Converter.java

/*
 * This file is part of NucleusFramework for Bukkit, licensed under the MIT License (MIT).
 *
 * Copyright (c) JCThePants (www.jcwhatever.com)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

package com.jcwhatever.nucleus.utils.converters;

import com.jcwhatever.nucleus.utils.PreCon;

import java.util.HashSet;
import java.util.Set;
import javax.annotation.Nullable;

/**
 * Abstract implementation of a value converter.
 */
public abstract class Converter<T> implements IConverter<Object, T> {

    private ConverterTracker _tracker;

    /**
     * Convert a value.
     *
     * @param value  The value to convert.
     *
     * @return  The converted value or null if failed.
     */
    @Override
    @Nullable
    public final T convert(@Nullable Object value) {

        if (value instanceof ConverterTracker) {
            _tracker = (ConverterTracker) value;

            if (_tracker.hasConverter(this))
                return null;

            _tracker.addConverter(this);
            value = _tracker.getValue();
        } else {
            _tracker = new ConverterTracker(this, value);
        }

        T result = onConvert(value);

        _tracker = null;

        return result;
    }

    /**
     * Invoked to convert a value.
     *
     * @param value  The value to convert.
     *
     * @return  The converted value or null if conversion failed.
     */
    @Nullable
    protected abstract T onConvert(@Nullable Object value);

    /**
     * Invoke to convert a value using another converter.
     *
     * <p>Prevents infinite loops by tracking which converters have already been used.</p>
     *
     * @param externalConverter  The external converter to use.
     * @param value              The value to convert.
     *
     * @param <V>  The value type.
     *
     * @return  The converted value or null if failed.
     */
    @Nullable
    protected final <V> V callConvert(Converter<V> externalConverter, Object value) {
        ConverterTracker tracker = new ConverterTracker(this, value, _tracker);
        return externalConverter.convert(tracker);
    }

    private static class ConverterTracker {

        private ConverterTracker _parentContainer;
        private Converter<?> _parentConverter;
        private Set<Converter<?>> _converters;
        private Object _value;

        /**
         * Constructor.
         *
         * @param parentConverter  The converter creating the instance.
         * @param value            The value
         */
        ConverterTracker(Converter<?> parentConverter, @Nullable Object value) {
            this(parentConverter, value, null);
        }

        /**
         * Constructor.
         *
         * @param parentConverter  The converter creating the instance.
         * @param value            The value.
         * @param container        The parent container.
         */
        ConverterTracker(Converter<?> parentConverter, @Nullable Object value,
                                        @Nullable ConverterTracker container) {
            PreCon.notNull(parentConverter);

            _parentContainer = container;
            _value = value;

            addConverter(parentConverter);
        }

        /**
         * Get the value.
         */
        @Nullable
        public Object getValue() {
            return _value;
        }

        /**
         * Determine if a converter is already in the hierarchy of converters
         * that have handled the value conversion path.
         *
         * @param converter  The converter to check.
         */
        public boolean hasConverter(Converter<?> converter) {
            PreCon.notNull(converter);

            if (converter == _parentConverter)
                return true;

            ConverterTracker topContainer = this;
            while (topContainer._parentContainer != null) {
                topContainer = topContainer._parentContainer;
            }

            return topContainer._converters != null && topContainer._converters.contains(converter);
        }

        void addConverter(Converter<?> converter) {

            // set parent converter if one is not set
            if (_parentConverter == null)
                _parentConverter = converter;

            // use parent container values if parent container exists
            ConverterTracker topContainer = this;
            while (topContainer._parentContainer != null) {
                topContainer = topContainer._parentContainer;
            }

            // add to list of converters
            if (topContainer._converters == null)
                topContainer._converters = new HashSet<>(5);

            topContainer._converters.add(converter);
        }
    }
}