westonpace commented on a change in pull request #9607:
URL: https://github.com/apache/arrow/pull/9607#discussion_r605521628
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File path: cpp/src/arrow/util/async_generator.h
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@@ -1029,6 +1102,65 @@ AsyncGenerator<T>
MakeConcatenatedGenerator(AsyncGenerator<AsyncGenerator<T>> so
return MergedGenerator<T>(std::move(source), 1);
}
+template <typename T>
+struct Enumerated {
+ util::optional<T> value;
+ int index;
+ bool last;
+};
+
+template <typename T>
+struct IterationTraits<Enumerated<T>> {
+ static Enumerated<T> End() { return Enumerated<T>{{}, -1, false}; }
+ static bool IsEnd(const Enumerated<T>& val) { return !val.value.has_value();
}
+};
+
+template <typename T>
+class EnumeratingGenerator {
+ public:
+ EnumeratingGenerator(AsyncGenerator<T> source, T initial_value)
+ : state_(std::make_shared<State>(std::move(source),
std::move(initial_value))) {}
+
+ Future<Enumerated<T>> operator()() {
Review comment:
Yeah, if we could somehow know up front how many scan tasks is in a
fragment or how many record batches are in a scan task then this could be
simplified. However, I'm not sure we can safely make that assumption.
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