/*
 * 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.api.java.typeutils.runtime;

import org.apache.flink.annotation.Internal;
import org.apache.flink.api.common.serialization.SerializerConfig;
import org.apache.flink.api.common.typeutils.TypeSerializer;
import org.apache.flink.api.java.typeutils.TypeExtractor;
import org.apache.flink.core.memory.DataInputView;
import org.apache.flink.core.memory.DataOutputView;
import org.apache.flink.util.CollectionUtil;

import javax.annotation.Nullable;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.Arrays;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Objects;
import java.util.stream.Collectors;

import static org.apache.flink.util.Preconditions.checkNotNull;

@Internal
public final class PojoSerializer<T> extends TypeSerializer<T> {

    // Flags for the header
    private static byte IS_NULL = 1;
    private static byte NO_SUBCLASS = 2;
    private static byte IS_SUBCLASS = 4;
    private static byte IS_TAGGED_SUBCLASS = 8;

    private static final long serialVersionUID = 1L;

    // --------------------------------------------------------------------------------------------
    // PojoSerializer parameters
    // --------------------------------------------------------------------------------------------

    /** The POJO type class. */
    private final Class<T> clazz;

    /**
     * Fields of the POJO and their serializers.
     *
     * <p>The fields are kept as a separate transient member, with their serialization handled with
     * the {@link #readObject(ObjectInputStream)} and {@link #writeObject(ObjectOutputStream)}
     * methods.
     */
    private transient Field[] fields;

    private final TypeSerializer<Object>[] fieldSerializers;
    private final int numFields;

    /**
     * Registered subclasses and their serializers. Each subclass to their registered class tag is
     * maintained as a separate map ordered by the class tag.
     */
    private final LinkedHashMap<Class<?>, Integer> registeredClasses;

    private final TypeSerializer<?>[] registeredSerializers;

    /** Cache of non-registered subclasses to their serializers, created on-the-fly. */
    private transient Map<Class<?>, TypeSerializer<?>> subclassSerializerCache;

    // --------------------------------------------------------------------------------------------

    /** Configuration of the current execution. */
    private final SerializerConfig serializerConfig;

    private transient ClassLoader cl;

    @Nullable private transient JavaRecordBuilderFactory<T> recordFactory;

    /** Constructor to create a new {@link PojoSerializer}. */
    @SuppressWarnings("unchecked")
    public PojoSerializer(
            Class<T> clazz,
            TypeSerializer<?>[] fieldSerializers,
            Field[] fields,
            SerializerConfig serializerConfig) {

        this.clazz = checkNotNull(clazz);
        this.fieldSerializers = (TypeSerializer<Object>[]) checkNotNull(fieldSerializers);
        this.fields = checkNotNull(fields);
        this.numFields = fieldSerializers.length;
        this.serializerConfig = checkNotNull(serializerConfig);

        for (int i = 0; i < numFields; i++) {
            this.fields[i].setAccessible(true);
        }

        this.cl = Thread.currentThread().getContextClassLoader();

        // We only want those classes that are not our own class and are actually sub-classes.
        LinkedHashSet<Class<?>> registeredSubclasses =
                getRegisteredSubclassesFromSerializerConfig(clazz, serializerConfig);

        this.registeredClasses = createRegisteredSubclassTags(registeredSubclasses);
        this.registeredSerializers =
                createRegisteredSubclassSerializers(registeredSubclasses, serializerConfig);

        this.subclassSerializerCache = new HashMap<>();
        if (TypeExtractor.isRecord(clazz)) {
            this.recordFactory = JavaRecordBuilderFactory.create(clazz, fields);
        }
    }

    /**
     * Constructor to create a restore serializer or a reconfigured serializer from a {@link
     * PojoSerializerSnapshot}.
     */
    PojoSerializer(
            Class<T> clazz,
            Field[] fields,
            TypeSerializer<Object>[] fieldSerializers,
            LinkedHashMap<Class<?>, Integer> registeredClasses,
            TypeSerializer<?>[] registeredSerializers,
            Map<Class<?>, TypeSerializer<?>> subclassSerializerCache,
            SerializerConfig serializerConfig) {

        this.clazz = checkNotNull(clazz);
        this.fields = checkNotNull(fields);
        this.numFields = fields.length;
        this.fieldSerializers = checkNotNull(fieldSerializers);
        this.registeredClasses = checkNotNull(registeredClasses);
        this.registeredSerializers = checkNotNull(registeredSerializers);
        this.subclassSerializerCache = checkNotNull(subclassSerializerCache);
        this.serializerConfig = checkNotNull(serializerConfig);
        this.cl = Thread.currentThread().getContextClassLoader();
        if (TypeExtractor.isRecord(clazz)) {
            this.recordFactory = JavaRecordBuilderFactory.create(clazz, fields);
        }
    }

