content: // Copyright (c) 2013, 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.
  
  part of dart.collection;
  
  /** Default function for equality comparison in customized HashMaps */
  bool _defaultEquals(a, b) => a == b;
  /** Default function for hash-code computation in customized HashMaps */
  int _defaultHashCode(a) => a.hashCode;
  
  /** Type of custom equality function */
  typedef bool _Equality<K>(K a, K b);
  /** Type of custom hash code function. */
  typedef int _Hasher<K>(K object);
  
  /**
   * A hash-table based implementation of [Map].
   *
   * The keys of a `HashMap` must have consistent [Object.operator==]
   * and [Object.hashCode] implementations. This means that the `==` operator
   * must define a stable equivalence relation on the keys (reflexive,
   * anti-symmetric, transitive, and consistent over time), and that `hashCode`
   * must be the same for objects that are considered equal by `==`.
   *
   * The map allows `null` as a key.
   */
  abstract class HashMap<K, V> implements Map<K, V> {
    /**
     * Creates a hash-table based [Map].
     *
     * The created map is not ordered in any way. When iterating the keys or
     * values, the iteration order is unspecified except that it will stay the
     * same as long as the map isn't changed.
     *
     * If [equals] is provided, it is used to compare the keys in the table with
     * new keys. If [equals] is omitted, the key's own [Object.operator==] is used
     * instead.
     *
     * Similar, if [hashCode] is provided, it is used to produce a hash value
     * for keys in order to place them in the hash table. If it is omitted, the
     * key's own [Object.hashCode] is used.
     *
     * If using methods like [operator[]], [remove] and [containsKey] together
     * with a custom equality and hashcode, an extra `isValidKey` function
     * can be supplied. This function is called before calling [equals] or
     * [hashCode] with an argument that may not be a [K] instance, and if the
     * call returns false, the key is assumed to not be in the set.
     * The [isValidKey] function defaults to just testing if the object is a
     * [K] instance.
     *
     * The used `equals` and `hashCode` method should always be consistent,
     * so that if `equals(a, b)` then `hashCode(a) == hashCode(b)`. The hash
     * of an object, or what it compares equal to, should not change while the
     * object is in the table. If it does change, the result is unpredictable.
     *
     * It is generally the case that if you supply one of [equals] and [hashCode],
     * you also want to supply the other. The only common exception is to pass
     * [identical] as the equality and use the default hash code.
     */
    external factory HashMap({bool equals(K key1, K key2),
                              int hashCode(K key),
                              bool isValidKey(potentialKey)});
  
    /**
     * Creates an unordered identity-based map.
     *
     * Effectively a shorthand for:
     *
     *     new HashMap(equals: identical, hashCode: identityHashCodeOf)
     */
    external factory HashMap.identity();
  
    /**
     * Creates a [HashMap] that contains all key value pairs of [other].
     */
    factory HashMap.from(Map<K, V> other) {
      return new HashMap<K, V>()..addAll(other);
    }
  
    /**
     * Creates a [HashMap] where the keys and values are computed from the
     * [iterable].
     *
     * For each element of the [iterable] this constructor computes a key/value
     * pair, by applying [key] and [value] respectively.
     *
     * The keys of the key/value pairs do not need to be unique. The last
     * occurrence of a key will simply overwrite any previous value.
     *
     * If no values are specified for [key] and [value] the default is the
     * identity function.
     */
    factory HashMap.fromIterable(Iterable iterable,
        {K key(element), V value(element)}) {
      HashMap<K, V> map = new HashMap<K, V>();
      Maps._fillMapWithMappedIterable(map, iterable, key, value);
      return map;
    }
  
    /**
     * Creates a [HashMap] associating the given [keys] to [values].
     *
     * This constructor iterates over [keys] and [values] and maps each element of
     * [keys] to the corresponding element of [values].
     *
     * If [keys] contains the same object multiple times, the last occurrence
     * overwrites the previous value.
     *
     * It is an error if the two [Iterable]s don't have the same length.
     */
    factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
      HashMap<K, V> map = new HashMap<K, V>();
      Maps._fillMapWithIterables(map, keys, values);
      return map;
    }
  }
  