sdk/lib/core/list.dart

// Copyright (c) 2012, 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.core;

/**
 * An indexable collection of objects with a length.
 *
 * Subclasses of this class implement different kinds of lists.
 * The most common kinds of lists are:
 *
 * * Fixed-length list.
 *   An error occurs when attempting to use operations
 *   that can change the length of the list.
 *
 * * Growable list. Full implementation of the API defined in this class.
 *
 * The default growable list, as created by `[]`, keeps
 * an internal buffer, and grows that buffer when necessary. This guarantees
 * that a sequence of [add] operations will each execute in amortized constant
 * time. Setting the length directly may take time proportional to the new
 * length, and may change the internal capacity so that a following add
 * operation will need to immediately increase the buffer capacity.
 * Other list implementations may have different performance behavior.
 *
 * The following code illustrates that some List implementations support
 * only a subset of the API.
 *
 *     List<int> fixedLengthList = new List(5);
 *     fixedLengthList.length = 0;  // Error
 *     fixedLengthList.add(499);    // Error
 *     fixedLengthList[0] = 87;
 *     List<int> growableList = [1, 2];
 *     growableList.length = 0;
 *     growableList.add(499);
 *     growableList[0] = 87;
 *
 * Lists are [Iterable]. Iteration occurs over values in index order. Changing
 * the values does not affect iteration, but changing the valid
 * indices&mdash;that is, changing the list's length&mdash;between iteration
 * steps causes a [ConcurrentModificationError]. This means that only growable
 * lists can throw ConcurrentModificationError. If the length changes
 * temporarily and is restored before continuing the iteration, the iterator
 * does not detect it.
 *
 * It is generally not allowed to modify the list's length (adding or removing
 * elements) while an operation on the list is being performed,
 * for example during a call to [forEach] or [sort].
 * Changing the list's length while it is being iterated, either by iterating it
 * directly or through iterating an [Iterable] that is backed by the list, will
 * break the iteration.
 */
abstract class List<E> implements EfficientLengthIterable<E> {
  /**
   * Creates a list of the given length.
   *
   * This constructor will throw an exception if [E] is not a nullable type.
   * In this case, another constructor such as [List.filled] must be used
   * instead.
   *
   * The created list is fixed-length if [length] is provided.
   *
   *     List fixedLengthList = new List(3);
   *     fixedLengthList.length;     // 3
   *     fixedLengthList.length = 1; // Error
   *
   * The list has length 0 and is growable if [length] is omitted.
   *
   *     List growableList = [];
   *     growableList.length; // 0;
   *     growableList.length = 3;
   *
   * To create a growable list with a given length, for a nullable element type,
   * just assign the length right after creation:
   *
   *     List growableList = []..length = 500;
   *
   * For a non-nullable element type, an alternative is the following:
   *
   *     List<int> growableList = List<int>.filled(500, 0, growable: true);
   *
   * The [length] must not be negative or null, if it is provided.
   *
   * If the element type is not nullable, [length] must not be greater than
   * zero.
   *
   * This constructor cannot be used in null-safe code.
   * Use [List.filled] to create a non-empty list.
   * This requires a fill value to initialize the list elements with.
   * To create an empty list, use `[]` for a growable list or
   * `List.empty` for a fixed length list (or where growability is determined
   * at run-time).
   */
  external factory List([int? length]);

  /**
   * Creates a list of the given length with [fill] at each position.
   *
   * The [length] must be a non-negative integer.
   *
   * Example:
   * ```dart
   * new List<int>.filled(3, 0, growable: true); // [0, 0, 0]
   * ```
   *
   * The created list is fixed-length if [growable] is false (the default)
   * and growable if [growable] is true.
   * If the list is growable, changing its length will not initialize new
   * entries with [fill].
   * After being created and filled, the list is no different from any other
   * growable or fixed-length list created using [List].
   *
   * All elements of the returned list share the same [fill] value.
   * ```
   * var shared = new List.filled(3, []);
   * shared[0].add(499);
   * print(shared);  // => [[499], [499], [499]]
   * ```
   *
   * You can use [List.generate] to create a list with a new object at
   * each position.
   * ```
   * var unique = new List.generate(3, (_) => []);
   * unique[0].add(499);
   * print(unique); // => [[499], [], []]
   * ```
   */
  external factory List.filled(int length, E fill, {bool growable = false});

