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Creating a Generic Data Type in C Binary Search Tree
For this assignment, you will implement a generic binary search tree in C. Search operations on a binary search tree require that the user data elements stored in the tree be compared. On the one hand, it's desirable to implement a data structure so that the implementation can be used to store whatever type of data the user wants. On the other hand, how can the tree do that without information about the user's data type?
In C, the problem can be solved by employing a function pointer. The name of a function is simply a pointer to the first instruction in the implementation of the function, and pointers can be passed as parameters and stored. We will implement a binary search tree as a collection of struct types (with appropriate associated functions); a first design might look like this:

/** Generic binary tree node type.
*
*A BSTNode object is proper iff:
*- it is encapsulated in a PayloadWrapper object, and
*- lchild is NULL or points to a proper BSTNode object, and
*- rchild is NULL or points to a proper BSTNode object
*/
struct _BSTNode {
struct _BSTNode* lchild;
struct _BSTNode* rchild;
};
typedef struct _BSTNode BSTNode;
/** Generic binary search tree type. The implementation depends on three
*user-supplied functions:
*
*compare takes pointers to two proper BSTNode objects; the function
*accesses the user payloads, compares them, and returns:
*< 0 if user payload on left < user payload on right
*0 if user payload on left == user payload on right
*> 0 if user payload on left > user payload on right
*(Of course what <, ==, and > mean is entirely up to the user.)
*
*destroy takes a pointer to a proper BSTNode object; the function deallocates all
*dynamic memory associated with the payload wrapper and the user's payload
*
*display takes an open file pointer and a pointer to a proper BSTNode
*object and writes a formatted display of the user-defined data object
*in the associated wrapper
*
*A BST object is proper iff:
*- compare, destroy, and display point to the user-supplied functions
*as described, and
*- root is NULL, or the user data object in *root is larger than every
*user data object in the left subtree of *root, and smaller than every
*user data object in the right subtree of *root, and the left and right
*subtrees of *root are both proper BST objects
*/
struct _BST {
// pointer to root node, if any struct _BSTNode* root;

// pointer to the user-supplied functions
int32_t (*compare)(const BSTNode* const left, const BSTNode* const right);
void (*destroy)(BSTNode* pNode);
void (*display)(FILE* fp, const BSTNode* const pNode);
};
typedef struct _BST BST;

Designing and Implementing a Generic Data Structure

This design requires some explanation. For example, how can user data be stored if the node type makes no provision for it? And, what's the deal with those function pointers?

If you have taken CS 2505 (a prerequisite for this course), you should have been exposed to the design of a generic linked list in C, and parts of the following discussion will be familiar.

Data structure code is usually more complex to implement and test than code for user data types, so we would like to produce an implementation of a binary search tree that could be used, without modification, to store user data of any type whatsoever. In other words, the implementation of the binary search tree should make minimal assumptions about the user's data type.

How can a user use this BST to store data objects? The answer is actually the key to making the implementation generic: the user data object will actually be stored in a "wrapper" object that encapsulates a user data object and a BSTNode. In addition, we'd like to make the wrapper type generic, if possible. We will use the following wrapper design:

struct _PayloadWrapper {
Payload* userdata; // pointer to a user data object
BSTNode node;
};
typedef struct _PayloadWrapper PayloadWrapper;

A user can make use of the PayloadWrapper type by creating a type called Payload to hold the desired data; for example, if the user wants to store character strings, the Payload type might look like this:

struct _Payload {
char* str;
};
typedef struct _Payload Payload;

Then, if the user wants to store the string "generic" in the BST, the user would create a Payload object and a corresponding PayloadWrapper object; after insertion, the tree would contain a node, embedded in a wrapper that also contained a pointer to the user's payload object:


"generic"

p





PayloadWrapper

Clearly, a user can customize the Payload type to support any content that's desired, so this is flexible. But there are issues to solve.

