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Homework 5 — Unrolled Linked Lists

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CSCI-1200 Data Structures
Homework 5 — Unrolled Linked Lists
In this assignment you will modify the dslist class from Lecture 11 to implement an unrolled linked list
data structure. This data structure is very similar to a standard doubly linked list, except that more than
one element may be stored at each node. This data structure can have performance advantages (both in
memory and running time) over a standard linked list when storing small items and can be used to better
align data in the cache. You’re welcome to read more about unrolled linked lists on Wikipedia (but you’re
not allowed to search for or use any code or pseudocode that might be available online!). Please read the
entire handout before beginning your implementation.
To complete the assignment you will modify the helper classes (Node and list_iterator) as well as the
main class, dslist, which will be renamed “UnrolledLL”. Below is a picture of the relationships between
the classes (compare this to the picture of the dslist class from Lecture 11):
NULL
6 2
elements:
num_elements:
next:
elements:
num_elements:
next:
prev:
elements:
num_elements:
next:
prev:
10 11 12 13 14 15 16 17 18 22 23
Node<int> Node<int> Node<int>
prev:
NULL
ptr:
list_iterator<int>
head:
UnrolledLL<int>
tail:
size:
offset: 3
14
6
19 20 21
Each Node object contains a fixed size array (size = 6 in the above example) that will store 1 or more elements
from the list. The elements are ordered from left to right. From the outside, this unrolled linked list should
perform exactly like an STL list containing the numbers 10 through 23 in sorted order. Note that to match
the behavior, the list_iterator object must also change. The iterator must keep track of not only which
Node it refers to, but also which element within the Node it’s on. This can be done with a simple offset index.
In the above example, the iterator refers to the element “19”.
Just like regular linked lists, the unrolled linked list supports speedy erase operations from the middle of
the list. For example, if we call erase with the iterator shown above, this is the resulting picture:
NULL
6 2
elements:
num_elements:
next:
elements:
num_elements:
next:
prev:
elements:
num_elements:
next:
prev:
10 11 12 13 14 15 16 17 18 22 23
Node<int> Node<int> Node<int>
prev:
NULL
ptr:
list_iterator<int>
head:
UnrolledLL<int>
tail:
size:
offset: 3
20 21
5
13
Note that the size of the list has decreased by one, the element 19 has been removed from the middle
node, the elements after 19 have been shifted to the left, and the number of elements in that node has been
decreased. It’s perfectly OK for some of the nodes to be only partially filled. (If we tried to enforce that
all nodes stay full, this operation would become just as expensive as the STL vector::erase operation!) If
removing an element causes a node to become empty, the entire node should be removed from the list.
Your Tasks
Your new data structure should be a templated class named UnrolledLL and its public interface should mimic
the STL list container class. It should support the member functions: size, front, back, push_back,
pop_back, push_front, pop_front, erase, and insert. Like an STL list, the public interface provides
bi-directional forward iterators (i.e., ++ and — operations), but you don’t need to do reverse iterators. Your
class constructors, assignment operator, & destructor should properly allocate and deallocate the necessary
dynamic memory. Similar to Homework 3, you must provide print functionality for debugging – this time
by implementing the stream output operator for your data structure that prints an ASCII version of the
internal data structures. We provide examples of the information that should be contained in this output,
but your formatting may be different.
The number of elements stored in each Node is fixed at compile time, but you should try different values as
you work. Please use a global constant defined at the top of your UnrolledLL class header file to control
this. Your submitted assignment should have 6 elements per node:
const int NUM_ELEMENTS_PER_NODE = 6;
To get started, we highly recommend that you study and heavily borrow from the dslist class that we
discussed in Lecture 11. The code is available on the course website. We provide a main.cpp file that will
exercise the required features of your class, but it does not include tests for each “corner” case. You should
add your own test cases to the bottom of the main.cpp file. Discuss in your README.txt file your strategy
for thoroughly debugging your assignment.
To earn full credit, your program must be free of memory errors and memory leaks. The homework server is
configured to run Dr. Memory on your program to check for memory problems.
Evaluation
What is the order notation of each of the member functions in your class? Discuss any differences between
the UnrolledLL, dslist, and STL list classes. Evaluate the memory usage of your initial implementation
in the worst case. Give a specific example sequence of operations where the data structure is inefficient.
What is the average number of elements per node in this case? Discuss in your README.txt.
Extra Credit
For extra credit, change your implementation to improve this worst case memory performance. The typical
strategy for this data structure is to incrementally merge or split the contents of neighboring nodes such that
each node is at least 50% full (except possibly the last node) at the end of each operation. Which operations
did you modify? The order notation of these operations should not change. Discuss in your README.txt.
Additional Information
You may not use STL vectors or lists for the implementation of your new data structure. STL
lists are only allowed in the main.cpp to test the new data structure and make sure it matches the standard
interface for lists.
Use good coding style when you design and implement your program. Be sure to make up new test cases to
fully test your program and don’t forget to comment your code! Use the template README.txt to list your
collaborators and any notes you want the grader to read. A one day extension will be granted for HW5 if
by Wednesday 11:59 PM you have 5 points on Test Case 10.
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