/*
 * 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.commons.configuration2.interpol;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.function.Function;

/**
 * <p>
 * A simple value class defining a {@link ConfigurationInterpolator}.
 * </p>
 * <p>
 * Objects of this class can be used for creating new {@code ConfigurationInterpolator} instances; they contain all
 * required properties. It is either possible to set a fully initialized {@code ConfigurationInterpolator} directly
 * which can be used as is. Alternatively, some or all properties of an instance to be newly created can be set. These
 * properties include
 * </p>
 * <ul>
 * <li>a map with {@code Lookup} objects associated with a specific prefix</li>
 * <li>a collection with default {@code Lookup} objects (without a prefix)</li>
 * <li>a parent {@code ConfigurationInterpolator}</li>
 * <li>a function used to convert interpolated values into strings</li>
 * </ul>
 * <p>
 * When setting up a configuration it is possible to define the {@code ConfigurationInterpolator} in terms of this
 * class. The configuration will then either use the {@code ConfigurationInterpolator} instance explicitly defined in
 * the {@code InterpolatorSpecification} instance or create a new one.
 * </p>
 * <p>
 * Instances are not created directly, but using the nested {@code Builder} class. They are then immutable.
 * </p>
 *
 * @since 2.0
 */
public final class InterpolatorSpecification {
    /** The {@code ConfigurationInterpolator} instance to be used directly. */
    private final ConfigurationInterpolator interpolator;

    /** The parent {@code ConfigurationInterpolator}. */
    private final ConfigurationInterpolator parentInterpolator;

    /** The map with prefix lookups. */
    private final Map<String, Lookup> prefixLookups;

    /** The collection with default lookups. */
    private final Collection<Lookup> defaultLookups;

    /** Function used to convert interpolated values to strings. */
    private final Function<Object, String> stringConverter;

    /**
     * Creates a new instance of {@code InterpolatorSpecification} with the properties defined by the given builder object.
     *
     * @param builder the builder
     */
    private InterpolatorSpecification(final Builder builder) {
        interpolator = builder.interpolator;
        parentInterpolator = builder.parentInterpolator;
        prefixLookups = Collections.unmodifiableMap(new HashMap<>(builder.prefixLookups));
        defaultLookups = Collections.unmodifiableCollection(new ArrayList<>(builder.defLookups));
        stringConverter = builder.stringConverter;
    }

    /**
     * Returns the {@code ConfigurationInterpolator} instance to be used directly.
     *
     * @return the {@code ConfigurationInterpolator} (can be <b>null</b>)
     */
    public ConfigurationInterpolator getInterpolator() {
        return interpolator;
    }

    /**
     * Returns the parent {@code ConfigurationInterpolator} object.
     *
     * @return the parent {@code ConfigurationInterpolator} (can be <b>null</b>)
     */
    public ConfigurationInterpolator getParentInterpolator() {
        return parentInterpolator;
    }

    /**
     * Returns a map with prefix lookups. The keys of the map are the prefix strings, its values are the corresponding
     * {@code Lookup} objects.
     *
     * @return the prefix lookups for a new {@code ConfigurationInterpolator} instance (never <b>null</b>)
     */
    public Map<String, Lookup> getPrefixLookups() {
        return prefixLookups;
    }

    /**
     * Returns a collection with the default lookups.
     *
     * @return the default lookups for a new {@code ConfigurationInterpolator} instance (never <b>null</b>)
     */
    public Collection<Lookup> getDefaultLookups() {
        return defaultLookups;
    }

    /**
     * Returns the function used to convert interpolated values to strings or {@code null}
     * if the default conversion function is to be used.
     *
     * @return function used to convert interpolated values to strings or {@code null} if
     *      the default conversion function is to be used
     */
    public Function<Object, String> getStringConverter() {
        return stringConverter;
    }

    /**
     * <p>
     * A <em>builder</em> class for creating instances of {@code InterpolatorSpecification}.
     * </p>
     * <p>
     * This class provides a fluent API for defining the various properties of an {@code InterpolatorSpecification} object.
     * <em>Note:</em> This builder class is not thread-safe.
     * </p>
     */
    public static class Builder {
        /** A map with prefix lookups. */
        private final Map<String, Lookup> prefixLookups;

        /** A collection with default lookups. */
        private final Collection<Lookup> defLookups;

        /** The {@code ConfigurationInterpolator}. */
        private ConfigurationInterpolator interpolator;

        /** The parent {@code ConfigurationInterpolator}. */
        private ConfigurationInterpolator parentInterpolator;

        /** Function used to convert interpolated values to strings. */
        private Function<Object, String> stringConverter;

        public Builder() {
            prefixLookups = new HashMap<>();
            defLookups = new LinkedList<>();
        }

