map.dart

// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// @dart = 2.6

part of dart.core;

/**
 * A collection of key/value pairs, from which you retrieve a value
 * using its associated key.
 *
 * There is a finite number of keys in the map,
 * and each key has exactly one value associated with it.
 *
 * Maps, and their keys and values, can be iterated.
 * The order of iteration is defined by the individual type of map.
 * Examples:
 *
 * * The plain [HashMap] is unordered (no order is guaranteed),
 * * the [LinkedHashMap] iterates in key insertion order,
 * * and a sorted map like [SplayTreeMap] iterates the keys in sorted order.
 *
 * It is generally not allowed to modify the map (add or remove keys) while
 * an operation is being performed on the map, for example in functions called
 * during a [forEach] or [putIfAbsent] call.
 * Modifying the map while iterating the keys or values
 * may also break the iteration.
 *
 * It is generally not allowed to modify the equality of keys (and thus not
 * their hashcode) while they are in the map. Some specialized subtypes may be
 * more permissive, in which case they should document this behavior.
 */
abstract class Map<K, V> {
  /**
   * Creates a Map instance with the default implementation, [LinkedHashMap].
   *
   * This constructor is equivalent to the non-const map literal `<K,V>{}`.
   *
   * A `LinkedHashMap` requires the keys to implement compatible
   * `operator==` and `hashCode`, and it allows null as a key.
   * It iterates in key insertion order.
   */
  external factory Map();

  /**
   * Creates a [LinkedHashMap] instance that contains all key/value pairs of
   * [other].
   *
   * The keys must all be instances of [K] and the values of [V].
   * The [other] map itself can have any type.
   *
   * A `LinkedHashMap` requires the keys to implement compatible
   * `operator==` and `hashCode`, and it allows `null` as a key.
   * It iterates in key insertion order.
   */
  factory Map.from(Map other) = LinkedHashMap<K, V>.from;

  /**
   * Creates a [LinkedHashMap] with the same keys and values as [other].
   *
   * A `LinkedHashMap` requires the keys to implement compatible
   * `operator==` and `hashCode`, and it allows `null` as a key.
   * It iterates in key insertion order.
   */
  factory Map.of(Map<K, V> other) = LinkedHashMap<K, V>.of;

  /**
   * Creates an unmodifiable hash based map containing the entries of [other].
   *
   * The keys must all be instances of [K] and the values of [V].
   * The [other] map itself can have any type.
   *
   * The map requires the keys to implement compatible
   * `operator==` and `hashCode`, and it allows `null` as a key.
   * The created map iterates keys in a fixed order,
   * preserving the order provided by [other].
   *
   * The resulting map behaves like the result of [Map.from],
   * except that the map returned by this constructor is not modifiable.
   */
  external factory Map.unmodifiable(Map other);

  /**
   * Creates an identity map with the default implementation, [LinkedHashMap].
   *
   * An identity map uses [identical] for equality and [identityHashCode]
   * for hash codes of keys instead of the intrinsic [Object.==] and
   * [Object.hashCode] of the keys.
   *
   * The returned map allows `null` as a key.
   * It iterates in key insertion order.
   */
  factory Map.identity() = LinkedHashMap<K, V>.identity;

  /**
   * Creates a Map instance in which the keys and values are computed from the
   * [iterable].
   *
   * The created map is a [LinkedHashMap].
   * A `LinkedHashMap` requires the keys to implement compatible
   * `operator==` and `hashCode`, and it allows null as a key.
   * It iterates in key insertion order.
   *
   * For each element of the [iterable] this constructor computes a key/value
   * pair, by applying [key] and [value] respectively.
   *
   * The example below creates a new Map from a List. The keys of `map` are
   * `list` values converted to strings, and the values of the `map` are the
   * squares of the `list` values:
   *
   *     List<int> list = [1, 2, 3];
   *     Map<String, int> map = new Map.fromIterable(list,
   *         key: (item) => item.toString(),
   *         value: (item) => item * item);
   *
   *     map['1'] + map['2']; // 1 + 4
   *     map['3'] - map['2']; // 9 - 4
   *
   * If no values are specified for [key] and [value] the default is the
   * identity function.
   *
   * In the following example, the keys and corresponding values of `map`
   * are `list` values:
   *
   *     map = new Map.fromIterable(list);
   *     map[1] + map[2]; // 1 + 2
   *     map[3] - map[2]; // 3 - 2
   *
   * The keys computed by the source [iterable] do not need to be unique. The
   * last occurrence of a key will simply overwrite any previous value.
   */
  factory Map.fromIterable(Iterable iterable,
      {K key(element), V value(element)}) = LinkedHashMap<K, V>.fromIterable;