    @Override
    public boolean isImmutableType() {
        return false;
    }

    private boolean isRecord() {
        return this.recordFactory != null;
    }

    @Override
    public PojoSerializer<T> duplicate() {
        TypeSerializer<Object>[] duplicateFieldSerializers = duplicateSerializers(fieldSerializers);
        TypeSerializer<Object>[] duplicateRegisteredSerializers =
                duplicateSerializers(registeredSerializers);

        return new PojoSerializer<>(
                clazz,
                fields,
                duplicateFieldSerializers,
                new LinkedHashMap<>(registeredClasses),
                duplicateRegisteredSerializers,
                subclassSerializerCache.entrySet().stream()
                        .collect(
                                Collectors.toMap(Map.Entry::getKey, e -> e.getValue().duplicate())),
                serializerConfig);
    }

    @SuppressWarnings("unchecked")
    private TypeSerializer<Object>[] duplicateSerializers(TypeSerializer<?>[] serializers) {
        boolean stateful = false;
        TypeSerializer<?>[] duplicateSerializers = new TypeSerializer[serializers.length];

        for (int i = 0; i < serializers.length; i++) {
            duplicateSerializers[i] = serializers[i].duplicate();
            if (duplicateSerializers[i] != serializers[i]) {
                // at least one of them is stateful
                stateful = true;
            }
        }

        if (!stateful) {
            // as a small memory optimization, we can share the same object between instances
            duplicateSerializers = serializers;
        }
        return (TypeSerializer<Object>[]) duplicateSerializers;
    }

    @Override
    public T createInstance() {
        if (clazz.isInterface() || Modifier.isAbstract(clazz.getModifiers()) || isRecord()) {
            return null;
        }
        try {
            T t = clazz.newInstance();
            initializeFields(t);
            return t;
        } catch (Exception e) {
            throw new RuntimeException("Cannot instantiate class.", e);
        }
    }

    protected void initializeFields(T t) {
        for (int i = 0; i < numFields; i++) {
            if (fields[i] != null) {
                try {
                    fields[i].set(t, fieldSerializers[i].createInstance());
                } catch (IllegalAccessException e) {
                    throw new RuntimeException("Cannot initialize fields.", e);
                }
            }
        }
    }

    @Override
    @SuppressWarnings({"unchecked", "rawtypes"})
    public T copy(T from) {
        if (from == null) {
            return null;
        }

        Class<?> actualType = from.getClass();
        if (isRecord()) {
            try {
                JavaRecordBuilderFactory<T>.JavaRecordBuilder builder = recordFactory.newBuilder();
                for (int i = 0; i < numFields; i++) {
                    if (fields[i] != null) {
                        builder.setField(i, copyField(i, from));
                    }
                }
                return builder.build();
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.");
            }
        } else if (actualType == clazz) {
            T target;
            try {
                target = (T) from.getClass().newInstance();
            } catch (Throwable t) {
                throw new RuntimeException("Cannot instantiate class.", t);
            }
            // no subclass
            try {
                for (int i = 0; i < numFields; i++) {
                    if (fields[i] != null) {
                        fields[i].set(target, copyField(i, from));
                    }
                }
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.");
            }
            return target;
        } else {
            // subclass
            TypeSerializer subclassSerializer = getSubclassSerializer(actualType);
            return (T) subclassSerializer.copy(from);
        }
    }

    @Override
    @SuppressWarnings({"unchecked", "rawtypes"})
    public T copy(T from, T reuse) {
        if (from == null) {
            return null;
        }

        Class<?> actualType = from.getClass();
        if (reuse == null || actualType != reuse.getClass()) {
            // cannot reuse, do a non-reuse copy
            return copy(from);
        }

        if (isRecord()) {
            try {
                JavaRecordBuilderFactory<T>.JavaRecordBuilder builder = recordFactory.newBuilder();
                for (int i = 0; i < numFields; i++) {
                    if (fields[i] != null) {
                        builder.setField(i, copyField(reuse, i, from));
                    }
                }
                return builder.build();
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.");
            }
        } else if (actualType == clazz) {
            try {
                for (int i = 0; i < numFields; i++) {
                    if (fields[i] != null) {
                        fields[i].set(reuse, copyField(reuse, i, from));
                    }
                }
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.",
                        e);
            }
        } else {
            TypeSerializer subclassSerializer = getSubclassSerializer(actualType);
            reuse = (T) subclassSerializer.copy(from, reuse);
        }

