/*
 * 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.tika.detect;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.Writer;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.ReadableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import org.apache.tika.io.TemporaryResources;
import org.apache.tika.metadata.Metadata;
import org.apache.tika.mime.MediaType;

import static java.nio.charset.StandardCharsets.UTF_8;

public abstract class TrainedModelDetector implements Detector {
    private final Map<MediaType, TrainedModel> MODEL_MAP = new HashMap<>();

    private static final long serialVersionUID = 1L;

    public TrainedModelDetector() {
        loadDefaultModels(getClass().getClassLoader());
    }

    public int getMinLength() {
        return Integer.MAX_VALUE;
    }

    public MediaType detect(InputStream input, Metadata metadata)
            throws IOException {
        // convert to byte-histogram
        if (input != null) {
            input.mark(getMinLength());
            float[] histogram = readByteFrequencies(input);
            // writeHisto(histogram); //on testing purpose
            /*
             * iterate the map to find out the one that gives the higher
             * prediction value.
             */
            Iterator<MediaType> iter = MODEL_MAP.keySet().iterator();
            float threshold = 0.5f;// probability threshold, any value below the
            // threshold will be considered as
            // MediaType.OCTET_STREAM
            float maxprob = threshold;
            MediaType maxType = MediaType.OCTET_STREAM;
            while (iter.hasNext()) {
                MediaType key = iter.next();
                TrainedModel model = MODEL_MAP.get(key);
                float prob = model.predict(histogram);
                if (maxprob < prob) {
                    maxprob = prob;
                    maxType = key;
                }
            }
            input.reset();
            return maxType;
        }
        return null;
    }

    /**
     * Read the {@code inputstream} and build a byte frequency histogram
     *
     * @param input stream to read from
     * @return byte frequencies array
     * @throws IOException
     */
    protected float[] readByteFrequencies(final InputStream input)
            throws IOException {
        ReadableByteChannel inputChannel;
        // TODO: any reason to avoid closing of input & inputChannel?
        try {
            inputChannel = Channels.newChannel(input);
            // long inSize = inputChannel.size();
            float histogram[] = new float[257];
            histogram[0] = 1;

            // create buffer with capacity of maxBufSize bytes
            ByteBuffer buf = ByteBuffer.allocate(1024 * 5);
            int bytesRead = inputChannel.read(buf); // read into buffer.

            float max = -1;
            while (bytesRead != -1) {

                buf.flip(); // make buffer ready for read

                while (buf.hasRemaining()) {
                    byte byt = buf.get();
                    int idx = byt;
                    idx++;
                    if (byt < 0) {
                        idx = 256 + idx;
                        histogram[idx]++;
                    } else {
                        histogram[idx]++;
                    }
                    max = max < histogram[idx] ? histogram[idx] : max;
                }

                buf.clear(); // make buffer ready for writing
                bytesRead = inputChannel.read(buf);
            }

            int i;
            for (i = 1; i < histogram.length; i++) {
                histogram[i] /= max;
                histogram[i] = (float) Math.sqrt(histogram[i]);
            }

            return histogram;
        } finally {
            // inputChannel.close();
        }
    }

    /**
     * for testing purposes; this method write the histogram vector to a file.
     *
     * @param histogram
     * @throws IOException
     */
    private void writeHisto(final float[] histogram)
            throws IOException {
        Path histPath = new TemporaryResources().createTempFile();
        try (Writer writer = Files.newBufferedWriter(histPath, UTF_8)) {
            for (float bin : histogram) {
                writer.write(String.valueOf(bin) + "\t");
                // writer.write(i + "\t");
            }
            writer.write("\r\n");
        }
    }

    public void loadDefaultModels(Path modelFile) {
        try (InputStream in = Files.newInputStream(modelFile)) {
            loadDefaultModels(in);
        } catch (IOException e) {
            throw new RuntimeException("Unable to read the default media type registry", e);
        }
    }

    public void loadDefaultModels(File modelFile) {
        loadDefaultModels(modelFile.toPath());
    }

    public abstract void loadDefaultModels(final InputStream modelStream);

    public abstract void loadDefaultModels(final ClassLoader classLoader);

    protected void registerModels(MediaType type, TrainedModel model) {
        MODEL_MAP.put(type, model);
    }
}