differential equations to integrate
      * @param t target time for the integration
      * (can be set to a value smaller than <code>t0</code> for backward integration)
      * @exception NumberIsTooSmallException if integration step is too small
      * @throws DimensionMismatchException if the dimension of the complete state does not
      * match the complete equations sets dimension
      * @exception MaxCountExceededException if the number of functions evaluations is exceeded
      * @exception NoBracketingException if the location of an event cannot be bracketed
      */
     public abstract void integrate(ExpandableStatefulODE equations, double t)
         throws NumberIsTooSmallException, DimensionMismatchException,
                MaxCountExceededException, NoBracketingException;
 
     /** Compute the derivatives and check the number of evaluations.
      * @param t current value of the independent <I>time</I> variable
      * @param y array containing the current value of the state vector
      * @param yDot placeholder array where to put the time derivative of the state vector
      * @exception MaxCountExceededException if the number of functions evaluations is exceeded
      * @exception DimensionMismatchException if arrays dimensions do not match equations settings
      * @exception NullPointerException if the ODE equations have not been set (i.e. if this method
      * is called outside of a call to {@link #integrate(ExpandableStatefulODE, double)} or {@link
      * #integrate(FirstOrderDifferentialEquations, double, double[], double, double[])})
      */
     public void computeDerivatives(final double t, final double[] y, final double[] yDot)
         throws MaxCountExceededException, DimensionMismatchException, NullPointerException {
         evaluations.increment();
         expandable.computeDerivatives(t, y, yDot);
     }
 
     /** Set the stateInitialized flag.
      * <p>This method must be called by integrators with the value
      * {@code false} before they start integration, so a proper lazy
      * initialization is done automatically on the first step.</p>
      * @param stateInitialized new value for the flag
      * @since 2.2
      */
     protected void setStateInitialized(final boolean stateInitialized) {
         this.statesInitialized = stateInitialized;
     }
 
     /** Accept a step, triggering events and step handlers.
      * @param interpolator step interpolator
      * @param y state vector at step end time, must be reset if an event
      * asks for resetting or if an events stops integration during the step
      * @param yDot placeholder array where to put the time derivative of the state vector
      * @param tEnd final integration time
      * @return time at end of step
      * @exception MaxCountExceededException if the interpolator throws one because
      * the number of functions evaluations is exceeded
      * @exception NoBracketingException if the location of an event cannot be bracketed
      * @exception DimensionMismatchException if arrays dimensions do not match equations settings
      * @since 2.2
      */
     protected double acceptStep(final AbstractStepInterpolator interpolator,
                                 final double[] y, final double[] yDot, final double tEnd)
         throws MaxCountExceededException, DimensionMismatchException, NoBracketingException {
 
             double previousT = interpolator.getGlobalPreviousTime();
             final double currentT = interpolator.getGlobalCurrentTime();
 
             // initialize the events states if needed
             if (! statesInitialized) {
                 for (EventState state : eventsStates) {
                     state.reinitializeBegin(interpolator);
                 }
                 statesInitialized = true;
             }
 
             // search for next events that may occur during the step
             final int orderingSign = interpolator.isForward() ? +1 : -1;
             SortedSet<EventState> occurringEvents = new TreeSet<>(new Comparator<EventState>() {
 
                 /** {@inheritDoc} */
                 @Override
                 public int compare(EventState es0, EventState es1) {
                     return orderingSign * Double.compare(es0.getEventTime(), es1.getEventTime());
                 }
             });
 
             for (final EventState state : eventsStates) {
                 if (state.evaluateStep(interpolator)) {
                     // the event occurs during the current step
                     occurringEvents.add(state);
                 }
             }
 
             while (!occurringEvents.isEmpty()) {
 
                 // handle the chronologically first event
                 final Iterator<EventState> iterator = occurringEvents.iterator();
                 final EventState currentEvent = iterator.next();
                 iterator.remove();
 
