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 * (the "License"); you may not use this file except in compliance with
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 *      http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.commons.math3.analysis.integration.gauss;

import java.util.Map;
import java.util.TreeMap;
import org.apache.commons.math3.util.Pair;
import org.apache.commons.math3.exception.DimensionMismatchException;
import org.apache.commons.math3.exception.NotStrictlyPositiveException;
import org.apache.commons.math3.exception.util.LocalizedFormats;

Base class for rules that determines the integration nodes and their weights. Subclasses must implement the computeRule method. 
Type parameters:
<T> – Type of the number used to represent the points and weights of
the quadrature rules.
Since:
3.1/**
 * Base class for rules that determines the integration nodes and their
 * weights.
 * Subclasses must implement the {@link #computeRule(int) computeRule} method.
 *
 * @param <T> Type of the number used to represent the points and weights of
 * the quadrature rules.
 *
 * @since 3.1
 */
public abstract class BaseRuleFactory<T extends Number> {
    
List of points and weights, indexed by the order of the rule. /** List of points and weights, indexed by the order of the rule. */
    private final Map<Integer, Pair<T[], T[]>> pointsAndWeights
        = new TreeMap<Integer, Pair<T[], T[]>>();
    
Cache for double-precision rules. /** Cache for double-precision rules. */
    private final Map<Integer, Pair<double[], double[]>> pointsAndWeightsDouble
        = new TreeMap<Integer, Pair<double[], double[]>>();

    
Gets a copy of the quadrature rule with the given number of integration
points.
Params:
numberOfPoints – Number of integration points.
Throws:
NotStrictlyPositiveException – if numberOfPoints < 1.
DimensionMismatchException – if the elements of the rule pair do not
have the same length.
Returns:
a copy of the integration rule./**
     * Gets a copy of the quadrature rule with the given number of integration
     * points.
     *
     * @param numberOfPoints Number of integration points.
     * @return a copy of the integration rule.
     * @throws NotStrictlyPositiveException if {@code numberOfPoints < 1}.
     * @throws DimensionMismatchException if the elements of the rule pair do not
     * have the same length.
     */
    public Pair<double[], double[]> getRule(int numberOfPoints)
        throws NotStrictlyPositiveException, DimensionMismatchException {

        if (numberOfPoints <= 0) {
            throw new NotStrictlyPositiveException(LocalizedFormats.NUMBER_OF_POINTS,
                                                   numberOfPoints);
        }

        // Try to obtain the rule from the cache.
        Pair<double[], double[]> cached = pointsAndWeightsDouble.get(numberOfPoints);

        if (cached == null) {
            // Rule not computed yet.

            // Compute the rule.
            final Pair<T[], T[]> rule = getRuleInternal(numberOfPoints);
            cached = convertToDouble(rule);

            // Cache it.
            pointsAndWeightsDouble.put(numberOfPoints, cached);
        }

        // Return a copy.
        return new Pair<double[], double[]>(cached.getFirst().clone(),
                                            cached.getSecond().clone());
    }

    
Gets a rule.
Synchronization ensures that rules will be computed and added to the
cache at most once.
The returned rule is a reference into the cache.
Params:
numberOfPoints – Order of the rule to be retrieved.
Throws:
DimensionMismatchException – if the elements of the rule pair do not
have the same length.
Returns:
the points and weights corresponding to the given order./**
     * Gets a rule.
     * Synchronization ensures that rules will be computed and added to the
     * cache at most once.
     * The returned rule is a reference into the cache.
     *
     * @param numberOfPoints Order of the rule to be retrieved.
     * @return the points and weights corresponding to the given order.
     * @throws DimensionMismatchException if the elements of the rule pair do not
     * have the same length.
     */
    protected synchronized Pair<T[], T[]> getRuleInternal(int numberOfPoints)
        throws DimensionMismatchException {
        final Pair<T[], T[]> rule = pointsAndWeights.get(numberOfPoints);
        if (rule == null) {
            addRule(computeRule(numberOfPoints));
            // The rule should be available now.
            return getRuleInternal(numberOfPoints);
        }
        return rule;
    }

    
Stores a rule.
Params:
rule – Rule to be stored.
Throws:
DimensionMismatchException – if the elements of the pair do not
have the same length./**
     * Stores a rule.
     *
     * @param rule Rule to be stored.
     * @throws DimensionMismatchException if the elements of the pair do not
     * have the same length.
     */
    protected void addRule(Pair<T[], T[]> rule) throws DimensionMismatchException {
        if (rule.getFirst().length != rule.getSecond().length) {
            throw new DimensionMismatchException(rule.getFirst().length,
                                                 rule.getSecond().length);
        }

        pointsAndWeights.put(rule.getFirst().length, rule);
    }

    
Computes the rule for the given order.
Params:
numberOfPoints – Order of the rule to be computed.
Throws:
DimensionMismatchException – if the elements of the pair do not
have the same length.
Returns:
the computed rule./**
     * Computes the rule for the given order.
     *
     * @param numberOfPoints Order of the rule to be computed.
     * @return the computed rule.
     * @throws DimensionMismatchException if the elements of the pair do not
     * have the same length.
     */
    protected abstract Pair<T[], T[]> computeRule(int numberOfPoints)
        throws DimensionMismatchException;

    
Converts the from the actual Number type to double 
Params:
rule – Points and weights.
Type parameters:
<T> – Type of the number used to represent the points and
weights of the quadrature rules.
Returns:
points and weights as doubles./**
     * Converts the from the actual {@code Number} type to {@code double}
     *
     * @param <T> Type of the number used to represent the points and
     * weights of the quadrature rules.
     * @param rule Points and weights.
     * @return points and weights as {@code double}s.
     */
    private static <T extends Number> Pair<double[], double[]> convertToDouble(Pair<T[], T[]> rule) {
        final T[] pT = rule.getFirst();
        final T[] wT = rule.getSecond();

        final int len = pT.length;
        final double[] pD = new double[len];
        final double[] wD = new double[len];

        for (int i = 0; i < len; i++) {
            pD[i] = pT[i].doubleValue();
            wD[i] = wT[i].doubleValue();
        }

        return new Pair<double[], double[]>(pD, wD);
    }
}