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Library providing immutable/persistent collection classes for Java. While collections are immutable they provide methods for adding and removing values by creating new modified copies of themselves. Each copy shares almost all of its structure with other copies to minimize memory consumption.
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///###////////////////////////////////////////////////////////////////////////
//
// Burton Computer Corporation
// http://www.burton-computer.com
//
// Copyright (c) 2021, Burton Computer Corporation
// All rights reserved.
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package org.javimmutable.collections;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.Immutable;
import java.io.Serializable;
import java.util.Iterator;
import java.util.stream.Collector;

/**
 * Interface for maps that map keys to lists of values.
 */
@Immutable
public interface JImmutableListMap
    extends Insertable, JImmutableListMap>,
            Mapped>,
            IterableStreamable>>,
            InvariantCheckable,
            Serializable
{
    /**
     * Return the list associated with key or an empty list if no list is associated.
     *
     * @param key identifies the value to retrieve
     * @return list associated with key or an empty list if no value is associated
     */
    @Nonnull
    JImmutableList getList(@Nonnull K key);

    /**
     * Sets the list associated with a specific key.  Key and value must be non-null.
     * If the key already has a list in the map the old list is discarded
     * and the new list is stored in its place.  Returns a new JImmutableListMap reflecting
     * any changes.  The original map is always left unchanged.
     *
     * @param key   non-null key
     * @param value list of possibly null values to use for this key
     * @return new map reflecting the change
     */
    @Nonnull
    JImmutableListMap assign(@Nonnull K key,
                                   @Nonnull JImmutableList value);

    /**
     * Add key/value entry to the map, replacing any existing entry with same key.
     */
    @Nonnull
    @Override
    JImmutableListMap insert(@Nonnull JImmutableMap.Entry value);

    /**
     * Add value to the list for the specified key.  Note that this can create duplicate values
     * in the list.
     */
    @Nonnull
    JImmutableListMap insert(@Nonnull K key,
                                   @Nullable V value);

    /**
     * Adds all of the elements of the specified collection to the List for the specified key.
     */
    @Nonnull
    default JImmutableListMap insertAll(@Nonnull K key,
                                              @Nonnull Iterable values)
    {
        return assign(key, getList(key).insertAll(values));
    }

    /**
     * Adds all of the elements of the specified collection to the List for the specified key.
     */
    @Nonnull
    default JImmutableListMap insertAll(@Nonnull K key,
                                              @Nonnull Iterator values)
    {
        return assign(key, getList(key).insertAll(values));
    }

    /**
     * Deletes the entry for the specified key (if any).  Returns a new map if the value
     * was deleted or the current map if the key was not contained in the map.
     *
     * @param key non-null key
     * @return same or different map depending on whether key was removed
     */
    @Nonnull
    JImmutableListMap delete(@Nonnull K key);

    /**
     * Deletes the list for every key in keys. Returns a new map if the keys were deleted or the
     * current map if the keys were contained in the map.
     */
    @Nonnull
    default JImmutableListMap deleteAll(@Nonnull Iterable keys)
    {
        return deleteAll(keys.iterator());
    }

    /**
     * Deletes the list for every key in keys. Returns a new map if the keys were deleted or the
     * current map if the keys were contained in the map.
     */
    @Nonnull
    default JImmutableListMap deleteAll(@Nonnull Iterator keys)
    {
        JImmutableListMap map = this;
        while (keys.hasNext()) {
            map = map.delete(keys.next());
        }
        return map;
    }

    /**
     * Apply the specified transform function to the List assigned to the specified key and assign the result
     * to the key in this map.  If no List is currently assigned to the key the transform function is called
     * with an empty list.
     *
     * @param key       key holding list to be updated
     * @param transform function to update the list
     * @return new map with update applied to list associated with key
     */
    default JImmutableListMap transform(@Nonnull K key,
                                              @Nonnull Func1, JImmutableList> transform)
    {
        final JImmutableList current = getList(key);
        final JImmutableList transformed = transform.apply(current);
        return (transformed == current) ? this : assign(key, transformed);
    }

    /**
     * Apply the specified transform function to the List assigned to the specified key and assign the result
     * to the key in this map.  If no list is currently assigned to the key the transform function is never
     * called and this map is returned unchanged.
     *
     * @param key       key holding list to be updated
     * @param transform function to update the list
     * @return new map with update applied to list associated with key
     */
    default JImmutableListMap transformIfPresent(@Nonnull K key,
                                                       @Nonnull Func1, JImmutableList> transform)
    {
        final JImmutableList current = get(key);
        if (current != null) {
            final JImmutableList transformed = transform.apply(current);
            if (transformed != current) {
                return assign(key, transformed);
            }
        }
        return this;
    }

    /**
     * Return the number of keys in the map.
     */
    int size();

    /**
     * @return true only if map contains no values
     */
    boolean isEmpty();

    /**
     * @return true only if map contains values
     */
    default boolean isNonEmpty()
    {
        return !isEmpty();
    }

    /**
     * @return an equivalent collection with no values
     */
    @Nonnull
    JImmutableListMap deleteAll();

    /**
     * Creates a Streamable to access all of the Map's keys.
     */
    @Nonnull
    IterableStreamable keys();

    /**
     * Creates a Streamable to access all of the specified key's list.
     * If no list exists for key an empty Streamable is returned.
     *
     * @return a (possibly empty) Streamable for traversing the values associated with key
     */
    @Nonnull
    IterableStreamable values(@Nonnull K key);

    /**
     * Creates a Streamable to access all of the Map's entries.
     */
    @Nonnull
    IterableStreamable> entries();

    /**
     * Processes every key/list pair in this map using the provided function.
     */
    default void forEach(@Nonnull Proc2> proc)
    {
        for (JImmutableMap.Entry> e : this) {
            proc.apply(e.getKey(), e.getValue());
        }
    }

    /**
     * Processes every key/list pair in this map using the provided function.
     */
    default  void forEachThrows(@Nonnull Proc2Throws, E> proc)
        throws E
    {
        for (JImmutableMap.Entry> e : this) {
            proc.apply(e.getKey(), e.getValue());
        }
    }

    /**
     * Returns a Collector that creates a listMap of the same type as this containing all
     * of the collected values inserted over whatever starting values this already contained.
     */
    @Nonnull
    default Collector, ?, JImmutableListMap> listMapCollector()
    {
        return GenericCollector.ordered(this, deleteAll(), a -> a.isEmpty(), (a, v) -> a.insert(v), (a, b) -> a.insertAll(b.entries()));
    }
}