                 // restrict the interpolator to the first part of the step, up to the event
                 final double eventT = currentEvent.getEventTime();
                 interpolator.setSoftPreviousTime(previousT);
                 interpolator.setSoftCurrentTime(eventT);
 
                 // get state at event time
                 interpolator.setInterpolatedTime(eventT);
                 final double[] eventYComplete = new double[y.length];
                 expandable.getPrimaryMapper().insertEquationData(interpolator.getInterpolatedState(),
                                                                  eventYComplete);
                 int index = 0;
                 for (EquationsMapper secondary : expandable.getSecondaryMappers()) {
                     secondary.insertEquationData(interpolator.getInterpolatedSecondaryState(index++),
                                                  eventYComplete);
                 }
 
                 // advance all event states to current time
                 for (final EventState state : eventsStates) {
                     state.stepAccepted(eventT, eventYComplete);
                     isLastStep = isLastStep || state.stop();
                 }
 
                 // handle the first part of the step, up to the event
                 for (final StepHandler handler : stepHandlers) {
                     handler.handleStep(interpolator, isLastStep);
                 }
 
                 if (isLastStep) {
                     // the event asked to stop integration
                     System.arraycopy(eventYComplete, 0, y, 0, y.length);
                     return eventT;
                 }
 
                 resetOccurred = false;
                 final boolean needReset = currentEvent.reset(eventT, eventYComplete);
                 if (needReset) {
                     // some event handler has triggered changes that
                     // invalidate the derivatives, we need to recompute them
                     interpolator.setInterpolatedTime(eventT);
                     System.arraycopy(eventYComplete, 0, y, 0, y.length);
                     computeDerivatives(eventT, y, yDot);
                     resetOccurred = true;
                     return eventT;
                 }
 
                 // prepare handling of the remaining part of the step
                 previousT = eventT;
                 interpolator.setSoftPreviousTime(eventT);
                 interpolator.setSoftCurrentTime(currentT);
 
                 // check if the same event occurs again in the remaining part of the step
                 if (currentEvent.evaluateStep(interpolator)) {
                     // the event occurs during the current step
                     occurringEvents.add(currentEvent);
                 }
             }
 
             // last part of the step, after the last event
             interpolator.setInterpolatedTime(currentT);
             final double[] currentY = new double[y.length];
             expandable.getPrimaryMapper().insertEquationData(interpolator.getInterpolatedState(),
                                                              currentY);
             int index = 0;
             for (EquationsMapper secondary : expandable.getSecondaryMappers()) {
                 secondary.insertEquationData(interpolator.getInterpolatedSecondaryState(index++),
                                              currentY);
             }
             for (final EventState state : eventsStates) {
                 state.stepAccepted(currentT, currentY);
                 isLastStep = isLastStep || state.stop();
             }
             isLastStep = isLastStep || Precision.equals(currentT, tEnd, 1);
 
             // handle the remaining part of the step, after all events if any
             for (StepHandler handler : stepHandlers) {
                 handler.handleStep(interpolator, isLastStep);
             }
 
             return currentT;
     }
 
     /** Check the integration span.
      * @param equations set of differential equations
      * @param t target time for the integration
      * @exception NumberIsTooSmallException if integration span is too small
      * @exception DimensionMismatchException if adaptive step size integrators
      * tolerance arrays dimensions are not compatible with equations settings
      */
     protected void sanityChecks(final ExpandableStatefulODE equations, final double t)
         throws NumberIsTooSmallException, DimensionMismatchException {
 
         final double threshold = 1000 * JdkMath.ulp(JdkMath.max(JdkMath.abs(equations.getTime()),
                                                                   JdkMath.abs(t)));
         final double dt = JdkMath.abs(equations.getTime() - t);
         if (dt <= threshold) {
             throw new NumberIsTooSmallException(LocalizedFormats.TOO_SMALL_INTEGRATION_INTERVAL,
                                                 dt, threshold, false);
         }
     }
 }