  /**
   * Creates a new empty list.
   *
   * If [growable] is `false`, which is the default,
   * the list is a fixed-length list of length zero.
   * If [growable] is `true`, the list is growable and equivalent to `<E>[]`.
   */
  @Since("2.8")
  external factory List.empty({bool growable = false});

  /**
   * Creates a list containing all [elements].
   *
   * The [Iterator] of [elements] provides the order of the elements.
   *
   * All the [elements] should be instances of [E].
   * The `elements` iterable itself may have any element type, so this
   * constructor can be used to down-cast a `List`, for example as:
   * ```dart
   * List<SuperType> superList = ...;
   * List<SubType> subList =
   *     new List<SubType>.from(superList.whereType<SubType>());
   * ```
   *
   * This constructor creates a growable list when [growable] is true;
   * otherwise, it returns a fixed-length list.
   */
  external factory List.from(Iterable elements, {bool growable = true});

  /**
   * Creates a list from [elements].
   *
   * The [Iterator] of [elements] provides the order of the elements.
   *
   * This constructor creates a growable list when [growable] is true;
   * otherwise, it returns a fixed-length list.
   */
  external factory List.of(Iterable<E> elements, {bool growable = true});

  /**
   * Generates a list of values.
   *
   * Creates a list with [length] positions and fills it with values created by
   * calling [generator] for each index in the range `0` .. `length - 1`
   * in increasing order.
   * ```dart
   * List<int>.generate(3, (int index) => index * index); // [0, 1, 4]
   * ```
   * The created list is fixed-length if [growable] is set to false.
   *
   * The [length] must be non-negative.
   */
  external factory List.generate(int length, E generator(int index),
      {bool growable = true});

  /**
   * Creates an unmodifiable list containing all [elements].
   *
   * The [Iterator] of [elements] provides the order of the elements.
   *
   * An unmodifiable list cannot have its length or elements changed.
   * If the elements are themselves immutable, then the resulting list
   * is also immutable.
   */
  external factory List.unmodifiable(Iterable elements);

  /**
   * Adapts [source] to be a `List<T>`.
   *
   * Any time the list would produce an element that is not a [T],
   * the element access will throw.
   *
   * Any time a [T] value is attempted stored into the adapted list,
   * the store will throw unless the value is also an instance of [S].
   *
   * If all accessed elements of [source] are actually instances of [T],
   * and if all elements stored into the returned list are actually instance
   * of [S],
   * then the returned list can be used as a `List<T>`.
   */
  static List<T> castFrom<S, T>(List<S> source) => CastList<S, T>(source);

  /**
   * Copy a range of one list into another list.
   *
   * This is a utility function that can be used to implement methods like
   * [setRange].
   *
   * The range from [start] to [end] must be a valid range of [source],
   * and there must be room for `end - start` elements from position [at].
   * If [start] is omitted, it defaults to zero.
   * If [end] is omitted, it defaults to [source.length].
   *
   * If [source] and [target] is the same list, overlapping source and target
   * ranges are respected so that the target range ends up containing the
   * initial content of the source range.
   * Otherwise the order of element copying is not guaranteed.
   */
  static void copyRange<T>(List<T> target, int at, List<T> source,
      [int? start, int? end]) {
    start ??= 0;
    end = RangeError.checkValidRange(start, end, source.length);
    if (end == null) {
      // TODO(dart-lang/language#440): Remove when promotion works.
      throw "unreachable";
    }
    int length = end - start;
    if (target.length < at + length) {
      throw ArgumentError.value(target, "target",
          "Not big enough to hold $length elements at position $at");
    }
    if (!identical(source, target) || start >= at) {
      for (int i = 0; i < length; i++) {
        target[at + i] = source[start + i];
      }
    } else {
      for (int i = length; --i >= 0;) {
        target[at + i] = source[start + i];
      }
    }
  }