How can the BST compare user data objects? Since a binary search tree must compare user data objects, the user must supply some way to do that; that means the user must supply a function that the binary search tree can call to perform the comparisons. Furthermore, when two user data values are compared there are three possible results (smaller, equal, greater). So, the user must provide a function that takes two data values and returns which of the three results occurs. This isn't imposing any severe restriction on the user, since a binary search tree can only store data values that can be compared in this manner.

The user can supply a comparison function as part of the Payload type; for our example:

int32_t Payload_compare(const Payload* const left, const Payload* const right) {

return strcmp(left->str, right->str);
}

Note this is customized according to the user's design of the contents of a Payload object, which is determined by the user. Remember that, when performing a search, the BST needs to determine, given two user data objects A and B, whether A < B or A == B, or A > B. The design of strcmp() provides exactly that information:

strcmp(A, B) < 0 if A < B (lexicographic ordering) strcmp(A, B) = 0 if A = B (lexicographic ordering) strcmp(A, B) > 0 if A > B (lexicographic ordering)

If the user chooses different content for a Payload object, then the user's comparison function must be designed accordingly, but this should still result in a comparison that behaves line strcmp().

But, BST doesn't know about the Payload interface (that’s really the whole point of making BST generic). Therefore, the BST cannot directly call the Payload function shown above.

The solution is to have PayloadWrapper provide an adaptor, a function that BST can call, and that itself calls the comparison function provided by Payload:

int32_t PayloadWrapper_compare(const BSTNode* const leftNode,
const BSTNode* const rightNode);

Note how this will work:

the BST can safely call the PayloadWrapper_compare() function, since the existence of and the interface of that function have been specified as a requirement by the BST
the BST receives the PayloadWrapper_compare() function when the BST is created, so the function name doesn't even need to match; of course, the return type and the parameters must match
the PayloadWrapper_compare() function only needs to call the function the user provides for the user's
Payload objects
the user never needs to make any changes to the BST itself
So, the user only needs to implement a data type that supplies an appropriate function (or set of functions) for the PayloadWrapper implementation to call. This may require making some adjustments in PayloadWrapper, but that isolates changes to the relatively simple code in PayloadWrapper. In essence, the PayloadWrapper type is an adaptor that makes it simple for the user to plug data objects of any type into the existing implementation of the BST.

The BST can then call the PayloadWrapper_compare() function just like any function, using the function name from the BST declaration:

BSTNode* BST_find(const BST* const pTree, const BSTNode* const userNode) {
. . .
return BST_findHelper(pTree->root, userNode, pTree->compare);
}

static BSTNode* BST_findHelper(const BSTNode* pNode, const BSTNode* const userNode,. . .) {

. . .
int32_t direction = compare(userNode, pNode);

if ( direction < 0 ) {
. . .

There are still some issues to work out.
A BST has BSTNode pointers, so PayloadWrapper_compare() must take BSTNode pointers as parameters. But the user-supplied Payload_compare()function needs pointers to Payload objects, not pointers to BSTNode
objects.

So, how can PayloadWrapper_compare() compute appropriate Payload pointers so they can be passed to
PayloadWrapper_compare()?

This can be accomplished by using pointer arithmetic:

/** Given a pointer to a BSTNode object, computes the address of the surrounding
*PayloadWrapper object.
*
*Pre: pNode points to a proper BSTNode, contained in a proper PayloadWrapper
*
*Returns: a pointer to the surrounding PayloadWrapper
*/
PayloadWrapper* PayloadWrapper_getPtr(const BSTNode* const pNode) {

return (PayloadWrapper*) ((uint8_t*) pNode - offsetof(PayloadWrapper, node));

}

The function uses Standard Library offsetof() macro to determine where the node field lies in a PayloadWrapper object, and then computes the address of the beginning of the surrounding PayloadWrapper object. So, we can use the Payload_getPtr() function to implement PayloadWrapper_compare().

