Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/4804#discussion_r25553306
--- Diff:
core/src/test/java/org/apache/spark/util/collection/TestTimSort.java ---
@@ -0,0 +1,133 @@
+/*
+ * 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.util.collection;
+
+import java.util.*;
+
+/**
+ * This codes generates a int array which fails the standard TimSort.
+ *
+ * The blog that reported the bug
+ * http://www.envisage-project.eu/timsort-specification-and-verification/
+ *
+ * The algorithms to reproduce the bug is obtained from the reporter of
the bug
+ * https://github.com/abstools/java-timsort-bug
+ *
+ * Licensed under Apache License 2.0
+ * https://github.com/abstools/java-timsort-bug/blob/master/LICENSE
+ */
+public class TestTimSort {
+
+ private static final int MIN_MERGE = 32;
+
+ /**
+ * Returns an array of integers that demonstrate the bug in TimSort
+ */
+ public static int[] getTimSortBugTestSet(int length) {
+ int minRun = minRunLength(length);
+ List<Long> runs = runsJDKWorstCase(minRun, length);
+ return createArray(runs, length);
+ }
+
+ private static int minRunLength(int n) {
+ int r = 0; // Becomes 1 if any 1 bits are shifted off
+ while (n >= MIN_MERGE) {
+ r |= (n & 1);
+ n >>= 1;
+ }
+ return n + r;
+ }
+
+ private static int[] createArray(List<Long> runs, int length) {
+ int[] a = new int[length];
+ Arrays.fill(a, 0);
+ int endRun = -1;
+ for (long len : runs)
+ a[endRun += len] = 1;
+ a[length - 1] = 0;
+ return a;
+ }
+
+ /**
+ * Fills <code>runs</code> with a sequence of run lengths of the form<br>
+ * Y_n x_{n,1} x_{n,2} ... x_{n,l_n} <br>
+ * Y_{n-1} x_{n-1,1} x_{n-1,2} ... x_{n-1,l_{n-1}} <br>
+ * ... <br>
+ * Y_1 x_{1,1} x_{1,2} ... x_{1,l_1}<br>
+ * The Y_i's are chosen to satisfy the invariant throughout execution,
+ * but the x_{i,j}'s are merged (by <code>TimSort.mergeCollapse</code>)
+ * into an X_i that violates the invariant.
+ *
+ * @param length The sum of all run lengths that will be added to
<code>runs</code>.
+ */
+ private static List<Long> runsJDKWorstCase(int minRun, int length) {
--- End diff --
Oh! can't believe I missed the file at the end here. OK. I see that the
test case that gets generated is really big, not something you can paste into
the source. Hm, OK well I suggest fixing the license situation here at a minimum
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