/*
 * 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.flink.connector.datagen.functions;

import org.apache.flink.annotation.Internal;
import org.apache.flink.annotation.VisibleForTesting;
import org.apache.flink.api.common.ExecutionConfig;
import org.apache.flink.api.common.typeinfo.TypeInformation;
import org.apache.flink.api.common.typeutils.TypeSerializer;
import org.apache.flink.api.connector.source.SourceReaderContext;
import org.apache.flink.connector.datagen.source.GeneratorFunction;
import org.apache.flink.core.memory.DataInputView;
import org.apache.flink.core.memory.DataInputViewStreamWrapper;
import org.apache.flink.core.memory.DataOutputViewStreamWrapper;
import org.apache.flink.util.Preconditions;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.annotation.Nullable;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.EOFException;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.NoSuchElementException;

/**
 * A stream generator function that returns elements from the collection based on their index.
 *
 * <p>This generator function serializes the elements using Flink's type information. That way, any
 * object transport using Java serialization will not be affected by the serializability of the
 * elements.
 *
 * @param <OUT> The type of elements returned by this function.
 */
@Internal
public class IndexLookupGeneratorFunction<OUT> implements GeneratorFunction<Long, OUT> {

    private static final long serialVersionUID = 1L;

    private static final Logger LOG = LoggerFactory.getLogger(IndexLookupGeneratorFunction.class);

    /** The (de)serializer to be used for the data elements. */
    private final TypeSerializer<OUT> serializer;

    /** The actual data elements, in serialized form. */
    private byte[] elementsSerialized;

    private int numElements;

    private transient DataInputView input;

    private transient Map<Long, OUT> lookupMap;

    public IndexLookupGeneratorFunction(TypeInformation<OUT> typeInfo, Iterable<OUT> elements) {
        this(typeInfo, new ExecutionConfig(), elements);
    }

    public IndexLookupGeneratorFunction(
            TypeInformation<OUT> typeInfo, ExecutionConfig config, Iterable<OUT> elements) {
        // must not have null elements and mixed elements
        checkIterable(elements, typeInfo.getTypeClass());
        this.serializer = typeInfo.createSerializer(config.getSerializerConfig());
        trySerialize(elements);
    }

    @VisibleForTesting
    @Nullable
    public TypeSerializer<OUT> getSerializer() {
        return serializer;
    }

    @Override
    public void open(SourceReaderContext readerContext) throws Exception {
        ByteArrayInputStream bais = new ByteArrayInputStream(elementsSerialized);
        this.input = new DataInputViewStreamWrapper(bais);
        lookupMap = new HashMap<>();
        buildLookup();
    }

    @Override
    public OUT map(Long index) throws Exception {
        return lookupMap.get(index);
    }

    /**
     * Verifies that all elements in the iterable are non-null, and are of the given class, or a
     * subclass thereof.
     *
     * @param elements The iterable to check.
     * @param viewedAs The class to which the elements must be assignable to.
     */
    private void checkIterable(Iterable<OUT> elements, Class<?> viewedAs) {
        for (OUT elem : elements) {
            numElements++;
            if (elem == null) {
                throw new IllegalArgumentException("The collection contains a null element");
            }

            if (!viewedAs.isAssignableFrom(elem.getClass())) {
                throw new IllegalArgumentException(
                        "The elements in the collection are not all subclasses of "
                                + viewedAs.getCanonicalName());
            }
        }
    }

    private void serializeElements(Iterable<OUT> elements) throws IOException {
        Preconditions.checkState(serializer != null, "serializer not set");
        LOG.info("Serializing elements using  {}", serializer);
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        DataOutputViewStreamWrapper wrapper = new DataOutputViewStreamWrapper(baos);

        try {
            for (OUT element : elements) {
                serializer.serialize(element, wrapper);
            }
        } catch (Exception e) {
            throw new IOException("Serializing the source elements failed: " + e.getMessage(), e);
        }
        this.elementsSerialized = baos.toByteArray();
    }

    private OUT tryDeserialize() throws IOException {
        try {
            return serializer.deserialize(input);
        } catch (EOFException eof) {
            throw new NoSuchElementException(
                    "Reached the end of the collection. This could be caused by issues with the "
                            + "serializer or by calling the map() function more times than there "
                            + "are elements in the collection. Make sure that you set the number "
                            + "of records to be produced by the DataGeneratorSource equal to the "
                            + "number of elements in the collection.");
        } catch (Exception e) {
            throw new IOException(
                    "Failed to deserialize an element from the source. "
                            + "If you are using user-defined serialization (Value and Writable "
                            + "types), check the serialization functions.\nSerializer is "
                            + serializer,
                    e);
        }
    }

    private void buildLookup() throws IOException {
        for (long i = 0; i < numElements; i++) {
            lookupMap.put(i, tryDeserialize());
        }
    }

    private void trySerialize(Iterable<OUT> elements) {
        try {
            serializeElements(elements);
        } catch (IOException e) {
            throw new RuntimeException(e.getMessage(), e);
        }
    }
}
