Remove dependency on package:collection by moving mergeSort into foundation/collections.dart (#59521)

This removes a dependency from Flutter (package:collection) by copying the implementation of mergeSort into Flutter's foundation/collections.dart.

Also, removed a reference to UnmodifiableSetView from the shortcuts code by just returning a copy instead.

packages/flutter/lib/src/foundation/collections.dart

@@ -117,3 +117,241 @@ int binarySearch<T extends Comparable<Object>>(List<T> sortedList, T value) {
   }
   return -1;
 }
+
+/// Limit below which merge sort defaults to insertion sort.
+const int _kMergeSortLimit = 32;
+
+/// Sorts a list between `start` (inclusive) and `end` (exclusive) using the
+/// merge sort algorithm.
+///
+/// If `compare` is omitted, this defaults to calling [Comparable.compareTo] on
+/// the objects. If any object is not [Comparable], this throws a [TypeError]
+/// (The stack trace may call it `_CastError` or `_TypeError`, but to catch it,
+/// use [TypeError]).
+///
+/// Merge-sorting works by splitting the job into two parts, sorting each
+/// recursively, and then merging the two sorted parts.
+///
+/// This takes on the order of `n * log(n)` comparisons and moves to sort `n`
+/// elements, but requires extra space of about the same size as the list being
+/// sorted.
+///
+/// This merge sort is stable: Equal elements end up in the same order as they
+/// started in.
+///
+/// For small lists (less than 32 elements), `mergeSort` automatically uses an
+/// insertion sort instead, as that is more efficient for small lists. The
+/// insertion sort is also stable.
+void mergeSort<T>(
+  List<T> list, {
+  int start = 0,
+  int end,
+  int Function(T, T) compare,
+}) {
+  end ??= list.length;
+  compare ??= _defaultCompare<T>();
+
+  final int length = end - start;
+  if (length < 2) {
+    return;
+  }
+  if (length < _kMergeSortLimit) {
+    _insertionSort(list, compare: compare, start: start, end: end);
+    return;
+  }
+  // Special case the first split instead of directly calling _mergeSort,
+  // because the _mergeSort requires its target to be different from its source,
+  // and it requires extra space of the same size as the list to sort. This
+  // split allows us to have only half as much extra space, and it ends up in
+  // the original place.
+  final int middle = start + ((end - start) >> 1);
+  final int firstLength = middle - start;
+  final int secondLength = end - middle;
+  // secondLength is always the same as firstLength, or one greater.
+  final List<T> scratchSpace = List<T>(secondLength);
+  _mergeSort(list, compare, middle, end, scratchSpace, 0);
+  final int firstTarget = end - firstLength;
+  _mergeSort(list, compare, start, middle, list, firstTarget);
+  _merge(compare, list, firstTarget, end, scratchSpace, 0, secondLength, list, start);
+}
+
+/// Returns a [Comparator] that asserts that its first argument is comparable.
+Comparator<T> _defaultCompare<T>() {
+  // If we specify Comparable<T> here, it fails if the type is an int, because
+  // int isn't a subtype of comparable. Leaving out the type implicitly converts
+  // it to a num, which is a comparable.
+  return (T value1, T value2) => (value1 as Comparable<dynamic>).compareTo(value2);
+}
+
+/// Sort a list between `start` (inclusive) and `end` (exclusive) using
+/// insertion sort.
+///
+/// If `compare` is omitted, this defaults to calling [Comparable.compareTo] on
+/// the objects. If any object is not [Comparable], this throws a [TypeError]
+/// (The stack trace may call it `_CastError` or `_TypeError`, but to catch it,
+/// use [TypeError]).
+///
+/// Insertion sort is a simple sorting algorithm. For `n` elements it does on
+/// the order of `n * log(n)` comparisons but up to `n` squared moves. The
+/// sorting is performed in-place, without using extra memory.
+///
+/// For short lists the many moves have less impact than the simple algorithm,
+/// and it is often the favored sorting algorithm for short lists.
+///
+/// This insertion sort is stable: Equal elements end up in the same order as
+/// they started in.
+void _insertionSort<T>(
+  List<T> list, {
+  int Function(T, T) compare,
+  int start = 0,
+  int end,
+}) {
+  // If the same method could have both positional and named optional
+  // parameters, this should be (list, [start, end], {compare}).
