Add top-level collection-manipulation functions.

lib/src/functions.dart

Index: lib/src/functions.dart
diff --git a/lib/src/functions.dart b/lib/src/functions.dart
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+// Copyright (c) 2016, 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.
+
+import 'utils.dart';
+
+// TODO(nweiz): When sdk#26488 is fixed, use overloads to ensure that if [key]
+// or [value] isn't passed, `K2`/`V2` defaults to `K1`/`V1`, respectively.
+/// Creates a new map from [map] with new keys and values. 
+///
+/// The return values of [key] are used as the keys and the return values of
+/// [value] are used as the values for the new map.
+Map/*<K2, V2>*/ mapMap/*<K1, V1, K2, V2>*/(Map/*<K1, V1>*/ map,
+    {/*=K2*/ key(/*=K1*/ key, /*=V1*/ value),
+    /*=V2*/ value(/*=K1*/ key, /*=V1*/ value)}) {
+  key ??= (mapKey, _) => mapKey as dynamic/*=K2*/;
+  value ??= (_, mapValue) => mapValue as dynamic/*=V2*/;
+
+  var result = /*<K2, V2>*/{};
+  map.forEach((mapKey, mapValue) {
+    result[key(mapKey, mapValue)] = value(mapKey, mapValue);
+  });
+  return result;
+}
+
+/// Returns a new map with all key/value pairs in both [map1] and [map2].
+///
+/// If there are keys that occur in both maps, the [value] function is used to
+/// select the value that goes into the resulting map based on the two original
+/// values. If [value] is omitted, the value from [map2] is used.
+Map/*<K, V>*/ mergeMaps/*<K, V>*/(Map/*<K, V>*/ map1, Map/*<K, V>*/ map2,
+    {/*=V*/ value(/*=V*/ value1, /*=V*/ value2)}) {
+  var result = new Map/*<K, V>*/.from(map1);
+  if (value == null) return result..addAll(map2);
+
+  map2.forEach((key, mapValue) {
+    result[key] = result.containsKey(key)
+        ? value(result[key], mapValue)
+        : mapValue;
+  });
+  return result;
+}
+
+/// Groups the elements in [values] by the value returned by [key].
+///
+/// Returns a map from keys computed by [key] to a list of all values for which
+/// [key] returns that key. The values appear in the list in the same relative
+/// order as in [values].
+Map<dynamic/*=T*/, List/*<S>*/> groupBy/*<S, T>*/(Iterable/*<S>*/ values,
+    /*=T*/ key(/*=S*/ element)) {
+  var map = /*<T, List<S>>*/{};
+  for (var element in values) {
+    var list = map.putIfAbsent(key(element), () => []);
+    list.add(element);
+  }
+  return map;
+}
+
+/// Returns the element of [values] for which [orderBy] returns the minimum
+/// value.
+///
+/// The values returned by [orderBy] are compared using the [compare] function.
+/// If [compare] is omitted, values must implement [Comparable<T>] and they are
+/// compared using their [Comparable.compareTo].
+/*=S*/ minBy/*<S, T>*/(Iterable/*<S>*/ values, /*=T*/ orderBy(/*=S*/ element),
+    {int compare(/*=T*/ value1, /*=T*/ value2)}) {
+  compare ??= defaultCompare/*<T>*/();
+
+  var/*=S*/ minValue;
+  var/*=T*/ minOrderBy;
+  for (var element in values) {
+    var elementOrderBy = orderBy(element);
+    if (minOrderBy == null || compare(elementOrderBy, minOrderBy) < 0) {
+      minValue = element;
+      minOrderBy = elementOrderBy;
+    }
+  }
+  return min;
+}
+
+/// Returns the element of [values] for which [orderBy] returns the maximum
+/// value.
+///
+/// The values returned by [orderBy] are compared using the [compare] function.
+/// If [compare] is omitted, values must implement [Comparable<T>] and they are
+/// compared using their [Comparable.compareTo].
+/*=S*/ maxBy/*<S, T>*/(Iterable/*<S>*/ values, /*=T*/ orderBy(/*=S*/ element),
+    {int compare(/*=T*/ value1, /*=T*/ value2)}) {
+  compare ??= defaultCompare/*<T>*/();
+
+  var/*=S*/ maxValue;
+  var/*=T*/ maxOrderBy;
+  for (var element in values) {
+    var elementOrderBy = orderBy(element);
+    if (maxOrderBy == null || compare(elementOrderBy, maxOrderBy) > 0) {
+      maxValue = element;
+      maxOrderBy = elementOrderBy;
+    }
+  }
+  return max;
+}
+
+/// Returns the [transitive closure][] of [graph].
+///
+/// [transitive closure]: https://en.wikipedia.org/wiki/Transitive_closure
+///
+/// This interprets [graph] as a directed graph with a vertex for each key and
+/// edges from each key to the values associated with that key.
+///
+/// Assumes that every vertex in the graph has a key to represent it, even if
+/// that vertex has no outgoing edges. For example, `{"a": ["b"]}` is not valid,
+/// but `{"a": ["b"], "b": []}` is.
+Map<dynamic/*=T*/, Set/*<T>*/> transitiveClosure/*<T>*/(
+    Map<dynamic/*=T*/, Iterable/*<T>*/> graph) {
+  // This uses [Warshall's algorithm][], modified not to add a vertex from each
+  // node to itself.
+  //
+  // [Warshall's algorithm]: https://en.wikipedia.org/wiki/Floyd%E2%80%93Warshall_algorithm#Applications_and_generalizations.
+  var result = /*<T, Set>*/{};
+  graph.forEach((vertex, edges) {
+    result[vertex] = new Set/*<T>*/.from(edges);
+  });
+
+  // Lists are faster to iterate than maps, so we create a list since we're
+  // iterating repeatedly.
+  var keys = graph.keys.toList();
+  for (var vertex1 in keys) {
+    for (var vertex2 in keys) {
+      for (var vertex3 in keys) {
+        if (result[vertex2].contains(vertex1) &&
+            result[vertex1].contains(vertex3)) {
+          result[vertex2].add(vertex3);
+        }
+      }
+    }
+  }
+
+  return result;
+}