SortingCompetition2018- UMN MORRIS

Table of contents

• Our Sorting Solutions
• Goal of the competition
• The data
• How is the data generated
• String Feature Background
• How do you need to sort the data
• Setup for sorting

Our Sorting Solutions

First we tried to implement counting sort, because it runs in linear time and there are a limited number of lrmus lengths. However, it couldn't handle tiebreakers like alphabetical order because those don't fit counting sort.

Next we tried radix sort, by considering the tiebreakers as digits and using Arrays.Sort for each, but it ended up being slower than using Arrays.sort with the original comparator.

Next we implemented quicksort (with a switch to insertion sort for the last 10) which runs slightly faster, and sorts the items correctly.

We also modified the compareTo function, and we hoped it would be faster by using subtracting instead of a comparison, but it made no difference.

Finding the LRMUS

We changed updateAt so that the string toMatch is always at least as long as the current lrmus, which saved about 100ms. Here's the change:

for(int end=start + length; end<n;end++){
//this used to be:
for(int end=start; end<n;end++){

This also required us to change the starting value for length to 0 instead of Integer.MIN_VALUE After making that change we removed the redundant comparisons in the function.

We tried to change the for loop in the findBest() function so that when you get near the end you don't call updateAt() if it can't produce an lrmus longer than the current best, but the operation in the for loop actually made it slower. Here's what it looked like:

public void findBest(){
  for(int i =0; i< referenceStr.length()-length;i++){
      updateAt(i);
  }
}

All of the code was done together at the lab. We used pair programming where we switched from time to time.

GroupZ was our attempt at radix sort. Our submission is Group2.

Goal of the competition

The Sorting Competition is a multi-lab exercise on developing the fastest sorting algorithm for a given type of data. By "fast" we mean the actual running time and not the Big-Theta approximation. The solutions are developed in Java and will be ran on a single processor.

The data

Data for this sorting oompetition consists of standard UTF-8 character strings of length 100 to 1000 characters passed to the sorting method as an array of strings. The strings will be randomly selected from the file corpus.txt (available on this repo) which is comprised of the ASCII versions of the works of Arthur Conan Doyle and the complete works of Shakespeare. In the first file source all newlines have been converted to spaces, and in the second, all newlines and tabs have been converted to spaces. The initial files were downloaded from https://sherlock-holm.es/stories/plain-text/cnus.txt and https://ocw.mit.edu/ans7870/6/6.006/s08/lecturenotes/files/t8.shakespeare.txt respectively.

How is the data generated

• A random number l between 100 and 1000 (uniform) is generated
• A random location t within the text file corpus.txt (positions 1 to n-l+1) is selected
• The substring covering t to t+l-1 is extracted

The generated strings are written into a data file one per line. Sample data files are: data1.txt (1000 elements), data2.txt (1000 elements), and data3.txt (10000 elements).

String feature background (LRMUS and M-LRMUS)

Every text string has at least one substring (possibly the entire string) that is unique. A left to right minimial unique substring (LRMUS) is one that becomes repetitive when the last letter is removed. For example, the sentence

the alien was confused by the assignment

Has many unique substrings: the alien, confused, assignment, l, li, ali, etc.

It also has many LRMUS's. Some include:

the al, he al, e al, al, l,

You can check by removing the last letters. Consider the two substrings the al and the as. Removing the last letter produces the a which is no longer unique.

Unique substrings of length 1, such as l, are also considered to be an LRMUS. Note that the space character in the sample string is not an LRMUS because that character appears more than once.

An LRMUS is maximal (M-LRMUS) if it is the longest LRMUS in a string. Every string will have at least one M-LRMUS, but it may have several. In our sample string the al and the as are both M-LRMUSs. If the string had been of the form

the alien was confused by the a

then only the al is an M-LRMUS.

How do you need to sort the data

The strings are compared as follows

1. The one with the shortest (M-LRMUS) comes first
2. In the event of a tie, the string with the left-most LRMUS comes first
3. In the event of a tie, the string whose LRMUS is alphabetically earliest comes first
4. In the event of a tie, the normal alphabetical ordering determines which comes first

For instance:

• Zunzibar comes before rorocco since the length of the shortest M-LRMUS in Zunzibar is 1, but in rorocco it is 2.
• Zunzibar comes before rOrocco: although both have a shortest M-LRMUS of 1, The left-most LRMUS in Zanzibar is Z in position 1 and the left-most LRMUS in rOrocco is O which appears in position 2.
• Zunzibar comes after Africa: The left-most M-LRMUS in the first string is Z in position 1 and it is A, also in position 1, in the second. Since A comes before Z.
• Aunzibar comes after Africa: Both have the same left-most M-RLMUS of A in position 1. So we revert to normal alphabetical order.

The file Group0.java provides a Comparator that implements this comparison and provides some tests. Please consult it as needed. However, note that this is a very slow implementation, and you should think of a way to make it much faster.

Once the data is sorted, it is written out to the output file, also one number per line, in the increasing order (according to the comparison given above). The files out1.txt, out2.txt, and out3.txt have the results of sorting for the three given data files.

Setup for sorting

The file Group0.java provides a template for the setup for your solution. Your class will be called GroupN, where N is the group number that is assigned to your group. The template class runs the sorting method once before the timing for the JVM warmup. It also pauses for 10ms before the actual test to let any leftover I/O or garbage collection to finish. Since the warmup and the actual sorting are done on the same array (for no reason other than simplicity), the array is cloned from the same input data.

The data reading, the array cloning, the warmup sorting, and writing out the output are all outside of the timed portion of the method, and thus do not affect the total time.

You may not use any global variables that depend on your data. You may, however, have global constants that are initialized to fixed values (no computation!) before the data is being read and stay the same throughout the run. These constants may be arrays of no more than 1000 long numberss or equivalent amount of memory. For instance, if you are storing an array of objects that contain two long fields, you can only have 500 of them. We consider one long to be the same as two int numbers, so you can store an array of 2000 int numbers.
If in doubt about specific cases, please discuss with me.

The method in the Group0.java files that you may modify is the sort method. It must take the array of strings. The return type of the method can be what it is now, which is the same as the parameter type String [], or it can be a different array type. If you are sorting in-place, i.e. the sorted result is in the same array, then you can just return a reference to that array, as my prototype method does, or make your sorting method void. If you are returning a different type of an array, the following rules have to be followed:

• Your sort method return type needs to be changed to whatever array you are returning, and consequently the type of sorted array in main needs to be changed.
• Your return type has to be an array (not an array list!) and it has to have the same number of elements as the original array.
• You need to supply a method to write out your result into a file. The file has to be exactly the same as in the prototype implementation; they will be compared using diff system command.

If you are not changing the return type, you don't need to modify anything other than sort method.

Even though you are not modifying anything other than the sort method, you still need to submit your entire class: copy the template, rename the Java class to your group number, and change thesort method. You may use supplementary classes, just don't forget to submit them. Make sure to add your names in comments when you submit.

Your program must print the only value, which is the time (as it currently does). Any other printed output disqualifies your submission. If you use test prints, make sure to turn them off for submission.

Important: if the sorting times may be too small to distinguish groups based on just one run of the sorting, so I may loop over the sorting section multiple times. If this is the case, I will let you know no later than a day after the preliminary competition and will modify Group0 file accordingly.