+  compare ??= _defaultCompare<T>();
+  end ??= list.length;
+
+  for (int pos = start + 1; pos < end; pos++) {
+    int min = start;
+    int max = pos;
+    final T element = list[pos];
+    while (min < max) {
+      final int mid = min + ((max - min) >> 1);
+      final int comparison = compare(element, list[mid]);
+      if (comparison < 0) {
+        max = mid;
+      } else {
+        min = mid + 1;
+      }
+    }
+    list.setRange(min + 1, pos + 1, list, min);
+    list[min] = element;
+  }
+}
+
+/// Performs an insertion sort into a potentially different list than the one
+/// containing the original values.
+///
+/// It will work in-place as well.
+void _movingInsertionSort<T>(
+  List<T> list,
+  int Function(T, T) compare,
+  int start,
+  int end,
+  List<T> target,
+  int targetOffset,
+) {
+  final int length = end - start;
+  if (length == 0) {
+    return;
+  }
+  target[targetOffset] = list[start];
+  for (int i = 1; i < length; i++) {
+    final T element = list[start + i];
+    int min = targetOffset;
+    int max = targetOffset + i;
+    while (min < max) {
+      final int mid = min + ((max - min) >> 1);
+      if (compare(element, target[mid]) < 0) {
+        max = mid;
+      } else {
+        min = mid + 1;
+      }
+    }
+    target.setRange(min + 1, targetOffset + i + 1, target, min);
+    target[min] = element;
+  }
+}
+
+/// Sorts `list` from `start` to `end` into `target` at `targetOffset`.
+///
+/// The `target` list must be able to contain the range from `start` to `end`
+/// after `targetOffset`.
+///
+/// Allows target to be the same list as `list`, as long as it's not overlapping
+/// the `start..end` range.
+void _mergeSort<T>(
+    List<T> list,
+    int Function(T, T) compare,
+    int start,
+    int end,
+    List<T> target,
+    int targetOffset,
+    ) {
+  final int length = end - start;
+  if (length < _kMergeSortLimit) {
+    _movingInsertionSort(list, compare, start, end, target, targetOffset);
+    return;
+  }
+  final int middle = start + (length >> 1);
+  final int firstLength = middle - start;
+  final int secondLength = end - middle;
+  // Here secondLength >= firstLength (differs by at most one).
+  final int targetMiddle = targetOffset + firstLength;
+  // Sort the second half into the end of the target area.
+  _mergeSort(list, compare, middle, end, target, targetMiddle);
+  // Sort the first half into the end of the source area.
+  _mergeSort(list, compare, start, middle, list, middle);
+  // Merge the two parts into the target area.
+  _merge(
+    compare,
+    list,
+    middle,
+    middle + firstLength,
+    target,
+    targetMiddle,
+    targetMiddle + secondLength,
+    target,
+    targetOffset,
+  );
+}
+
+/// Merges two lists into a target list.
+///
+/// One of the input lists may be positioned at the end of the target list.
+///
+/// For equal object, elements from `firstList` are always preferred. This
+/// allows the merge to be stable if the first list contains elements that
+/// started out earlier than the ones in `secondList`.
+void _merge<T>(
+  int Function(T, T) compare,
+  List<T> firstList,
+  int firstStart,
+  int firstEnd,
+  List<T> secondList,
+  int secondStart,
+  int secondEnd,
+  List<T> target,
+  int targetOffset,
+) {
+  // No empty lists reaches here.
+  assert(firstStart < firstEnd);
+  assert(secondStart < secondEnd);
+  int cursor1 = firstStart;
+  int cursor2 = secondStart;
+  T firstElement = firstList[cursor1++];
+  T secondElement = secondList[cursor2++];
+  while (true) {
+    if (compare(firstElement, secondElement) <= 0) {
+      target[targetOffset++] = firstElement;
+      if (cursor1 == firstEnd) {
+        // Flushing second list after loop.
+        break;
+      }
+      firstElement = firstList[cursor1++];
+    } else {
+      target[targetOffset++] = secondElement;
+      if (cursor2 != secondEnd) {
+        secondElement = secondList[cursor2++];
+        continue;
+      }
+      // Second list empties first. Flushing first list here.
+      target[targetOffset++] = firstElement;
+      target.setRange(targetOffset, targetOffset + (firstEnd - cursor1), firstList, cursor1);
+      return;
+    }
+  }
+  // First list empties first. Reached by break above.
+  target[targetOffset++] = secondElement;
+  target.setRange(targetOffset, targetOffset + (secondEnd - cursor2), secondList, cursor2);
+}