Now, some other considerations reveal the need for some additional "external" functions that allow a BST to take actions on user data objects:

deallocate any dynamic memory related to the storage of user data objects
print a useful display of user data objects (primarily for quick debugging)

We'll discuss the details of this later, but we will require PayloadWrapper and Payload to implement the following pairs of functions. First, we need a pair of functions to support deallocating dynamic memory:

/** Destroys a user payload object
*
*Pre: pLoad points to a proper Payload object
*
*Post: all dynamic memory associated with *pPayload, and *pLoad have been freed
*/
void Payload_destroy(Payload* const pLoad);

/** Deallocates all memory associated with a PayloadWrapper object.
*
*Pre: pNode points to a proper BSTNode, contained in a proper PayloadWrapper object
*
*Post: the PayloadWrapper object containing *pNode and the user data object associated
*with that wrapper object have been deallocated
*
*Calls: PayloadWrapper_GetPtr(), Payload_destroy()
*/
void PayloadWrapper_destroy(BSTNode* const pNode);

Here, the PayloadWrapper_destroy() function simply needs to compute an appropriate Payload pointer, from the given BSTNode pointer, and then call the user's Payload_destroy() function.

Similarly, we need two functions to support displaying the user's data:

/** Writes a formatted representation of a Payload object.
*
*Pre: fp is open on an output device
*pLoad points to a proper Payload object
*
*Calls: Payload_display()
*/
void PayloadWrapper_display(FILE* fp, const BSTNode* const pNode);

/** Writes a formatted representation of a Payload object.
*
*Pre: fp is open on an output device
*pLoad points to a proper Payload object
*/
void Payload_display(FILE* fp, const Payload* const pLoad);

The user will have designed the actual user data, contained in the user's Payload objects, so it's up to the user to:

determine what elements in the Payload objects must be freed
determine whether the Payload objects themselves need to be freed
determine how to format the contents of a Payload object when printing it
The implementation of PayloadWrapper will assume the user has implemented these functions correctly.

With this design, it's possible to supply the BST with the necessary functionality to compare, destroy, and display the user's data without telling the BST any details about that data, simply by plugging the supporting functions provided by PayloadWrapper into the BST:

/** Create a proper, empty binary search tree object.
*
*Pre: compare is the name of a user-defined function satisfying the BST specification
*display is the name of a user-defined function satisfying the BST specification
*destroy is the name of a user-defined function satisfying the BST specification
*
*Returns: a BST object with NULL root and configured to use the three user-supplied
*functions for comparing, destroying and displaying user-defined data objects
*stored in the tree
*/
BST BST_create(int32_t (*compare)(const BSTNode* const left, const BSTNode* const right),
void (*display)(FILE* fp, const BSTNode* const pD),
void (*destroy)(BSTNode* pNode)) {

BST newTree; newTree.root = NULL;
newTree.compare = compare; newTree.display = display; newTree.destroy = destroy; return newTree;
}

A user can then create a new BST by calling BST_create() as follows:

BST T1 = BST_create(PayloadWrapper_compare, PayloadWrapper_display, PayloadWrapper_destroy);

BST_create() installs the PayloadWrapper functions into the new BST; this allows a user to create several different binary search trees, each storing different data and using different functions. Note that the user would have to create specialized Payload and PayloadWrapper types in order to do this. But the underlying BST implementation will not have to change, since it is blissfully unaware of any details of Payload and PayloadWrapper.

The BST Interface

For this assignment, the BST will have the following "public" functions. The first is essentially a constructor that makes a new, empty proper BST. An implementation was shown earlier and you are free to use it, but it's not guaranteed to be fully correct, so you should check it carefully.