  /**
   * Creates a Map instance associating the given [keys] to [values].
   *
   * The created map is a [LinkedHashMap].
   * A `LinkedHashMap` requires the keys to implement compatible
   * `operator==` and `hashCode`, and it allows null as a key.
   * It iterates in key insertion order.
   *
   * This constructor iterates over [keys] and [values] and maps each element of
   * [keys] to the corresponding element of [values].
   *
   *     List<String> letters = ['b', 'c'];
   *     List<String> words = ['bad', 'cat'];
   *     Map<String, String> map = new Map.fromIterables(letters, words);
   *     map['b'] + map['c'];  // badcat
   *
   * If [keys] contains the same object multiple times, the last occurrence
   * overwrites the previous value.
   *
   * The two [Iterable]s must have the same length.
   */
  factory Map.fromIterables(Iterable<K> keys, Iterable<V> values) =
      LinkedHashMap<K, V>.fromIterables;

  /**
   * Adapts [source] to be a `Map<K2, V2>`.
   *
   * Any time the set would produce a key or value that is not a [K2] or [V2],
   * the access will throw.
   *
   * Any time [K2] key or [V2] value is attempted added into the adapted map,
   * the store will throw unless the key is also an instance of [K] and
   * the value is also an instance of [V].
   *
   * If all accessed entries of [source] are have [K2] keys and [V2] values
   * and if all entries added to the returned map have [K] keys and [V]] values,
   * then the returned map can be used as a `Map<K2, V2>`.
   */
  static Map<K2, V2> castFrom<K, V, K2, V2>(Map<K, V> source) =>
      CastMap<K, V, K2, V2>(source);

  /**
   * Creates a new map and adds all entries.
   *
   * Returns a new `Map<K, V>` where all entries of [entries]
   * have been added in iteration order.
   *
   * If multiple [entries] have the same key,
   * later occurrences overwrite the earlier ones.
   */
  factory Map.fromEntries(Iterable<MapEntry<K, V>> entries) =>
      <K, V>{}..addEntries(entries);

  /**
   * Provides a view of this map as having [RK] keys and [RV] instances,
   * if necessary.
   *
   * If this map is already a `Map<RK, RV>`, it is returned unchanged.
   *
   * If this set contains only keys of type [RK] and values of type [RV],
   * all read operations will work correctly.
   * If any operation exposes a non-[RK] key or non-[RV] value,
   * the operation will throw instead.
   *
   * Entries added to the map must be valid for both a `Map<K, V>` and a
   * `Map<RK, RV>`.
   */
  Map<RK, RV> cast<RK, RV>();
  /**
   * Returns true if this map contains the given [value].
   *
   * Returns true if any of the values in the map are equal to `value`
   * according to the `==` operator.
   */
  bool containsValue(Object value);

  /**
   * Returns true if this map contains the given [key].
   *
   * Returns true if any of the keys in the map are equal to `key`
   * according to the equality used by the map.
   */
  bool containsKey(Object key);

  /**
   * Returns the value for the given [key] or null if [key] is not in the map.
   *
   * Some maps allow keys to have `null` as a value.
   * For those maps, a lookup using this operator cannot distinguish between a
   * key not being in the map and the key having a `null` value.
   * Methods like [containsKey] or [putIfAbsent] can be used if the distinction
   * is important.
   */
  V operator [](Object key);

  /**
   * Associates the [key] with the given [value].
   *
   * If the key was already in the map, its associated value is changed.
   * Otherwise the key/value pair is added to the map.
   */
  void operator []=(K key, V value);

  /**
   * The map entries of [this].
   */
  Iterable<MapEntry<K, V>> get entries;

  /**
   * Returns a new map where all entries of this map are transformed by
   * the given [f] function.
   */
  Map<K2, V2> map<K2, V2>(MapEntry<K2, V2> f(K key, V value));

