add a repeat-n-times overload to folly::window

Summary:
Add the ability to use folly::window without providing a vector ahead of time.
This allows window to scale to a large number of iterations.

Reviewed By: yfeldblum

Differential Revision: D7863481

fbshipit-source-id: 86c6b7926e9c3b7510233fac20a50c1fb38f5b57

folly/futures/Future-inl.h

@@ -547,6 +547,25 @@ class DeferredExecutor final : public Executor {
   Executor* executor_;
   folly::Synchronized<std::shared_ptr<FutureBatonType>> baton_;
 };
+
+// Vector-like structure to play with window,
+// which otherwise expects a vector of size `times`,
+// which would be expensive with large `times` sizes.
+struct WindowFakeVector {
+  using iterator = std::vector<size_t>::iterator;
+
+  WindowFakeVector(int size) : size_(size) {}
+
+  size_t operator[](const size_t index) const {
+    return index;
+  }
+  size_t size() const {
+    return size_;
+  }
+
+ private:
+  size_t size_;
+};
 } // namespace detail
 } // namespace futures
 
@@ -1451,6 +1470,12 @@ window(Collection input, F func, size_t n) {
   return window(executor, std::move(input), std::move(func), n);
 }
 
+template <class F>
+auto window(size_t times, F func, size_t n)
+    -> std::vector<invoke_result_t<F, size_t>> {
+  return window(futures::detail::WindowFakeVector(times), std::move(func), n);
+}
+
 template <class Collection, class F, class ItT, class Result>
 std::vector<Future<Result>>
 window(Executor* executor, Collection input, F func, size_t n) {

folly/futures/test/WindowTest.cpp

@@ -82,6 +82,20 @@ TEST(Window, basic) {
       }).get();
     EXPECT_EQ(6, res);
   }
+  {
+    SCOPED_TRACE("repeat same fn");
+    auto res = reduce(
+      window(
+        5UL,
+        [](size_t iteration) {
+          return folly::makeFuture(iteration); },
+        2),
+        0UL,
+        [](size_t sum, const Try<size_t>& b) {
+          return sum + b.value();
+        }).get();
+    EXPECT_EQ(0 + 1 + 2 + 3 + 4, res);
+  }
 }
 
 TEST(Window, exception) {