/*
 * 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.math.random;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Serializable;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

import org.apache.commons.math.MathRuntimeException;
import org.apache.commons.math.exception.util.LocalizedFormats;
import org.apache.commons.math.stat.descriptive.StatisticalSummary;
import org.apache.commons.math.stat.descriptive.SummaryStatistics;
import org.apache.commons.math.util.FastMath;

/**
 * Implements <code>EmpiricalDistribution</code> interface.  This implementation
 * uses what amounts to the
 * <a href="http://nedwww.ipac.caltech.edu/level5/March02/Silverman/Silver2_6.html">
 * Variable Kernel Method</a> with Gaussian smoothing:<p>
 * <strong>Digesting the input file</strong>
 * <ol><li>Pass the file once to compute min and max.</li>
 * <li>Divide the range from min-max into <code>binCount</code> "bins."</li>
 * <li>Pass the data file again, computing bin counts and univariate
 *     statistics (mean, std dev.) for each of the bins </li>
 * <li>Divide the interval (0,1) into subintervals associated with the bins,
 *     with the length of a bin's subinterval proportional to its count.</li></ol>
 * <strong>Generating random values from the distribution</strong><ol>
 * <li>Generate a uniformly distributed value in (0,1) </li>
 * <li>Select the subinterval to which the value belongs.
 * <li>Generate a random Gaussian value with mean = mean of the associated
 *     bin and std dev = std dev of associated bin.</li></ol></p><p>
 *<strong>USAGE NOTES:</strong><ul>
 *<li>The <code>binCount</code> is set by default to 1000.  A good rule of thumb
 *    is to set the bin count to approximately the length of the input file divided
 *    by 10. </li>
 *<li>The input file <i>must</i> be a plain text file containing one valid numeric
 *    entry per line.</li>
 * </ul></p>
 *
 * @version $Revision: 1003886 $ $Date: 2010-10-02 23:04:44 +0200 (sam. 02 oct. 2010) $
 */
public class EmpiricalDistributionImpl implements Serializable, EmpiricalDistribution {

    /** Serializable version identifier */
    private static final long serialVersionUID = 5729073523949762654L;

    /** List of SummaryStatistics objects characterizing the bins */
    private final List<SummaryStatistics> binStats;

    /** Sample statistics */
    private SummaryStatistics sampleStats = null;

    /** Max loaded value */
    private double max = Double.NEGATIVE_INFINITY;

    /** Min loaded value */
    private double min = Double.POSITIVE_INFINITY;

    /** Grid size */
    private double delta = 0d;

    /** number of bins */
    private final int binCount;

    /** is the distribution loaded? */
    private boolean loaded = false;

    /** upper bounds of subintervals in (0,1) "belonging" to the bins */
    private double[] upperBounds = null;

    /** RandomData instance to use in repeated calls to getNext() */
    private final RandomData randomData = new RandomDataImpl();

    /**
     * Creates a new EmpiricalDistribution with the default bin count.
     */
    public EmpiricalDistributionImpl() {
        binCount = 1000;
        binStats = new ArrayList<SummaryStatistics>();
    }

    /**
     * Creates a new EmpiricalDistribution  with the specified bin count.
     *
     * @param binCount number of bins
     */
    public EmpiricalDistributionImpl(int binCount) {
        this.binCount = binCount;
        binStats = new ArrayList<SummaryStatistics>();
    }

     /**
     * Computes the empirical distribution from the provided
     * array of numbers.
     *
     * @param in the input data array
     */
    public void load(double[] in) {
        DataAdapter da = new ArrayDataAdapter(in);
        try {
            da.computeStats();
            fillBinStats(in);
        } catch (IOException e) {
            throw new MathRuntimeException(e);
        }
        loaded = true;

    }

    /**
     * Computes the empirical distribution using data read from a URL.
     * @param url  url of the input file
     *
     * @throws IOException if an IO error occurs
     */
    public void load(URL url) throws IOException {
        BufferedReader in =
            new BufferedReader(new InputStreamReader(url.openStream()));
        try {
            DataAdapter da = new StreamDataAdapter(in);
            da.computeStats();
            if (sampleStats.getN() == 0) {
                throw MathRuntimeException.createEOFException(LocalizedFormats.URL_CONTAINS_NO_DATA,
                                                              url);
            }
            in = new BufferedReader(new InputStreamReader(url.openStream()));
            fillBinStats(in);
            loaded = true;
        } finally {
           try {
               in.close();
           } catch (IOException ex) {
               // ignore
           }
        }
    }

