The goal of this assignment is to work with priority queues. A secondary goal is to start diving into data structures at a lower level than we have been, implementing some of the essential operations of the priority queue from scratch.

You are provided here with a dynamic array implementation (dynarray.h and dynarray.c) and the template for an implementation of a priority queue (pq.h and pq.c). You should use the dynamic array implementation to implement a minimizing binary heap underlying the priority queue. Note that in a minimizing binary hear, the highest priority node is smallest integral priority.

Functions to implement

You must fill out the functions below in the priority queue implementation in pq.c. When you are implementing these functions you should not break the encapsulation of the dynamic array. That is, you should not access the internal details of the dynamic array but should instead use the functions comprising the dynamic array's public interface.

void pq_insert(struct pq* pq, void* item, int priority)

This function inserts a given piece of data (item) into the priority queue with the specified priority (priority). This entails finding the correct location in the heap array in which to insert the new element, inserting it there, and then making sure the minimizing heap property (that every element's value is less than the values of its children) is maintained.

To implement this function, you will be introduced to the new concept of a void pointer (void*). This is simply a generic pointer that can point to a memory address of any type. The dynamic array implementation stores void pointers. We will use these to hold pointers to an auxiliary structure (struct pq_elem) representing a single element in the priority queue, including its priority value and its associated data.

void* pq_first(struct pq* pq)

This function simply returns the data represented by the highest-priority element in the priority queue. To do this, you will have to figure out the location in the heap array corresponding to the highest-priority element in the priority queue and return the data represented by that element (i.e. the item field of the struct pq_elem stored at that location in the priority queue).

void* pq_remove_first(struct pq* pq)

This function removes the highest-priority element from the priority queue and returns the data it represents. To implement this, you will have to find the same element in the heap array you found when implementing pq_first(). Here, however, you will also have to find the appropriate element to replace that element in the heap array, perform the replacement, and ensure that the minimizing heap property (that every element's value is less than the values of its children) is maintained

Example sequence of calls

Suppose the user made this sequence of insertions into an empty struct pq called pq:

pq_insert(pq, "sixteen", 16);
pq_insert(pq, "eight", 8);
pq_insert(pq, "twenty", 20);
pq_insert(pq, "four", 4);
pq_insert(pq, "twelve", 16);

Then suppose they ran this loop:

while (!pq_isempty(pq)) {
printf("%sn", pq_remove_first(pq));
}

The following should be the output of that loop:

four
eight
twelve
sixteen
twenty

Test program

A program to test your priority queue is available in test_pq.c. You can compile this using the provided Makefile and then run it like so:

make
./test_pq

The program will run a series of insertions and a series of removals. After each operation, it will check to make sure the priority queue has the appropriate structure (in particular that the first element is what's expected).

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