        return reuse;
    }

    private Object copyField(int i, Object from) throws IllegalAccessException {
        Object value = fields[i].get(from);
        if (value != null) {
            return fieldSerializers[i].copy(value);
        } else {
            return null;
        }
    }

    private Object copyField(T reuse, int i, Object from) throws IllegalAccessException {
        Object value = fields[i].get(from);
        if (value != null) {
            Object reuseValue = fields[i].get(reuse);

            if (reuseValue != null) {
                return fieldSerializers[i].copy(value, reuseValue);
            } else {
                return fieldSerializers[i].copy(value);
            }
        } else {
            return null;
        }
    }

    @Override
    public int getLength() {
        return -1;
    }

    @Override
    @SuppressWarnings({"unchecked", "rawtypes"})
    public void serialize(T value, DataOutputView target) throws IOException {
        int flags = 0;
        // handle null values
        if (value == null) {
            flags |= IS_NULL;
            target.writeByte(flags);
            return;
        }

        Integer subclassTag = -1;
        Class<?> actualClass = value.getClass();
        TypeSerializer subclassSerializer = null;
        if (clazz != actualClass) {
            subclassTag = registeredClasses.get(actualClass);
            if (subclassTag != null) {
                flags |= IS_TAGGED_SUBCLASS;
                subclassSerializer = registeredSerializers[subclassTag];
            } else {
                flags |= IS_SUBCLASS;
                subclassSerializer = getSubclassSerializer(actualClass);
            }
        } else {
            flags |= NO_SUBCLASS;
        }

        target.writeByte(flags);

        // if its a registered subclass, write the class tag id, otherwise write the full classname
        if ((flags & IS_SUBCLASS) != 0) {
            target.writeUTF(actualClass.getName());
        } else if ((flags & IS_TAGGED_SUBCLASS) != 0) {
            target.writeByte(subclassTag);
        }

        // if its a subclass, use the corresponding subclass serializer,
        // otherwise serialize each field with our field serializers
        if ((flags & NO_SUBCLASS) != 0) {
            try {
                for (int i = 0; i < numFields; i++) {
                    Object o = (fields[i] != null) ? fields[i].get(value) : null;
                    if (o == null) {
                        target.writeBoolean(true); // null field handling
                    } else {
                        target.writeBoolean(false);
                        fieldSerializers[i].serialize(o, target);
                    }
                }
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.",
                        e);
            }
        } else {
            // subclass
            if (subclassSerializer != null) {
                subclassSerializer.serialize(value, target);
            }
        }
    }

    @Override
    @SuppressWarnings({"unchecked", "rawtypes"})
    public T deserialize(DataInputView source) throws IOException {
        int flags = source.readByte();
        if ((flags & IS_NULL) != 0) {
            return null;
        }

        T target;

        Class<?> actualSubclass = null;
        TypeSerializer subclassSerializer = null;

        if ((flags & IS_SUBCLASS) != 0) {
            String subclassName = source.readUTF();
            try {
                actualSubclass = Class.forName(subclassName, true, cl);
            } catch (ClassNotFoundException e) {
                throw new RuntimeException("Cannot instantiate class.", e);
            }
            subclassSerializer = getSubclassSerializer(actualSubclass);
            target = (T) subclassSerializer.createInstance();
            // also initialize fields for which the subclass serializer is not responsible
            initializeFields(target);
        } else if ((flags & IS_TAGGED_SUBCLASS) != 0) {

            int subclassTag = source.readByte();
            subclassSerializer = registeredSerializers[subclassTag];
            target = (T) subclassSerializer.createInstance();
            // also initialize fields for which the subclass serializer is not responsible
            initializeFields(target);
        } else {
            target = createInstance();
        }

        if (isRecord()) {
            JavaRecordBuilderFactory<T>.JavaRecordBuilder builder = recordFactory.newBuilder();
            for (int i = 0; i < numFields; i++) {
                boolean isNull = source.readBoolean();
                Object fieldValue = isNull ? null : fieldSerializers[i].deserialize(source);
                if (fields[i] != null) {
                    builder.setField(i, fieldValue);
                }
            }
            target = builder.build();
        } else if ((flags & NO_SUBCLASS) != 0) {
            try {
                for (int i = 0; i < numFields; i++) {
                    boolean isNull = source.readBoolean();
                    Object fieldValue = isNull ? null : fieldSerializers[i].deserialize(source);
                    if (fields[i] != null) {
                        fields[i].set(target, fieldValue);
                    }
                }
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.",
                        e);
            }
        } else {
            if (subclassSerializer != null) {
                target = (T) subclassSerializer.deserialize(target, source);
            }
        }
        return target;
    }

