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https://issues.apache.org/jira/browse/FLINK-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15140656#comment-15140656
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ASF GitHub Bot commented on FLINK-2991:
---------------------------------------
Github user tillrohrmann commented on a diff in the pull request:
https://github.com/apache/flink/pull/1605#discussion_r52445422
--- Diff:
flink-core/src/main/java/org/apache/flink/api/common/state/FoldingStateDescriptor.java
---
@@ -0,0 +1,108 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you 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.
+ */
+
+package org.apache.flink.api.common.state;
+
+import org.apache.flink.api.common.functions.FoldFunction;
+import org.apache.flink.api.common.functions.RichFunction;
+import org.apache.flink.api.common.typeinfo.TypeInformation;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+
+import static java.util.Objects.requireNonNull;
+
+/**
+ * {@link StateDescriptor} for {@link ReducingState}. This can be used to
create partitioned
--- End diff --
for `FoldingState`
> Extend Window Operators to Allow Efficient Fold Operation
> ---------------------------------------------------------
>
> Key: FLINK-2991
> URL: https://issues.apache.org/jira/browse/FLINK-2991
> Project: Flink
> Issue Type: Improvement
> Components: Streaming
> Reporter: Aljoscha Krettek
> Assignee: Aljoscha Krettek
>
> Right now, a window fold is implemented as a WindowFunction that gets all the
> elements as input. No pre-aggregation is performed. The window operator
> should be extended to also allow the fold to also be pre-aggregated.
> This requires changing the signature of the {{WindowBuffer}} so that it can
> emit a type other than the input type.
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