  /**
   * Write the elements of an iterable into a list.
   *
   * This is a utility function that can be used to implement methods like
   * [setAll].
   *
   * The elements of [source] are written into [target] from position [at].
   * The [source] must not contain more elements after writing the last
   * position of [target].
   *
   * If the source is a list, the [copyRange] function is likely to be more
   * efficient.
   */
  static void writeIterable<T>(List<T> target, int at, Iterable<T> source) {
    RangeError.checkValueInInterval(at, 0, target.length, "at");
    int index = at;
    int targetLength = target.length;
    for (var element in source) {
      if (index == targetLength) {
        throw IndexError(targetLength, target);
      }
      target[index] = element;
      index++;
    }
  }

  /**
   * Returns a view of this list as a list of [R] instances.
   *
   * If this list contains only instances of [R], all read operations
   * will work correctly. If any operation tries to access an element
   * that is not an instance of [R], the access will throw instead.
   *
   * Elements added to the list (e.g., by using [add] or [addAll])
   * must be instance of [R] to be valid arguments to the adding function,
   * and they must be instances of [E] as well to be accepted by
   * this list as well.
   *
   * Typically implemented as `List.castFrom<E, R>(this)`.
   */
  List<R> cast<R>();

  /**
   * Returns the object at the given [index] in the list
   * or throws a [RangeError] if [index] is out of bounds.
   */
  E operator [](int index);

  /**
   * Sets the value at the given [index] in the list to [value]
   * or throws a [RangeError] if [index] is out of bounds.
   */
  void operator []=(int index, E value);

  /**
   * Updates the first position of the list to contain [value].
   *
   * Equivalent to `theList[0] = value;`.
   *
   * The list must be non-empty.
   */
  void set first(E value);

  /**
   * Updates the last position of the list to contain [value].
   *
   * Equivalent to `theList[theList.length - 1] = value;`.
   *
   * The list must be non-empty.
   */
  void set last(E value);

  /**
   * The number of objects in this list.
   *
   * The valid indices for a list are `0` through `length - 1`.
   */
  int get length;

  /**
   * Changes the length of this list.
   *
   * If [newLength] is greater than
   * the current length, entries are initialized to `null`.
   * Increasing the length fails if the element type does not allow `null`.
   *
   * Throws an [UnsupportedError] if the list is fixed-length or
   * if attempting tp enlarge the list when `null` is not a valid element.
   */
  set length(int newLength);

  /**
   * Adds [value] to the end of this list,
   * extending the length by one.
   *
   * Throws an [UnsupportedError] if the list is fixed-length.
   */
  void add(E value);

  /**
   * Appends all objects of [iterable] to the end of this list.
   *
   * Extends the length of the list by the number of objects in [iterable].
   * Throws an [UnsupportedError] if this list is fixed-length.
   */
  void addAll(Iterable<E> iterable);

  /**
   * Returns an [Iterable] of the objects in this list in reverse order.
   */
  Iterable<E> get reversed;

  /**
   * Sorts this list according to the order specified by the [compare] function.
   *
   * The [compare] function must act as a [Comparator].
   *
   *     List<String> numbers = ['two', 'three', 'four'];
   *     // Sort from shortest to longest.
   *     numbers.sort((a, b) => a.length.compareTo(b.length));
   *     print(numbers);  // [two, four, three]
   *
   * The default List implementations use [Comparable.compare] if
   * [compare] is omitted.
   *
   *     List<int> nums = [13, 2, -11];
   *     nums.sort();
   *     print(nums);  // [-11, 2, 13]
   *
   * A [Comparator] may compare objects as equal (return zero), even if they
   * are distinct objects.
   * The sort function is not guaranteed to be stable, so distinct objects
   * that compare as equal may occur in any order in the result:
   *
   *     List<String> numbers = ['one', 'two', 'three', 'four'];
   *     numbers.sort((a, b) => a.length.compareTo(b.length));
   *     print(numbers);  // [one, two, four, three] OR [two, one, four, three]
   */
  void sort([int compare(E a, E b)?]);

  /**
   * Shuffles the elements of this list randomly.
   */
  void shuffle([Random? random]);

