Github user mpetruska commented on a diff in the pull request:
https://github.com/apache/spark/pull/19659#discussion_r149696337
--- Diff:
mllib/src/main/scala/org/apache/spark/ml/extensions/seq/package.scala ---
@@ -0,0 +1,121 @@
+/*
+ * 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.spark.ml.extensions
+
+import scala.annotation.tailrec
+import scala.collection.immutable.Queue
+
+import org.apache.spark.annotation.Since
+
+package object seq {
+
+ /**
+ * Calculates sliding windows over multiple length parameters
simultaneously.
+ * @param x the input sequence over which sliding windows will be
collected
+ * @param min the inclusive minimal length of the window to be collected
+ * @param max the inclusive maximal length of the window to be collected
+ * @return the collected windows
+ *
+ * @example {{{
+ * multiSliding(1 to 5, min = 2, max = 4) == Seq(
+ * Seq(1, 2), Seq(1, 2, 3), Seq(1, 2, 3, 4),
+ * Seq(2, 3), Seq(2, 3, 4), Seq(2, 3, 4, 5),
+ * Seq(3, 4), Seq(3, 4, 5),
+ * Seq(4, 5)
+ * )
+ *
+ * multiSliding(1 to 10, min = 2, max = 5) == Seq(
+ * Seq(1, 2), Seq(1, 2, 3), Seq(1, 2, 3, 4), Seq(1, 2, 3, 4, 5),
+ * Seq(2, 3), Seq(2, 3, 4), Seq(2, 3, 4, 5), Seq(2, 3, 4, 5, 6),
+ * Seq(3, 4), Seq(3, 4, 5), Seq(3, 4, 5, 6), Seq(3, 4, 5, 6, 7),
+ * Seq(4, 5), Seq(4, 5, 6), Seq(4, 5, 6, 7), Seq(4, 5, 6, 7, 8),
+ * Seq(5, 6), Seq(5, 6, 7), Seq(5, 6, 7, 8), Seq(5, 6, 7, 8, 9),
+ * Seq(6, 7), Seq(6, 7, 8), Seq(6, 7, 8, 9), Seq(6, 7, 8, 9, 10),
+ * Seq(7, 8), Seq(7, 8, 9), Seq(7, 8, 9, 10),
+ * Seq(8, 9), Seq(8, 9, 10),
+ * Seq(9, 10)
+ * )
+ * }}}
+ */
+ @Since("From which version?")
+ def multiSliding[A](x: Seq[A], min: Int, max: Int): Seq[Seq[A]] = {
+ type B = Seq[A]
+
+ def addWindowsFromBuffer(acc: List[B], buffer: Queue[A]): List[B] = {
+ buffer.drop(min - 1).foldLeft((acc, buffer.take(min - 1))) {
+ case ((a, b), current) =>
+ val newB = b.enqueue(current)
+ (newB :: a, newB)
+ }._1
+ }
+
+ @tailrec
+ def addWindowsFromFinalBuffer(acc: List[B], buffer: Queue[A]): List[B]
= {
+ buffer.dequeueOption match {
+ case Some((_, tail)) =>
addWindowsFromFinalBuffer(addWindowsFromBuffer(acc, tail), tail)
+ case None => acc
+ }
+ }
+
+ def calculateMultiSliding(): List[B] = {
--- End diff --
Nope, unfortunately it's not the case:
```Scala
scala> def slidingOriginal(x: Seq[String], n: Int): Seq[String] = {
| x.iterator.sliding(n).withPartial(false).map(_.mkString("
")).toSeq
| }
slidingOriginal: (x: Seq[String], n: Int)Seq[String]
scala> def slidingNew(x: Seq[String], n: Int): Seq[String] = {
| multiSliding(x, n, n).map(_.mkString(" "))
| }
slidingNew: (x: Seq[String], n: Int)Seq[String]
scala> time(slidingOriginal((1 to 10000).map(_.toString), 5))
res9: scala.concurrent.duration.FiniteDuration = 2427473 nanoseconds
scala> time(slidingOriginal((1 to 10000).map(_.toString), 5)).toMillis
res10: Long = 3
scala> time(slidingNew((1 to 10000).map(_.toString), 5)).toMillis
res11: Long = 15
scala> time(slidingOriginal((1 to 100000).map(_.toString), 5)).toMillis
res12: Long = 11
scala> time(slidingNew((1 to 100000).map(_.toString), 5)).toMillis
res13: Long = 479
scala> time(slidingOriginal((1 to 10000).map(_.toString), 50)).toMillis
res14: Long = 1
scala> time(slidingNew((1 to 10000).map(_.toString), 50)).toMillis
res15: Long = 136
```
Adding code to fall back to the original implementation if `n == maxN`...
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