    @Override
    @SuppressWarnings({"unchecked", "rawtypes"})
    public T deserialize(T reuse, DataInputView source) throws IOException {

        // handle null values
        int flags = source.readByte();
        if ((flags & IS_NULL) != 0) {
            return null;
        }

        Class<?> subclass = null;
        TypeSerializer subclassSerializer = null;
        if ((flags & IS_SUBCLASS) != 0) {
            String subclassName = source.readUTF();
            try {
                subclass = Class.forName(subclassName, true, cl);
            } catch (ClassNotFoundException e) {
                throw new RuntimeException("Cannot instantiate class.", e);
            }
            subclassSerializer = getSubclassSerializer(subclass);

            if (reuse == null || subclass != reuse.getClass()) {
                // cannot reuse
                reuse = (T) subclassSerializer.createInstance();
                // also initialize fields for which the subclass serializer is not responsible
                initializeFields(reuse);
            }
        } else if ((flags & IS_TAGGED_SUBCLASS) != 0) {
            int subclassTag = source.readByte();
            subclassSerializer = registeredSerializers[subclassTag];

            if (reuse == null || ((PojoSerializer) subclassSerializer).clazz != reuse.getClass()) {
                // cannot reuse
                reuse = (T) subclassSerializer.createInstance();
                // also initialize fields for which the subclass serializer is not responsible
                initializeFields(reuse);
            }
        } else {
            if (reuse == null || clazz != reuse.getClass()) {
                reuse = createInstance();
            }
        }

        if (isRecord()) {
            try {
                JavaRecordBuilderFactory<T>.JavaRecordBuilder builder = recordFactory.newBuilder();
                for (int i = 0; i < numFields; i++) {
                    boolean isNull = source.readBoolean();

                    if (fields[i] != null) {
                        if (isNull) {
                            builder.setField(i, null);
                        } else {
                            Object reuseField = reuse == null ? null : fields[i].get(reuse);
                            builder.setField(i, deserializeField(reuseField, i, source));
                        }
                    } else if (!isNull) {
                        // read and dump a pre-existing field value
                        fieldSerializers[i].deserialize(source);
                    }
                }

                reuse = builder.build();
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.",
                        e);
            }
        } else if ((flags & NO_SUBCLASS) != 0) {
            try {
                for (int i = 0; i < numFields; i++) {
                    boolean isNull = source.readBoolean();

                    if (fields[i] != null) {
                        if (isNull) {
                            fields[i].set(reuse, null);
                        } else {
                            fields[i].set(reuse, deserializeField(fields[i].get(reuse), i, source));
                        }
                    } else if (!isNull) {
                        // read and dump a pre-existing field value
                        fieldSerializers[i].deserialize(source);
                    }
                }
            } catch (IllegalAccessException e) {
                throw new RuntimeException(
                        "Error during POJO copy, this should not happen since we check the fields before.",
                        e);
            }
        } else {
            if (subclassSerializer != null) {
                reuse = (T) subclassSerializer.deserialize(reuse, source);
            }
        }

        return reuse;
    }

    private Object deserializeField(Object reuseField, int i, DataInputView source)
            throws IllegalAccessException, IOException {
        if (reuseField != null) {
            return fieldSerializers[i].deserialize(reuseField, source);
        } else {
            return fieldSerializers[i].deserialize(source);
        }
    }

    @Override
    public void copy(DataInputView source, DataOutputView target) throws IOException {
        // copy the flags
        int flags = source.readByte();
        target.writeByte(flags);

        if ((flags & IS_NULL) != 0) {
            // is a null value, nothing further to copy
            return;
        }