  /**
   * Returns the first index of [element] in this list.
   *
   * Searches the list from index [start] to the end of the list.
   * The first time an object [:o:] is encountered so that [:o == element:],
   * the index of [:o:] is returned.
   *
   *     List<String> notes = ['do', 're', 'mi', 're'];
   *     notes.indexOf('re');    // 1
   *     notes.indexOf('re', 2); // 3
   *
   * Returns -1 if [element] is not found.
   *
   *     notes.indexOf('fa');    // -1
   */
  int indexOf(E element, [int start = 0]);

  /**
   * Returns the first index in the list that satisfies the provided [test].
   *
   * Searches the list from index [start] to the end of the list.
   * The first time an object `o` is encountered so that `test(o)` is true,
   * the index of `o` is returned.
   *
   * ```
   * List<String> notes = ['do', 're', 'mi', 're'];
   * notes.indexWhere((note) => note.startsWith('r'));       // 1
   * notes.indexWhere((note) => note.startsWith('r'), 2);    // 3
   * ```
   *
   * Returns -1 if [element] is not found.
   * ```
   * notes.indexWhere((note) => note.startsWith('k'));    // -1
   * ```
   */
  int indexWhere(bool test(E element), [int start = 0]);

  /**
   * Returns the last index in the list that satisfies the provided [test].
   *
   * Searches the list from index [start] to 0.
   * The first time an object `o` is encountered so that `test(o)` is true,
   * the index of `o` is returned.
   * If [start] is omitted, it defaults to the [length] of the list.
   *
   * ```
   * List<String> notes = ['do', 're', 'mi', 're'];
   * notes.lastIndexWhere((note) => note.startsWith('r'));       // 3
   * notes.lastIndexWhere((note) => note.startsWith('r'), 2);    // 1
   * ```
   *
   * Returns -1 if [element] is not found.
   * ```
   * notes.lastIndexWhere((note) => note.startsWith('k'));    // -1
   * ```
   */
  int lastIndexWhere(bool test(E element), [int? start]);

  /**
   * Returns the last index of [element] in this list.
   *
   * Searches the list backwards from index [start] to 0.
   *
   * The first time an object [:o:] is encountered so that [:o == element:],
   * the index of [:o:] is returned.
   *
   *     List<String> notes = ['do', 're', 'mi', 're'];
   *     notes.lastIndexOf('re', 2); // 1
   *
   * If [start] is not provided, this method searches from the end of the
   * list.
   *
   *     notes.lastIndexOf('re');  // 3
   *
   * Returns -1 if [element] is not found.
   *
   *     notes.lastIndexOf('fa');  // -1
   */
  int lastIndexOf(E element, [int? start]);

  /**
   * Removes all objects from this list;
   * the length of the list becomes zero.
   *
   * Throws an [UnsupportedError], and retains all objects, if this
   * is a fixed-length list.
   */
  void clear();

  /**
   * Inserts the object at position [index] in this list.
   *
   * This increases the length of the list by one and shifts all objects
   * at or after the index towards the end of the list.
   *
   * The list must be growable.
   * The [index] value must be non-negative and no greater than [length].
   */
  void insert(int index, E element);

  /**
   * Inserts all objects of [iterable] at position [index] in this list.
   *
   * This increases the length of the list by the length of [iterable] and
   * shifts all later objects towards the end of the list.
   *
   * The list must be growable.
   * The [index] value must be non-negative and no greater than [length].
   */
  void insertAll(int index, Iterable<E> iterable);

  /**
   * Overwrites objects of `this` with the objects of [iterable], starting
   * at position [index] in this list.
   *
   *     List<String> list = ['a', 'b', 'c'];
   *     list.setAll(1, ['bee', 'sea']);
   *     list.join(', '); // 'a, bee, sea'
   *
   * This operation does not increase the length of `this`.
   *
   * The [index] must be non-negative and no greater than [length].
   *
   * The [iterable] must not have more elements than what can fit from [index]
   * to [length].
   *
   * If `iterable` is based on this list, its values may change /during/ the
   * `setAll` operation.
   */
  void setAll(int index, Iterable<E> iterable);

