Add collection-helpers library.

pkg/collection_helpers/lib/algorithms.dart

+// 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.
+
+/**
+ * Operations on collections.
+ */
+library dart.collection_helper.algorithms;
+
+import "dart:math" show Random;
+
+/** Version of [binarySearch] optimized for comparable keys */
+int _comparableBinarySearch(List<Comparable> list, Comparable key) {
+  int min = 0;
+  int max = list.length;
+  while (min < max) {
+    int mid = min + ((max - min) >> 1);
+    var element = list[mid];
+    int comp = element.compareTo(key);
+    if (comp == 0) return mid;
+    if (comp < 0) {
+      min = mid + 1;
+    } else {
+      max = mid;
+    }
+  }
+  return -1;
+}
+
+/**
+ * Returns a position of the [key] in [sortedList], if it is there.
+ *
+ * If the list isn't sorted according to the [compare] function, the result
+ * is unpredictable.
+ *
+ * If [compare] is omitted, it defaults to calling [Comparable.compareTo] on
+ * the objects.
+ *
+ * Returns -1 if [key] is not in the list by default.
+ */
+int binarySearch(List sortedList, var key,
+                 { int compare(var a, var b) }) {
+  if (compare == null) {
+    return _comparableBinarySearch(sortedList, key);
+  }
+  int min = 0;
+  int max = sortedList.length;
+  while (min < max) {
+    int mid = min + ((max - min) >> 1);
+    var element = sortedList[mid];
+    int comp = compare(element, key);
+    if (comp == 0) return mid;
+    if (comp < 0) {
+      min = mid + 1;
+    } else {
+      max = mid;
+    }
+  }
+  return -1;
+}
+
+
+/**
+ * Shuffles a list randomly.
+ *
+ * A sub-range of a list can be shuffled by providing [start] and [end].
+ */
+void shuffle(List list, [int start = 0, int end = null]) {
+  Random random = new Random();
+  if (end == null) end = list.length;
+  int length = end - start;
+  while (length > 1) {
+    int pos = random.nextInt(length);
+    length--;
+    var tmp1 = list[start + pos];
+    list[start + pos] = list[start + length];
+    list[start + length] = tmp1;
+  }
+}
+
+
+/**
+ * Reverses a list, or a part of a list, in-place.
+ */
+void reverse(List list, [int start = 0, int end = null]) {
+  if (end == null) end = list.length;
+  _reverse(list, start, end);
+}
+
+// Internal helper function that assumes valid arguments.
+void _reverse(List list, int start, int end) {
+  for (int i = start, j = end - 1; i < j; i++, j--) {
+    var tmp = list[i];
+    list[i] = list[j];
+    list[j] = tmp;
+  }
+}
+
+/**
+ * Sort a list using insertion sort.
+ *
+ * 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(List list,
+                   { int compare(a, b),
+                     int start: 0,
+                     int end: null }) {
+  // If the same method could have both positional and named optional
+  // parameters, this should be (list, [start, end], {compare}).
+  if (end == null) end = list.length;
+  if (compare == null) compare = Comparable.compare;
+  _insertionSort(list, compare, start, end, start + 1);
+}
+
+/**
+ * Internal helper function that assumes arguments correct.
+ *
+ * Assumes that the elements up to [sortedUntil] (not inclusive) are
+ * already sorted. The [sortedUntil] values should always be at least
+ * `start + 1`.
+ */
+void _insertionSort(List list, int compare(a, b), int start, int end,
+                    int sortedUntil) {
+  for (int pos = sortedUntil; pos < end; pos++) {
+    int min = start;
+    int max = pos;
+    var element = list[pos];
+    while (min < max) {
+      int mid = min + ((max - min) >> 1);
+      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;
+  }
+}
+
+/** Limit below which merge sort defaults to insertion sort. */
+const int _MERGE_SORT_LIMIT = 32;
+
+/**
+ * Sorts a list, or a range of a list, using the merge sort algorithm.
+ *
+ * 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.
+ */
+void mergeSort(List list, {int start: 0, int end: null, int compare(a, b)}) {
+  if (end == null) end = list.length;
+  if (compare == null) compare = Comparable.compare;
+  int length = end - start;
+  if (length < 2) return;
+  if (length < _MERGE_SORT_LIMIT) {
+    _insertionSort(list, compare, start, end, start + 1);
+    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.
+  int middle = start + ((end - start) >> 1);
+  int firstLength = middle - start;
+  int secondLength = end - middle;
+  // secondLength is always the same as firstLength, or one greater.
+  List scratchSpace = new List(secondLength);
+  _mergeSort(list, compare, middle, end, scratchSpace, 0);
+  int firstTarget = end - firstLength;
+  _mergeSort(list, compare, start, middle, list, firstTarget);
+  _merge(compare,
+         list, firstTarget, end,
+         scratchSpace, 0, secondLength,
+         list, start);
+}
+
+/**
+ * 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(List list, int compare(a, b), int start, int end,
+                          List target, int targetOffset) {
+  int length = end - start;
+  if (length == 0) return;
+  target[targetOffset] = list[start];
+  for (int i = 1; i < length; i++) {
+    var element = list[start + i];
+    int min = targetOffset;
+    int max = targetOffset + i;
+    while (min < max) {
+      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(List list, int compare(a, b), int start, int end,
+                List target, int targetOffset) {
+  int length = end - start;
+  if (length < _MERGE_SORT_LIMIT) {
+    _movingInsertionSort(list, compare, start, end, target, targetOffset);
+    return;
+  }
+  int middle = start + (length >> 1);
+  int firstLength = middle - start;
+  int secondLength = end - middle;
+  // Here secondLength >= firstLength (differs by at most one).
+  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(int compare(a, b),
+            List firstList, int firstStart, int firstEnd,
+            List secondList, int secondStart, int secondEnd,
+            List target, int targetOffset) {
+  // No empty lists reaches here.
+  assert(firstStart < firstEnd);
+  assert(secondStart < secondEnd);
+  int cursor1 = firstStart;
+  int cursor2 = secondStart;
+  var firstElement = firstList[cursor1++];
+  var secondElement = secondList[cursor2++];
+  while (true) {
+    if (compare(firstElement, secondElement) <= 0) {
+      target[targetOffset++] = firstElement;
+      if (cursor1 == firstEnd) break;  // Flushing second list after loop.
+      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);
+}