packages/flutter/lib/src/widgets/focus_traversal.dart

@@ -6,7 +6,6 @@
 
 import 'dart:ui';
 
-import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart';
 

packages/flutter/lib/src/widgets/shortcuts.dart

@@ -6,7 +6,6 @@
 
 import 'dart:collection';
 
-import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
@@ -79,8 +78,8 @@ class KeySet<T extends KeyboardKey> {
         assert(!keys.contains(null)),
         _keys = HashSet<T>.from(keys);
 
-  /// Returns an unmodifiable view of the [KeyboardKey]s in this [KeySet].
-  Set<T> get keys => UnmodifiableSetView<T>(_keys);
+  /// Returns a copy of the [KeyboardKey]s in this [KeySet].
+  Set<T> get keys => _keys.toSet();
   final HashSet<T> _keys;
 
   @override

packages/flutter/test/foundation/collections_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 // @dart = 2.8
+import 'dart:math';
 
 import 'package:flutter/src/foundation/collections.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -60,4 +61,120 @@ void main() {
     expect(binarySearch(items, 3), 2);
     expect(binarySearch(items, 12), -1);
   });
+  test('MergeSortRandom', () {
+    final Random random = Random();
+    for (int i = 0; i < 250; i += 1) {
+      final List<int> list = List<int>(i);
+      for (int j = 0; j < i; j++) {
+        list[j] = random.nextInt(i); // Expect some equal elements.
+      }
+      mergeSort(list);
+      for (int j = 1; j < i; j++) {
+        expect(list[j - 1], lessThanOrEqualTo(list[j]));
+      }
+    }
+  });
+  test('MergeSortPreservesOrder', () {
+    final Random random = Random();
+    // Small case where only insertion call is called,
+    // larger case where the internal moving insertion sort is used
+    // larger cases with multiple splittings, numbers just around a power of 2.
+    for (final int size in <int>[8, 50, 511, 512, 513]) {
+      final List<OrderedComparable> list = List<OrderedComparable>(size);
+      // Class OC compares using id.
+      // With size elements with id's in the range 0..size/4, a number of
+      // collisions are guaranteed. These should be sorted so that the 'order'
+      // part of the objects are still in order.
+      for (int i = 0; i < size; i++) {
+        list[i] = OrderedComparable(random.nextInt(size >> 2), i);
+      }
+      mergeSort(list);
+      OrderedComparable prev = list[0];
+      for (int i = 1; i < size; i++) {
+        final OrderedComparable next = list[i];
+        expect(prev.id, lessThanOrEqualTo(next.id));
+        if (next.id == prev.id) {
+          expect(prev.order, lessThanOrEqualTo(next.order));
+        }
+        prev = next;
+      }
+      // Reverse compare on part of list.
+      final List<OrderedComparable> copy = list.toList();
+      final int min = size >> 2;
+      final int max = size - min;
+      mergeSort<OrderedComparable>(
+        list,
+        start: min,
+        end: max,
+        compare: (OrderedComparable a, OrderedComparable b) => b.compareTo(a),
+      );
+      prev = list[min];
+      for (int i = min + 1; i < max; i++) {
+        final OrderedComparable next = list[i];
+        expect(prev.id, greaterThanOrEqualTo(next.id));
+        if (next.id == prev.id) {
+          expect(prev.order, lessThanOrEqualTo(next.order));
+        }
+        prev = next;
+      }
+      // Equals on OC objects is identity, so this means the parts before min,
+      // and the parts after max, didn't change at all.
+      expect(list.sublist(0, min), equals(copy.sublist(0, min)));
+      expect(list.sublist(max), equals(copy.sublist(max)));
+    }
+  });
+  test('MergeSortSpecialCases', () {
+    for (final int size in <int>[511, 512, 513]) {
+      // All equal.
+      final List<OrderedComparable> list = List<OrderedComparable>(size);
+      for (int i = 0; i < size; i++) {
+        list[i] = OrderedComparable(0, i);
+      }
+      mergeSort(list);
+      for (int i = 0; i < size; i++) {
+        expect(list[i].order, equals(i));
+      }
+      // All but one equal, first.
+      list[0] = OrderedComparable(1, 0);
+      for (int i = 1; i < size; i++) {
+        list[i] = OrderedComparable(0, i);
+      }
+      mergeSort(list);
+      for (int i = 0; i < size - 1; i++) {
+        expect(list[i].order, equals(i + 1));
+      }
+      expect(list[size - 1].order, equals(0));
+
+      // All but one equal, last.
+      for (int i = 0; i < size - 1; i++) {
+        list[i] = OrderedComparable(0, i);
+      }
+      list[size - 1] = OrderedComparable(-1, size - 1);
+      mergeSort(list);
+      expect(list[0].order, equals(size - 1));
+      for (int i = 1; i < size; i++) {
+        expect(list[i].order, equals(i - 1));
+      }
+
+      // Reversed.
+      for (int i = 0; i < size; i++) {
+        list[i] = OrderedComparable(size - 1 - i, i);
+      }
+      mergeSort(list);
+      for (int i = 0; i < size; i++) {
+        expect(list[i].id, equals(i));
+        expect(list[i].order, equals(size - 1 - i));
+      }
+    }
+  });
+}
+
+class OrderedComparable implements Comparable<OrderedComparable> {
+  OrderedComparable(this.id, this.order);
+  final int id;
+  final int order;
+  @override
+  int compareTo(OrderedComparable other) => id - other.id;
+  @override
+  String toString() => 'OverrideComparable[$id,$order]';
 }