  /**
   * Removes the first occurrence of [value] from this list.
   *
   * Returns true if [value] was in the list, false otherwise.
   *
   *     List<String> parts = ['head', 'shoulders', 'knees', 'toes'];
   *     parts.remove('head'); // true
   *     parts.join(', ');     // 'shoulders, knees, toes'
   *
   * The method has no effect if [value] was not in the list.
   *
   *     // Note: 'head' has already been removed.
   *     parts.remove('head'); // false
   *     parts.join(', ');     // 'shoulders, knees, toes'
   *
   * An [UnsupportedError] occurs if the list is fixed-length.
   */
  bool remove(Object? value);

  /**
   * Removes the object at position [index] from this list.
   *
   * This method reduces the length of `this` by one and moves all later objects
   * down by one position.
   *
   * Returns the removed object.
   *
   * The [index] must be in the range `0 ≤ index < length`.
   *
   * Throws an [UnsupportedError] if this is a fixed-length list. In that case
   * the list is not modified.
   */
  E removeAt(int index);

  /**
   * Pops and returns the last object in this list.
   *
   * The list must not be empty.
   *
   * Throws an [UnsupportedError] if this is a fixed-length list.
   */
  E removeLast();

  /**
   * Removes all objects from this list that satisfy [test].
   *
   * An object [:o:] satisfies [test] if [:test(o):] is true.
   *
   *     List<String> numbers = ['one', 'two', 'three', 'four'];
   *     numbers.removeWhere((item) => item.length == 3);
   *     numbers.join(', '); // 'three, four'
   *
   * Throws an [UnsupportedError] if this is a fixed-length list.
   */
  void removeWhere(bool test(E element));

  /**
   * Removes all objects from this list that fail to satisfy [test].
   *
   * An object [:o:] satisfies [test] if [:test(o):] is true.
   *
   *     List<String> numbers = ['one', 'two', 'three', 'four'];
   *     numbers.retainWhere((item) => item.length == 3);
   *     numbers.join(', '); // 'one, two'
   *
   * Throws an [UnsupportedError] if this is a fixed-length list.
   */
  void retainWhere(bool test(E element));

  /**
   * Returns the concatenation of this list and [other].
   *
   * Returns a new list containing the elements of this list followed by
   * the elements of [other].
   *
   * The default behavior is to return a normal growable list.
   * Some list types may choose to return a list of the same type as themselves
   * (see [Uint8List.+]);
   */
  List<E> operator +(List<E> other);

  /**
   * Returns a new list containing the elements between [start] and [end].
   *
   * The new list is a `List<E>` containing the elements of this list at
   * positions greater than or equal to [start] and less than [end] in the same
   * order as they occur in this list.
   *
   * ```dart
   * var colors = ["red", "green", "blue", "orange", "pink"];
   * print(colors.sublist(1, 3)); // [green, blue]
   * ```
   *
   * If [end] is omitted, it defaults to the [length] of this list.
   *
   * ```dart
   * print(colors.sublist(1)); // [green, blue, orange, pink]
   * ```
   *
   * The `start` and `end` positions must satisfy the relations
   * 0 ≤ `start` ≤ `end` ≤ `this.length`
   * If `end` is equal to `start`, then the returned list is empty.
   */
  List<E> sublist(int start, [int? end]);

  /**
   * Returns an [Iterable] that iterates over the objects in the range
   * [start] inclusive to [end] exclusive.
   *
   * The provided range, given by [start] and [end], must be valid at the time
   * of the call.
   *
   * A range from [start] to [end] is valid if `0 <= start <= end <= len`, where
   * `len` is this list's `length`. The range starts at `start` and has length
   * `end - start`. An empty range (with `end == start`) is valid.
   *
   * The returned [Iterable] behaves like `skip(start).take(end - start)`.
   * That is, it does *not* throw if this list changes size.
   *
   *     List<String> colors = ['red', 'green', 'blue', 'orange', 'pink'];
   *     Iterable<String> range = colors.getRange(1, 4);
   *     range.join(', ');  // 'green, blue, orange'
   *     colors.length = 3;
   *     range.join(', ');  // 'green, blue'
   */
  Iterable<E> getRange(int start, int end);