        TypeSerializer<?> subclassSerializer = null;
        if ((flags & IS_SUBCLASS) != 0) {
            String className = source.readUTF();
            target.writeUTF(className);
            try {
                Class<?> subclass =
                        Class.forName(
                                className, true, Thread.currentThread().getContextClassLoader());
                subclassSerializer = getSubclassSerializer(subclass);
            } catch (ClassNotFoundException e) {
                throw new RuntimeException("Cannot instantiate class.", e);
            }
        } else if ((flags & IS_TAGGED_SUBCLASS) != 0) {
            int subclassTag = source.readByte();
            target.writeByte(subclassTag);
            subclassSerializer = registeredSerializers[subclassTag];
        }

        if ((flags & NO_SUBCLASS) != 0) {
            for (int i = 0; i < numFields; i++) {
                boolean isNull = source.readBoolean();
                target.writeBoolean(isNull);
                if (!isNull) {
                    fieldSerializers[i].copy(source, target);
                }
            }
        } else {
            if (subclassSerializer != null) {
                subclassSerializer.copy(source, target);
            }
        }
    }

    @Override
    public int hashCode() {
        return 31
                        * (31 * Arrays.hashCode(fieldSerializers)
                                + Arrays.hashCode(registeredSerializers))
                + Objects.hash(clazz, numFields, registeredClasses);
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof PojoSerializer) {
            PojoSerializer<?> other = (PojoSerializer<?>) obj;

            return clazz == other.clazz
                    && Arrays.equals(fieldSerializers, other.fieldSerializers)
                    && Arrays.equals(registeredSerializers, other.registeredSerializers)
                    && numFields == other.numFields
                    && registeredClasses.equals(other.registeredClasses);
        } else {
            return false;
        }
    }

    // --------------------------------------------------------------------------------------------
    // Serializer configuration snapshotting & compatibility
    // --------------------------------------------------------------------------------------------

    @Override
    public PojoSerializerSnapshot<T> snapshotConfiguration() {
        return buildSnapshot(
                clazz,
                registeredClasses,
                registeredSerializers,
                fields,
                fieldSerializers,
                subclassSerializerCache,
                serializerConfig);
    }

    // --------------------------------------------------------------------------------------------

    private void writeObject(ObjectOutputStream out) throws IOException, ClassNotFoundException {
        out.defaultWriteObject();
        out.writeInt(fields.length);
        for (Field field : fields) {
            FieldSerializer.serializeField(field, out);
        }
    }

    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        int numFields = in.readInt();
        fields = new Field[numFields];
        for (int i = 0; i < numFields; i++) {
            // the deserialized Field may be null if the field no longer exists in the POJO;
            // in this case, when de-/serializing and copying instances using this serializer
            // instance, the missing fields will simply be skipped
            fields[i] = FieldSerializer.deserializeField(in);
        }

        cl = Thread.currentThread().getContextClassLoader();
        subclassSerializerCache = new HashMap<>();
        if (TypeExtractor.isRecord(clazz)) {
            this.recordFactory = JavaRecordBuilderFactory.create(clazz, fields);
        }
    }

    // --------------------------------------------------------------------------------------------
    // Configuration access
    // --------------------------------------------------------------------------------------------

    Class<T> getPojoClass() {
        return clazz;
    }

    Field[] getFields() {
        return fields;
    }

    TypeSerializer<?>[] getFieldSerializers() {
        return fieldSerializers;
    }

    TypeSerializer<?> getFieldSerializer(Field targetField) {
        int fieldIndex = findField(targetField.getName());
        if (fieldIndex == -1) {
            return null;
        }
        return fieldSerializers[fieldIndex];
    }

    SerializerConfig getSerializerConfig() {
        return serializerConfig;
    }

    LinkedHashMap<Class<?>, Integer> getRegisteredClasses() {
        return registeredClasses;
    }

    TypeSerializer<?>[] getRegisteredSerializers() {
        return registeredSerializers;
    }

    LinkedHashMap<Class<?>, TypeSerializer<?>> getBundledSubclassSerializerRegistry() {
        final LinkedHashMap<Class<?>, TypeSerializer<?>> result =
                CollectionUtil.newLinkedHashMapWithExpectedSize(registeredClasses.size());
        registeredClasses.forEach(
                (registeredClass, id) -> result.put(registeredClass, registeredSerializers[id]));
        return result;
    }

    Map<Class<?>, TypeSerializer<?>> getSubclassSerializerCache() {
        return subclassSerializerCache;
    }

    // --------------------------------------------------------------------------------------------
    // Utilities
    // --------------------------------------------------------------------------------------------

    /** Extracts the subclasses of the base POJO class registered in the execution config. */
    private static LinkedHashSet<Class<?>> getRegisteredSubclassesFromSerializerConfig(
            Class<?> basePojoClass, SerializerConfig serializerConfig) {

