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/*
 * 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.runtime.state.heap;

import org.apache.flink.api.common.functions.ReduceFunction;
import org.apache.flink.api.common.state.ReducingState;
import org.apache.flink.api.common.state.ReducingStateDescriptor;
import org.apache.flink.api.common.state.State;
import org.apache.flink.api.common.state.StateDescriptor;
import org.apache.flink.api.common.typeutils.TypeSerializer;
import org.apache.flink.runtime.state.StateTransformationFunction;
import org.apache.flink.runtime.state.internal.InternalReducingState;
import org.apache.flink.util.Preconditions;

import java.io.IOException;

/**
 * Heap-backed partitioned {@link ReducingState} that is snapshotted into files.
 *
 * @param  The type of the key.
 * @param  The type of the namespace.
 * @param  The type of the value.
 */
class HeapReducingState extends AbstractHeapMergingState
        implements InternalReducingState {

    private final ReduceTransformation reduceTransformation;

    /**
     * Creates a new key/value state for the given hash map of key/value pairs.
     *
     * @param stateTable The state table for which this state is associated to.
     * @param keySerializer The serializer for the keys.
     * @param valueSerializer The serializer for the state.
     * @param namespaceSerializer The serializer for the namespace.
     * @param defaultValue The default value for the state.
     * @param reduceFunction The reduce function used for reducing state.
     */
    private HeapReducingState(
            StateTable stateTable,
            TypeSerializer keySerializer,
            TypeSerializer valueSerializer,
            TypeSerializer namespaceSerializer,
            V defaultValue,
            ReduceFunction reduceFunction) {

        super(stateTable, keySerializer, valueSerializer, namespaceSerializer, defaultValue);
        this.reduceTransformation = new ReduceTransformation<>(reduceFunction);
    }

    @Override
    public TypeSerializer getKeySerializer() {
        return keySerializer;
    }

    @Override
    public TypeSerializer getNamespaceSerializer() {
        return namespaceSerializer;
    }

    @Override
    public TypeSerializer getValueSerializer() {
        return valueSerializer;
    }

    // ------------------------------------------------------------------------
    //  state access
    // ------------------------------------------------------------------------

    @Override
    public V get() {
        return getInternal();
    }

    @Override
    public void add(V value) throws IOException {

        if (value == null) {
            clear();
            return;
        }

        try {
            stateTable.transform(currentNamespace, value, reduceTransformation);
        } catch (Exception e) {
            throw new IOException("Exception while applying ReduceFunction in reducing state", e);
        }
    }

    // ------------------------------------------------------------------------
    //  state merging
    // ------------------------------------------------------------------------

    @Override
    protected V mergeState(V a, V b) throws Exception {
        return reduceTransformation.apply(a, b);
    }

    static final class ReduceTransformation implements StateTransformationFunction {

        private final ReduceFunction reduceFunction;

        ReduceTransformation(ReduceFunction reduceFunction) {
            this.reduceFunction = Preconditions.checkNotNull(reduceFunction);
        }

        @Override
        public V apply(V previousState, V value) throws Exception {
            return previousState != null ? reduceFunction.reduce(previousState, value) : value;
        }
    }

    @SuppressWarnings("unchecked")
    static  IS create(
            StateDescriptor stateDesc,
            StateTable stateTable,
            TypeSerializer keySerializer) {
        return (IS)
                new HeapReducingState<>(
                        stateTable,
                        keySerializer,
                        stateTable.getStateSerializer(),
                        stateTable.getNamespaceSerializer(),
                        stateDesc.getDefaultValue(),
                        ((ReducingStateDescriptor) stateDesc).getReduceFunction());
    }
}