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quiver/lib/src/async/iteration.dart

-// Copyright 2013 Google Inc. All Rights Reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-part of quiver.async;
-
-/**
- * An asynchronous callback that returns a value.
- */
-typedef Future<T> AsyncAction<T>(e);
-
-/**
- * An asynchronous funcuntion that combines an element [e] with a previous value
- * [previous], for use with [reduceAsync].
- */
-typedef Future<T> AsyncCombiner<T>(T previous, e);
-
-/**
- * Calls [action] for each item in [iterable] in turn, waiting for the Future
- * returned by action to complete.
- *
- * If the Future completes to [true], iteration continues.
- *
- * The Future returned completes to [true] if the entire iterable was processed,
- * otherwise [false].
- */
-Future doWhileAsync(Iterable iterable, AsyncAction<bool> action) =>
-    _doWhileAsync(iterable.iterator, action);
-
-Future _doWhileAsync(
-    Iterator iterator, AsyncAction<bool> action) => (iterator.moveNext())
-    ? action(iterator.current).then((bool result) =>
-        (result) ? _doWhileAsync(iterator, action) : new Future.value(false))
-    : new Future.value(true);
-
-/**
- * Reduces a collection to a single value by iteratively combining elements
- * of the collection using the provided [combine] function. Similar to
- * [Iterable.reduce], except that [combine] is an async function that returns a
- * [Future].
- */
-Future reduceAsync(Iterable iterable, initialValue, AsyncCombiner combine) =>
-    _reduceAsync(iterable.iterator, initialValue, combine);
-
-Future _reduceAsync(Iterator iterator, currentValue, AsyncCombiner combine) {
-  if (iterator.moveNext()) {
-    return combine(currentValue, iterator.current)
-        .then((result) => _reduceAsync(iterator, result, combine));
-  }
-  return new Future.value(currentValue);
-}
-
-/**
- * Schedules calls to [action] for each element in [iterable]. No more than
- * [maxTasks] calls to [action] will be pending at once.
- */
-Future forEachAsync(Iterable iterable, AsyncAction action, {int maxTasks: 1}) {
-  if (maxTasks == null || maxTasks < 1) {
-    throw new ArgumentError("maxTasks must be greater than 0, was: $maxTasks");
-  }
-
-  if (iterable == null) {
-    throw new ArgumentError("iterable must not be null");
-  }
-
-  if (iterable.isEmpty) return new Future.value();
-
-  var completer = new Completer();
-  var iterator = iterable.iterator;
-  int pending = 0;
-  bool failed = false;
-
-  bool scheduleTask() {
-    if (pending < maxTasks && iterator.moveNext()) {
-      pending++;
-      var item = iterator.current;
-      scheduleMicrotask(() {
-        var task = action(item);
-        task.then((_) {
-          pending--;
-          if (failed) return;
-          if (!scheduleTask() && pending == 0) {
-            completer.complete();
-          }
-        }).catchError((e) {
-          if (failed) return;
-          failed = true;
-          completer.completeError(e);
-        });
-      });
-      return true;
-    }
-    return false;
-  }
-  while (scheduleTask()) {}
-  return completer.future;
-}