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https://issues.apache.org/jira/browse/FLINK-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15140666#comment-15140666
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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_r52446045
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/state/GenericFoldingState.java
---
@@ -0,0 +1,126 @@
+/**
+ * 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
+ * <p/>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p/>
+ * 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.runtime.state;
+
+import org.apache.flink.api.common.functions.FoldFunction;
+import org.apache.flink.api.common.state.FoldingState;
+import org.apache.flink.api.common.state.FoldingStateDescriptor;
+import org.apache.flink.api.common.state.ValueState;
+import org.apache.flink.api.common.state.ValueStateDescriptor;
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+
+/**
+ * Generic implementation of {@link FoldingState} based on a wrapped
{@link ValueState}.
+ *
+ * @param <K> The type of the key.
+ * @param <N> The type of the namespace.
+ * @param <T> The type of the values that can be folded into the state.
+ * @param <ACC> The type of the value in the folding state.
+ * @param <Backend> The type of {@link AbstractStateBackend} that manages
this {@code KvState}.
+ * @param <W> Generic type that extends both the underlying {@code
ValueState} and {@code KvState}.
+ */
+public class GenericFoldingState<K, N, T, ACC, Backend extends
AbstractStateBackend, W extends ValueState<ACC> & KvState<K, N,
ValueState<ACC>, ValueStateDescriptor<ACC>, Backend>>
+ implements FoldingState<T, ACC>, KvState<K, N, FoldingState<T, ACC>,
FoldingStateDescriptor<T, ACC>, Backend> {
+
+ private final W wrappedState;
+ private final FoldFunction<T, ACC> foldFunction;
+
+ @SuppressWarnings("unchecked")
+ public GenericFoldingState(ValueState<ACC> wrappedState,
FoldFunction<T, ACC> foldFunction) {
--- End diff --
java docs
> 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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