    /**
     * Computes the empirical distribution from the input file.
     *
     * @param file the input file
     * @throws IOException if an IO error occurs
     */
    public void load(File file) throws IOException {
        BufferedReader in = new BufferedReader(new FileReader(file));
        try {
            DataAdapter da = new StreamDataAdapter(in);
            da.computeStats();
            in = new BufferedReader(new FileReader(file));
            fillBinStats(in);
            loaded = true;
        } finally {
            try {
                in.close();
            } catch (IOException ex) {
                // ignore
            }
        }
    }

    /**
     * Provides methods for computing <code>sampleStats</code> and
     * <code>beanStats</code> abstracting the source of data.
     */
    private abstract class DataAdapter{

        /**
         * Compute bin stats.
         *
         * @throws IOException  if an error occurs computing bin stats
         */
        public abstract void computeBinStats() throws IOException;

        /**
         * Compute sample statistics.
         *
         * @throws IOException if an error occurs computing sample stats
         */
        public abstract void computeStats() throws IOException;

    }

    /**
     * Factory of <code>DataAdapter</code> objects. For every supported source
     * of data (array of doubles, file, etc.) an instance of the proper object
     * is returned.
     */
    private class DataAdapterFactory{
        /**
         * Creates a DataAdapter from a data object
         *
         * @param in object providing access to the data
         * @return DataAdapter instance
         */
        public DataAdapter getAdapter(Object in) {
            if (in instanceof BufferedReader) {
                BufferedReader inputStream = (BufferedReader) in;
                return new StreamDataAdapter(inputStream);
            } else if (in instanceof double[]) {
                double[] inputArray = (double[]) in;
                return new ArrayDataAdapter(inputArray);
            } else {
                throw MathRuntimeException.createIllegalArgumentException(
                      LocalizedFormats.INPUT_DATA_FROM_UNSUPPORTED_DATASOURCE,
                      in.getClass().getName(),
                      BufferedReader.class.getName(), double[].class.getName());
            }
        }
    }
    /**
     * <code>DataAdapter</code> for data provided through some input stream
     */
    private class StreamDataAdapter extends DataAdapter{

        /** Input stream providing access to the data */
        private BufferedReader inputStream;

        /**
         * Create a StreamDataAdapter from a BufferedReader
         *
         * @param in BufferedReader input stream
         */
        public StreamDataAdapter(BufferedReader in){
            super();
            inputStream = in;
        }

        /** {@inheritDoc} */
        @Override
        public void computeBinStats() throws IOException {
            String str = null;
            double val = 0.0d;
            while ((str = inputStream.readLine()) != null) {
                val = Double.parseDouble(str);
                SummaryStatistics stats = binStats.get(findBin(val));
                stats.addValue(val);
            }

            inputStream.close();
            inputStream = null;
        }

        /** {@inheritDoc} */
        @Override
        public void computeStats() throws IOException {
            String str = null;
            double val = 0.0;
            sampleStats = new SummaryStatistics();
            while ((str = inputStream.readLine()) != null) {
                val = Double.valueOf(str).doubleValue();
                sampleStats.addValue(val);
            }
            inputStream.close();
            inputStream = null;
        }
    }

    /**
     * <code>DataAdapter</code> for data provided as array of doubles.
     */
    private class ArrayDataAdapter extends DataAdapter {

        /** Array of input  data values */
        private double[] inputArray;

        /**
         * Construct an ArrayDataAdapter from a double[] array
         *
         * @param in double[] array holding the data
         */
        public ArrayDataAdapter(double[] in){
            super();
            inputArray = in;
        }

        /** {@inheritDoc} */
        @Override
        public void computeStats() throws IOException {
            sampleStats = new SummaryStatistics();
            for (int i = 0; i < inputArray.length; i++) {
                sampleStats.addValue(inputArray[i]);
            }
        }

        /** {@inheritDoc} */
        @Override
        public void computeBinStats() throws IOException {
            for (int i = 0; i < inputArray.length; i++) {
                SummaryStatistics stats =
                    binStats.get(findBin(inputArray[i]));
                stats.addValue(inputArray[i]);
            }
        }
    }

    /**
     * Fills binStats array (second pass through data file).
     *
     * @param in object providing access to the data
     * @throws IOException  if an IO error occurs
     */
    private void fillBinStats(Object in) throws IOException {
        // Set up grid
        min = sampleStats.getMin();
        max = sampleStats.getMax();
        delta = (max - min)/(Double.valueOf(binCount)).doubleValue();

        // Initialize binStats ArrayList
        if (!binStats.isEmpty()) {
            binStats.clear();
        }
        for (int i = 0; i < binCount; i++) {
            SummaryStatistics stats = new SummaryStatistics();
            binStats.add(i,stats);
        }