        /**
         * Adds a {@code Lookup} object for a given prefix.
         *
         * @param prefix the prefix (must not be <b>null</b>)
         * @param lookup the {@code Lookup} (must not be <b>null</b>)
         * @return a reference to this builder for method chaining
         * @throws IllegalArgumentException if a required parameter is missing
         */
        public Builder withPrefixLookup(final String prefix, final Lookup lookup) {
            if (prefix == null) {
                throw new IllegalArgumentException("Prefix must not be null!");
            }
            checkLookup(lookup);
            prefixLookups.put(prefix, lookup);
            return this;
        }

        /**
         * Adds the content of the given map to the prefix lookups managed by this builder. The map can be <b>null</b>, then
         * this method has no effect.
         *
         * @param lookups the map with prefix lookups to be added
         * @return a reference to this builder for method chaining
         * @throws IllegalArgumentException if the map contains <b>null</b> values
         */
        public Builder withPrefixLookups(final Map<String, ? extends Lookup> lookups) {
            if (lookups != null) {
                for (final Map.Entry<String, ? extends Lookup> e : lookups.entrySet()) {
                    withPrefixLookup(e.getKey(), e.getValue());
                }
            }
            return this;
        }

        /**
         * Adds the given {@code Lookup} object to the list of default lookups.
         *
         * @param lookup the {@code Lookup} (must not be <b>null</b>)
         * @return a reference to this builder for method chaining
         * @throws IllegalArgumentException if the {@code Lookup} is <b>null</b>
         */
        public Builder withDefaultLookup(final Lookup lookup) {
            checkLookup(lookup);
            defLookups.add(lookup);
            return this;
        }

        /**
         * Adds the content of the given collection to the default lookups managed by this builder. The collection can be
         * <b>null</b>, then this method has no effect.
         *
         * @param lookups the collection with lookups to be added
         * @return a reference to this builder for method chaining
         * @throws IllegalArgumentException if the collection contains <b>null</b> entries
         */
        public Builder withDefaultLookups(final Collection<? extends Lookup> lookups) {
            if (lookups != null) {
                for (final Lookup l : lookups) {
                    withDefaultLookup(l);
                }
            }
            return this;
        }

        /**
         * Sets the {@code ConfigurationInterpolator} instance for the {@code InterpolatorSpecification}. This means that a
         * {@code ConfigurationInterpolator} has been created and set up externally and can be used directly.
         *
         * @param ci the {@code ConfigurationInterpolator} (can be <b>null</b>)
         * @return a reference to this builder for method chaining
         */
        public Builder withInterpolator(final ConfigurationInterpolator ci) {
            interpolator = ci;
            return this;
        }

        /**
         * Sets an optional parent {@code ConfigurationInterpolator}. If defined, this object is set as parent of a newly
         * created {@code ConfigurationInterpolator} instance.
         *
         * @param parent the parent {@code ConfigurationInterpolator} (can be <b>null</b>)
         * @return a reference to this builder for method chaining
         */
        public Builder withParentInterpolator(final ConfigurationInterpolator parent) {
            parentInterpolator = parent;
            return this;
        }

        /**
         * Sets the function used to convert interpolated values to strings. Pass {@code null}
         * if the default conversion function is to be used.
         *
         * @param fn function used to convert interpolated values to string or {@code null} if the
         *      default conversion function is to be used
         * @return a reference to this builder for method chaining
         */
        public Builder withStringConverter(final Function<Object, String> fn) {
            this.stringConverter = fn;
            return this;
        }


        /**
         * Creates a new {@code InterpolatorSpecification} instance with the properties set so far. After that this builder
         * instance is reset so that it can be reused for creating further specification objects.
         *
         * @return the newly created {@code InterpolatorSpecification}
         */
        public InterpolatorSpecification create() {
            final InterpolatorSpecification spec = new InterpolatorSpecification(this);
            reset();
            return spec;
        }

        /**
         * Removes all data from this builder. Afterwards it can be used to define a brand new {@code InterpolatorSpecification}
         * object.
         */
        public void reset() {
            interpolator = null;
            parentInterpolator = null;
            prefixLookups.clear();
            defLookups.clear();
            stringConverter = null;
        }

        /**
         * Helper method for checking a lookup. Throws an exception if the lookup is <b>null</b>.
         *
         * @param lookup the lookup to be checked
         * @throws IllegalArgumentException if the lookup is <b>null</b>
         */
        private static void checkLookup(final Lookup lookup) {
            if (lookup == null) {
                throw new IllegalArgumentException("Lookup must not be null!");
            }
        }
    }
}