/** Create a proper, empty binary search tree object.
*
*Pre: compare is the name of a user-defined function satisfying the BST specification
*display is the name of a user-defined function satisfying the BST specification
*destroy is the name of a user-defined function satisfying the BST specification
*
*Returns: a BST object with NULL root and configured to use the three user-supplied
*functions for comparing, destroying and displaying user-defined data objects
*stored in the tree
*/
BST BST_create(int32_t (*compare)(const BSTNode* const left, const BSTNode* const right),
void (*display)(FILE* fp, const BSTNode* const pD),
void (*destroy)(BSTNode* pNode));

The second function inserts a new element into the tree. Note that userNode must point to a BSTNode that's embedded in a PayloadWrapper object that also contains a pointer to the user's actual data object.
/** Inserts user data object into a BST, unless it duplicates an existing object.
*
*Pre: pTree points to a proper BST object
*userNode points to a proper BSTNode object
*
*Returns: true iff the insertion was performed; the implementation will not
*insert a new element that is equal to one that's already in the
*BST (according to the user-supplied comparison function)
*/
bool BST_insert(BST* const pTree, const BSTNode* const userNode);

The third function searches for a given element in a BST. Note that userNode must point to a BSTNode that's embedded in a PayloadWrapper object that also contains a pointer to the user's actual data object. You might wonder why the user would be searching for a data object if the user already has it. In a typical situation, the user's data object will contain multiple fields, only one of which (the key) is used for comparisons; in that case, the user will only initialize the key field when creating the wrapper to be used when calling the search function.

/** Searches a proper BST for an occurence of a user data object that equals
**pData (according to the user-supplied comparison function).
*
*Pre: pTree points to a proper BST object
*pData points to a proper BSTNode object
*
*Returns: pointer to matching user data object; NULL if no match is found
*/
BSTNode* BST_find(const BST* const pTree, const BSTNode* const userNode);

The fourth function is a destructor; that is, a function that completely deallocates the dynamic memory for an object:

/** Deallocates all dynamic memory associated with a proper BST object.
*
*Pre: *pTree is a proper BST object
*Post: all the user payloads and payload wrappers associated with *pTree
*have been freed
*the BST object itself is NOT freed since it may or may not have
*been allocated dynamically; that's the responsibility of the caller
*
*Calls: Payload_destroy() to handle destruction of the user's data object
*/
void BST_destroy(BST* const pTree);

The final function is a display function, which is supplied as part of the base code you'll be given for the assignment.

/** Writes a formatted display of the contents of a proper BST.
*
*Pre: fp is open on an output device
*pTree points to a proper BST object
*/
void BST_display(FILE* fp, const BST* const pTree);

You might wonder why no deletion function is listed above; the reason is simply to scale back the scope of the assignment. A complete implementation of a binary search tree would certainly include a deletion function.

The natural way to implement search/insertion/destruction for a tree is to use recursion; that will require that each of those "public" functions be paired with a static recursive helper function. The design of those is up to you. That said, you should have encountered binary search tree implementations in earlier courses (albeit in a different programming language), and you should feel free to use that knowledge here.

A tar file will be posted containing base code for you to start with, along with testing code that is described later. Be sure to examine what's given closely, since that may give useful hints for how to implement the parts for which you are responsible.

Some Notes on Function Interface Design

Consider the interface for this function:

bool BST_insert(BST* const pTree, const BSTNode* const userNode);

In this case, the use of a bool return type allows the function to indicate whether the operation was completed successfully; an insertion fails if a duplicate value is being inserted, and might fail for other reasons as well.

The function is designed to pass a pointer to a BST object, rather than to pass by copy. In this case, it's logically necessary, since the insertion of a new value may modify the original BST object. (You should think about why I said "may modify" instead of "will modify".)

The interface also illustrates the complexities of using the const qualifier in C. The specification of the parameter

BST* const pTree

implies that the value of the parameter pTree cannot be modified within the function, but the value of the pointer's target can be changed via a use of that pointer. This means that if the implementation of the function attempts to change where pTree points, there will be a warning (alas, not an error) at compile time.