  /**
   * Copies the objects of [iterable], skipping [skipCount] objects first,
   * into the range [start], inclusive, to [end], exclusive, of the list.
   *
   *     List<int> list1 = [1, 2, 3, 4];
   *     List<int> list2 = [5, 6, 7, 8, 9];
   *     // Copies the 4th and 5th items in list2 as the 2nd and 3rd items
   *     // of list1.
   *     list1.setRange(1, 3, list2, 3);
   *     list1.join(', '); // '1, 8, 9, 4'
   *
   * The provided range, given by [start] and [end], must be valid.
   * A range from [start] to [end] is valid if `0 <= start <= end <= len`, where
   * `len` is this list's `length`. The range starts at `start` and has length
   * `end - start`. An empty range (with `end == start`) is valid.
   *
   * The [iterable] must have enough objects to fill the range from `start`
   * to `end` after skipping [skipCount] objects.
   *
   * If `iterable` is this list, the operation copies the elements
   * originally in the range from `skipCount` to `skipCount + (end - start)` to
   * the range `start` to `end`, even if the two ranges overlap.
   *
   * If `iterable` depends on this list in some other way, no guarantees are
   * made.
   */
  void setRange(int start, int end, Iterable<E> iterable, [int skipCount = 0]);

  /**
   * Removes the objects in the range [start] inclusive to [end] exclusive.
   *
   * The provided range, given by [start] and [end], must be valid.
   * A range from [start] to [end] is valid if `0 <= start <= end <= len`, where
   * `len` is this list's `length`. The range starts at `start` and has length
   * `end - start`. An empty range (with `end == start`) is valid.
   *
   * Throws an [UnsupportedError] if this is a fixed-length list. In that case
   * the list is not modified.
   */
  void removeRange(int start, int end);

  /**
   * Sets the objects in the range [start] inclusive to [end] exclusive
   * to the given [fillValue].
   *
   * The provided range, given by [start] and [end], must be valid.
   * A range from [start] to [end] is valid if `0 <= start <= end <= len`, where
   * `len` is this list's `length`. The range starts at `start` and has length
   * `end - start`. An empty range (with `end == start`) is valid.
   *
   * Example:
   * ```dart
   *  List<int> list = new List(3);
   *     list.fillRange(0, 2, 1);
   *     print(list); //  [1, 1, null]
   * ```
   *
   * If the element type is not nullable, omitting [fillValue] or passing `null`
   * as [fillValue] will make the `fillRange` fail.
   */
  void fillRange(int start, int end, [E? fillValue]);

  /**
   * Removes the objects in the range [start] inclusive to [end] exclusive
   * and inserts the contents of [replacement] in its place.
   *
   *     List<int> list = [1, 2, 3, 4, 5];
   *     list.replaceRange(1, 4, [6, 7]);
   *     list.join(', '); // '1, 6, 7, 5'
   *
   * The provided range, given by [start] and [end], must be valid.
   * A range from [start] to [end] is valid if `0 <= start <= end <= len`, where
   * `len` is this list's `length`. The range starts at `start` and has length
   * `end - start`. An empty range (with `end == start`) is valid.
   *
   * This method does not work on fixed-length lists, even when [replacement]
   * has the same number of elements as the replaced range. In that case use
   * [setRange] instead.
   */
  void replaceRange(int start, int end, Iterable<E> replacement);

  /**
   * Returns an unmodifiable [Map] view of `this`.
   *
   * The map uses the indices of this list as keys and the corresponding objects
   * as values. The `Map.keys` [Iterable] iterates the indices of this list
   * in numerical order.
   *
   *     List<String> words = ['fee', 'fi', 'fo', 'fum'];
   *     Map<int, String> map = words.asMap();
   *     map[0] + map[1];   // 'feefi';
   *     map.keys.toList(); // [0, 1, 2, 3]
   */
  Map<int, E> asMap();

  /**
  * Whether this list is equal to [other].
  *
  * Lists are, by default, only equal to themselves.
  * Even if [other] is also a list, the equality comparison
  * does not compare the elements of the two lists.
  */
  bool operator ==(Object other);
}