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Pods/PromisesSwift/Sources/Promises/Promise+Reduce.swift

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+// Copyright 2018 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.
+
+import Dispatch
+
+public extension Promise {
+  typealias Reducer<Element> = (Value, Element) throws -> Promise<Value>
+
+  /// Sequentially reduces a collection of values to a single promise using a given combining block
+  /// and the value `self` resolves with as initial value.
+  /// - parameters:
+  ///   - queue: A queue to execute `reducer` block on.
+  ///   - items: A sequence of values to process in order.
+  ///   - reducer: A block to combine an accumulating value and an element of the sequence into
+  ///              a promise resolved with the new accumulating value, to be used in the next call
+  ///              of the `reducer` or returned to the caller.
+  /// - returns: A new pending promise returned from the last `reducer` invocation.
+  ///            Or `self` if `items` is empty.
+  @discardableResult
+  func reduce<Element>(
+    on queue: DispatchQueue = .promises,
+    _ items: Element...,
+    combine reducer: @escaping Reducer<Element>
+  ) -> Promise<Value> {
+    return reduce(on: queue, items, reducer)
+  }
+
+  /// Sequentially reduces a collection of values to a single promise using a given combining block
+  /// and the value `self` resolves with as initial value.
+  /// - parameters:
+  ///   - queue: A queue to execute `reducer` block on.
+  ///   - items: A sequence of values to process in order.
+  ///   - reducer: A block to combine an accumulating value and an element of the sequence into
+  ///              a promise resolved with the new accumulating value, to be used in the next call
+  ///              of the `reducer` or returned to the caller.
+  /// - returns: A new pending promise returned from the last `reducer` invocation.
+  ///            Or `self` if `items` is empty.
+  @discardableResult
+  func reduce<Container: Sequence>(
+    on queue: DispatchQueue = .promises,
+    _ items: Container,
+    _ reducer: @escaping Reducer<Container.Element>
+  ) -> Promise<Value> {
+    return items.reduce(self) { promise, item in
+      promise.then { value in
+        try reducer(value, item)
+      }
+    }
+  }
+}