Apply a function to each value in the list in turn, accumulating the results in an accumulator. The final value of the accumulator is returned as the value of reduce()
.
The framework calls this method for each <key, (list of values)>
pair in the grouped inputs. Output values must be of the same type as input values. Input keys must not be altered. Typically all values are combined into zero or one value.
Output pairs are collected with calls to {@link OutputCollector#collect(WritableComparable,Writable)}.
Applications can use the {@link Reporter} provided to report progress or just indicate that they are alive. In scenarios where the application takes an insignificant amount of time to process individual key/value pairs, this is crucial since the framework might assume that the task has timed-out and kill that task. The other way of avoiding this is to set mapred.task.timeout to a high-enough value (or even zero for no time-outs).
@param key the key. @param values the list of values to reduce. @param output to collect keys and combined values. @param reporter facility to report progress.The {@link Envelope#getCoordinateReferenceSystem coordinate reference system} of thesource envelope is ignored. The coordinate reference system of the target envelope will be {@link #getCoordinateReferenceSystem2D} or {@code null}. @param envelope The envelope to reduce, or {@code null}. This envelope will not be modified. @return An envelope with exactly 2 dimensions, or {@code null} if {@code envelope} was null.The returned envelope is always a new instance, so it can be modified safely. @since 2.5
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