  /**
   * Adds all key/value pairs of [newEntries] to this map.
   *
   * If a key of [newEntries] is already in this map,
   * the corresponding value is overwritten.
   *
   * The operation is equivalent to doing `this[entry.key] = entry.value`
   * for each [MapEntry] of the iterable.
   */
  void addEntries(Iterable<MapEntry<K, V>> newEntries);

  /**
   * Updates the value for the provided [key].
   *
   * Returns the new value of the key.
   *
   * If the key is present, invokes [update] with the current value and stores
   * the new value in the map.
   *
   * If the key is not present and [ifAbsent] is provided, calls [ifAbsent]
   * and adds the key with the returned value to the map.
   *
   * It's an error if the key is not present and [ifAbsent] is not provided.
   */
  V update(K key, V update(V value), {V ifAbsent()});

  /**
   * Updates all values.
   *
   * Iterates over all entries in the map and updates them with the result
   * of invoking [update].
   */
  void updateAll(V update(K key, V value));

  /**
   * Removes all entries of this map that satisfy the given [predicate].
   */
  void removeWhere(bool predicate(K key, V value));

  /**
   * Look up the value of [key], or add a new value if it isn't there.
   *
   * Returns the value associated to [key], if there is one.
   * Otherwise calls [ifAbsent] to get a new value, associates [key] to
   * that value, and then returns the new value.
   *
   *     Map<String, int> scores = {'Bob': 36};
   *     for (var key in ['Bob', 'Rohan', 'Sophena']) {
   *       scores.putIfAbsent(key, () => key.length);
   *     }
   *     scores['Bob'];      // 36
   *     scores['Rohan'];    //  5
   *     scores['Sophena'];  //  7
   *
   * Calling [ifAbsent] must not add or remove keys from the map.
   */
  V putIfAbsent(K key, V ifAbsent());

  /**
   * Adds all key/value pairs of [other] to this map.
   *
   * If a key of [other] is already in this map, its value is overwritten.
   *
   * The operation is equivalent to doing `this[key] = value` for each key
   * and associated value in other. It iterates over [other], which must
   * therefore not change during the iteration.
   */
  void addAll(Map<K, V> other);

  /**
   * Removes [key] and its associated value, if present, from the map.
   *
   * Returns the value associated with `key` before it was removed.
   * Returns `null` if `key` was not in the map.
   *
   * Note that values can be `null` and a returned `null` value doesn't
   * always mean that the key was absent.
   */
  V remove(Object key);

  /**
   * Removes all pairs from the map.
   *
   * After this, the map is empty.
   */
  void clear();

  /**
   * Applies [f] to each key/value pair of the map.
   *
   * Calling `f` must not add or remove keys from the map.
   */
  void forEach(void f(K key, V value));

  /**
   * The keys of [this].
   *
   * The returned iterable has efficient `length` and `contains` operations,
   * based on [length] and [containsKey] of the map.
   *
   * The order of iteration is defined by the individual `Map` implementation,
   * but must be consistent between changes to the map.
   *
   * Modifying the map while iterating the keys
   * may break the iteration.
   */
  Iterable<K> get keys;

  /**
   * The values of [this].
   *
   * The values are iterated in the order of their corresponding keys.
   * This means that iterating [keys] and [values] in parallel will
   * provide matching pairs of keys and values.
   *
   * The returned iterable has an efficient `length` method based on the
   * [length] of the map. Its [Iterable.contains] method is based on
   * `==` comparison.
   *
   * Modifying the map while iterating the
   * values may break the iteration.
   */
  Iterable<V> get values;

  /**
   * The number of key/value pairs in the map.
   */
  int get length;

  /**
   * Returns true if there is no key/value pair in the map.
   */
  bool get isEmpty;

  /**
   * Returns true if there is at least one key/value pair in the map.
   */
  bool get isNotEmpty;
}

/**
 * A key/value pair representing an entry in a [Map].
 */
class MapEntry<K, V> {
  /** The key of the entry. */
  final K key;

  /** The value associated to [key] in the map. */
  final V value;

  /** Creates an entry with [key] and [value]. */
  const factory MapEntry(K key, V value) = MapEntry<K, V>._;

  const MapEntry._(this.key, this.value);

  String toString() => "MapEntry($key: $value)";
}