westonpace commented on a change in pull request #9945:
URL: https://github.com/apache/arrow/pull/9945#discussion_r609930251
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File path: cpp/src/arrow/util/async_generator.h
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@@ -1063,6 +1063,86 @@ 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;
Review comment:
The need for this originated from trying to resequence merged streams.
So given a generator that returns streams (A, B, C), each of which return items
(1, 2), the merged generator might return something like...
```
A1
B1
A2
C1
C2
B2
```
Any end tokens are removed by the merge operation. Otherwise the combined
stream would terminate after A2.
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