        LinkedHashSet<Class<?>> subclassesInRegistrationOrder =
                CollectionUtil.newLinkedHashSetWithExpectedSize(
                        serializerConfig.getRegisteredPojoTypes().size());
        for (Class<?> registeredClass : serializerConfig.getRegisteredPojoTypes()) {
            if (registeredClass.equals(basePojoClass)) {
                continue;
            }
            if (!basePojoClass.isAssignableFrom(registeredClass)) {
                continue;
            }
            subclassesInRegistrationOrder.add(registeredClass);
        }

        return subclassesInRegistrationOrder;
    }

    /**
     * Builds map of registered subclasses to their class tags. Class tags will be integers starting
     * from 0, assigned incrementally with the order of provided subclasses.
     */
    private static LinkedHashMap<Class<?>, Integer> createRegisteredSubclassTags(
            LinkedHashSet<Class<?>> registeredSubclasses) {
        final LinkedHashMap<Class<?>, Integer> classToTag = new LinkedHashMap<>();

        int id = 0;
        for (Class<?> registeredClass : registeredSubclasses) {
            classToTag.put(registeredClass, id);
            id++;
        }

        return classToTag;
    }

    /**
     * Creates an array of serializers for provided list of registered subclasses. Order of returned
     * serializers will correspond to order of provided subclasses.
     */
    private static TypeSerializer<?>[] createRegisteredSubclassSerializers(
            LinkedHashSet<Class<?>> registeredSubclasses, SerializerConfig serializerConfig) {

        final TypeSerializer<?>[] subclassSerializers =
                new TypeSerializer[registeredSubclasses.size()];

        int i = 0;
        for (Class<?> registeredClass : registeredSubclasses) {
            subclassSerializers[i] =
                    TypeExtractor.createTypeInfo(registeredClass)
                            .createSerializer(serializerConfig);
            i++;
        }

        return subclassSerializers;
    }

    /**
     * Fetches cached serializer for a non-registered subclass; also creates the serializer if it
     * doesn't exist yet.
     *
     * <p>This method is also exposed to package-private access for testing purposes.
     */
    TypeSerializer<?> getSubclassSerializer(Class<?> subclass) {
        TypeSerializer<?> result = subclassSerializerCache.get(subclass);
        if (result == null) {
            result = createSubclassSerializer(subclass);
            subclassSerializerCache.put(subclass, result);
        }
        return result;
    }

    private TypeSerializer<?> createSubclassSerializer(Class<?> subclass) {
        TypeSerializer<?> serializer =
                TypeExtractor.createTypeInfo(subclass).createSerializer(serializerConfig);

        if (serializer instanceof PojoSerializer) {
            PojoSerializer<?> subclassSerializer = (PojoSerializer<?>) serializer;
            subclassSerializer.copyBaseFieldOrder(this);
        }

        return serializer;
    }

    /**
     * Finds and returns the order (0-based) of a POJO field. Returns -1 if the field does not exist
     * for this POJO.
     */
    private int findField(String fieldName) {
        int foundIndex = 0;
        for (Field field : fields) {
            if (field != null && fieldName.equals(field.getName())) {
                return foundIndex;
            }

            foundIndex++;
        }

        return -1;
    }

    private void copyBaseFieldOrder(PojoSerializer<?> baseSerializer) {
        // do nothing for now, but in the future, adapt subclass serializer to have same
        // ordering as base class serializer so that binary comparison on base class fields
        // can work
    }

    /**
     * Build and return a snapshot of the serializer's parameters and currently cached serializers.
     */
    private static <T> PojoSerializerSnapshot<T> buildSnapshot(
            Class<T> pojoType,
            LinkedHashMap<Class<?>, Integer> registeredSubclassesToTags,
            TypeSerializer<?>[] registeredSubclassSerializers,
            Field[] fields,
            TypeSerializer<?>[] fieldSerializers,
            Map<Class<?>, TypeSerializer<?>> nonRegisteredSubclassSerializerCache,
            SerializerConfig serializerConfig) {

        final LinkedHashMap<Class<?>, TypeSerializer<?>> subclassRegistry =
                CollectionUtil.newLinkedHashMapWithExpectedSize(registeredSubclassesToTags.size());

        for (Map.Entry<Class<?>, Integer> entry : registeredSubclassesToTags.entrySet()) {
            subclassRegistry.put(entry.getKey(), registeredSubclassSerializers[entry.getValue()]);
        }

        return new PojoSerializerSnapshot<>(
                pojoType,
                fields,
                fieldSerializers,
                subclassRegistry,
                nonRegisteredSubclassSerializerCache,
                serializerConfig);
    }
}
