Lab 05: Sorting
sorting faster than ever before
This Assignment is Optional. If you would like to receive credit for the assignment, please submit it by Wednesday, May 19th, 23:59 AoE.
Background
Sorting is a fundamental task in computer science. As such, most generalpurpose programming languages provide a builtin sorting function. For example, Java provides the Arrays.sort(...)
method. The sort
method will takes an array numbers (int
, double
, float
, etc.) and sorts the array inplace in ascending order. For example, after calling
1
2
double[] data = {4.0, 2.0, 3.0, 1.0};
Arrays.sort(data)
data
will store the values {1.0, 2.0. 3.0, 4.0}
. The Arrays.sort
method is quite efficient: on my laptop, it sorts a random array of 10 million double
s in about 1 second. Nonetheless, there is always room for improvement!
Your Task
In this lab, you will use multithreading and parallelism in order to implement a faster sorting procedure than Arrays.sort
(at least for large arrays). Specifically, you should implement a method Sorting.parallelSort(double[] data)
that sorts an array of double
s in place. To get started, download the following archive, which includes a program SortingTester.java
to test your implementation.
Grading
Your program will be graded on a 6 point scale according to the following criteria:
 Correctness (2 pts). Your program employs multithreading and sorts all tested inputs correctly.
 Note. Be sure to test your program on “easy” inputs such as those repeated values.

Performance (2 pts). Your program sorts large randomly generated inputs significantly faster than the provided baseline implementation. Your program should run at least twice as fast as
Arrays.sort()
on arrays of aroud 10 million entries.  Style & Documentation (2 pts). Code is wellorganized and commented. Submission includes a
README
file that describes your program at a high level, and briefly discusses optimizations and tests performed.
Suggestions and Resources
Many of the fastest practical sorting algorithms use some variant of quicksort. We briefly describe a highlevel description of quicksort. For simplicity we assume that elements in the array are pairwise distinct (i.e., not value is repeated), but your program should handle the case where repeated values are allowed.
 Choose a
pivot
element in the array—typically either some fixed element or the a random element in the array.  Reorder the array so that
pivot
is at indexp
and all elements at indices
i < p
are smaller thanpivot
 all elements at indices
i > p
are greater thanpivot
 Recursively sort
 the subarray from indices
0
top1
 the subarray from indices
p+1
ton
(wheren
is the original size of the array).
 the subarray from indices
The recursive sorting in step 3 can again be performed using quicksort. To this end, it is useful to define the general method to take three parameters: the array data
to be sorted, as well as indices i
and j
. A call to quicksort(data, i, j)
will return an array such that the elements of data
with indices i, i+1,..., j1
are sorted, while the remaining entries are unchanged.
For our purposes, the quicksort algorithm is appealing for two reasons. First, it sorts elements inplace. That is, all of the operations can be performed on the original array without duplicating data. (In particular, this is true of step 2). Second, the substeps of step 3 of the procedure can be performed in parallel.
For a detailed account of implementing and parallelizing quicksort (and other sorting algorithms), see
 Sorting and Selection in Sequential and Parallel Data Structures and Algorithms (book chapter available here).
Since quicksort follows a divideandconquer approach, you may find the following documentation/tutorial helpful:
Note that there are many different variants of quicksort. Exactly which version you use and how it is implemented can have a significant impact on your program’s performance.
Tips

For reasonably small arrays,
Arrays.sort()
is likely to be faster than a more complex multithreaded procedure. You may freely useArrays.sort()
as a subroutine/base case for your sorting procedure. 
If you choose to implement quicksort, the most costly part of the execution is the “reordering” step (step 2). Even though the procedure can be conceptually simple, there are a lot of opportunities for inefficiency. In particular, for a large array you will want to consider the cache performance and memory access pattern to the input.