        // Filling data in binStats Array
        DataAdapterFactory aFactory = new DataAdapterFactory();
        DataAdapter da = aFactory.getAdapter(in);
        da.computeBinStats();

        // Assign upperBounds based on bin counts
        upperBounds = new double[binCount];
        upperBounds[0] =
        ((double) binStats.get(0).getN()) / (double) sampleStats.getN();
        for (int i = 1; i < binCount-1; i++) {
            upperBounds[i] = upperBounds[i-1] +
            ((double) binStats.get(i).getN()) / (double) sampleStats.getN();
        }
        upperBounds[binCount-1] = 1.0d;
    }

    /**
     * Returns the index of the bin to which the given value belongs
     *
     * @param value  the value whose bin we are trying to find
     * @return the index of the bin containing the value
     */
    private int findBin(double value) {
        return FastMath.min(
                FastMath.max((int) FastMath.ceil((value- min) / delta) - 1, 0),
                binCount - 1);
        }

    /**
     * Generates a random value from this distribution.
     *
     * @return the random value.
     * @throws IllegalStateException if the distribution has not been loaded
     */
    public double getNextValue() throws IllegalStateException {

        if (!loaded) {
            throw MathRuntimeException.createIllegalStateException(LocalizedFormats.DISTRIBUTION_NOT_LOADED);
        }

        // Start with a uniformly distributed random number in (0,1)
        double x = FastMath.random();

        // Use this to select the bin and generate a Gaussian within the bin
        for (int i = 0; i < binCount; i++) {
           if (x <= upperBounds[i]) {
               SummaryStatistics stats = binStats.get(i);
               if (stats.getN() > 0) {
                   if (stats.getStandardDeviation() > 0) {  // more than one obs
                        return randomData.nextGaussian
                            (stats.getMean(),stats.getStandardDeviation());
                   } else {
                       return stats.getMean(); // only one obs in bin
                   }
               }
           }
        }
        throw new MathRuntimeException(LocalizedFormats.NO_BIN_SELECTED);
    }

    /**
     * Returns a {@link StatisticalSummary} describing this distribution.
     * <strong>Preconditions:</strong><ul>
     * <li>the distribution must be loaded before invoking this method</li></ul>
     *
     * @return the sample statistics
     * @throws IllegalStateException if the distribution has not been loaded
     */
    public StatisticalSummary getSampleStats() {
        return sampleStats;
    }

    /**
     * Returns the number of bins.
     *
     * @return the number of bins.
     */
    public int getBinCount() {
        return binCount;
    }

    /**
     * Returns a List of {@link SummaryStatistics} instances containing
     * statistics describing the values in each of the bins.  The list is
     * indexed on the bin number.
     *
     * @return List of bin statistics.
     */
    public List<SummaryStatistics> getBinStats() {
        return binStats;
    }

    /**
     * <p>Returns a fresh copy of the array of upper bounds for the bins.
     * Bins are: <br/>
     * [min,upperBounds[0]],(upperBounds[0],upperBounds[1]],...,
     *  (upperBounds[binCount-2], upperBounds[binCount-1] = max].</p>
     *
     * <p>Note: In versions 1.0-2.0 of commons-math, this method
     * incorrectly returned the array of probability generator upper
     * bounds now returned by {@link #getGeneratorUpperBounds()}.</p>
     *
     * @return array of bin upper bounds
     * @since 2.1
     */
    public double[] getUpperBounds() {
        double[] binUpperBounds = new double[binCount];
        binUpperBounds[0] = min + delta;
        for (int i = 1; i < binCount - 1; i++) {
            binUpperBounds[i] = binUpperBounds[i-1] + delta;
        }
        binUpperBounds[binCount - 1] = max;
        return binUpperBounds;
    }

    /**
     * <p>Returns a fresh copy of the array of upper bounds of the subintervals
     * of [0,1] used in generating data from the empirical distribution.
     * Subintervals correspond to bins with lengths proportional to bin counts.</p>
     *
     * <p>In versions 1.0-2.0 of commons-math, this array was (incorrectly) returned
     * by {@link #getUpperBounds()}.</p>
     *
     * @since 2.1
     * @return array of upper bounds of subintervals used in data generation
     */
    public double[] getGeneratorUpperBounds() {
        int len = upperBounds.length;
        double[] out = new double[len];
        System.arraycopy(upperBounds, 0, out, 0, len);
        return out;
    }

    /**
     * Property indicating whether or not the distribution has been loaded.
     *
     * @return true if the distribution has been loaded
     */
    public boolean isLoaded() {
        return loaded;
    }
}