On the other hand, the specification of the parameter

const BSTNode* const userNode

prevents the implementation of the function from either changing where userNode points or changing the value of the target of userNode (again, without a warning).

The use of const is a good way to avoid careless errors in C code, and is probably underused. But, const is certainly not foolproof, since you can even eliminate the warnings about violations by writing a suitable typecast.

Testing/Grading Harness

Download the posted tar file, BSTFiles.tar from the course website and unpack it on a CentOS 8 system. You should receive the following files:

README brief usage instructions
dev
c01driver.c driver for running all the tests and scoring; see embedded comments
BST.h declaration of BST type; do NOT modify this file!
BST.c implementation file for BST type; complete the functions as needed
Payload.h declaration of Payload type; do NOT modify this file!
Payload.c implementation file for the Payload type; complete functions as needed
PayloadWrapper.h declaration of PayloadWrapper type; do NOT modify this file! PayloadWrapper.c implementation file for the PayloadWrapper type; complete functions as needed Monk.h interface for testing/grading code; do NOT modify this file!
Monk.o 64-bit CentOS 8 binary for testing/grading code
runvalgrind.sh shell script for testing your solution with Valgrind; see header comment in file
grading
gradec01.sh shell script for performing grading; see embedded comments!
BSTGrader.tar containing the following files used in grading:
c01driver.c Payload.h PayloadWrapper.h BST.h
Monk.c Monk.h Monk.o

Unpack the posted tar file, and copy the files from BSTGrader.tar into a working directory. You can use the ./dev subdirectory to work on your solution. Edit the files BST.c, Payload.c, and PayloadWrapper.c as needed to implement your solution. Note that you can enable or disable certain tests by commenting parts of the driver file. You can then compile with the following command:

gcc -o c01driver -std=c11 -Wall -W –ggdb3 c01driver.c BST.c Payload.c Payloadwrapper.c Monk.o

If you have only implemented some of the specified functions (which would be a consequence of wisely following an incremental development pattern), you should edit c01driver.c to comment out the calls to test functions you have not yet completed. You will find it valuable to implement some static ("private") helper functions. Those must be placed in your .c file, and declared there.

Do not modify any of the .h files; the original versions will be used in grading your submission, so any changes you have made will be lost, and your submission may not even compile.

If compilation succeeds, you can run tests by executing c01driver; examine the various log files that are created for details of the testing. Test data is randomized for each run. There are four components involved in computing your score. The first three are analyzed by running c01driver, the fourth will be depend on a Valgrind analysis of your solution.
The components will be weighted as follows:

BST creation 10%
BST insertion 40%
BST find 30%
Valgrind 20%

The fourth is based entirely on whether you achieve a clean run on Valgrind. This will depend on the correctness of your implementation of BST_destroy(), and the functions it depends on.

When you have tested your implementation enough to have confidence in it, create a tar file containing only your BST.c, PayloadWrapper.c, and Payload.c files, and copy that tar file into the same directory as gradec01.sh and

BSTGrader.tar. We recommend naming your tar file as PID.tar, where PID is your VT email PID. Execute the grading script:
./gradec01.sh

If all goes well, this will create a file, PID.txt, containing complete details of the grading process.

We are supplying the test/grading harness, including the grading script gradec01.sh, so that you can be certain that your submission is complete and correct before you make a submission. We will make no allowances for special treatment if you fail to do this.

The requirements related to memory management will be checked by the supplied grading script. Your score for that part will then be assessed manually.

What to Submit

You will submit your solution as a single tar file to Canvas.

You will submit your completed versions of BST.c, PayloadWrapper.c, and Payload.c, in a flat tar file. The tar file should contain nothing but the specified C files. The gradec01.sh file will not work correctly with your tar file if your tar file is incomplete or not structured correctly; that's why we have supplied it.

Your submission will be compiled with the test driver using the command shown above.

Again, we have supplied the testing/grading harness so that you can be sure your solution is complete and correct before you submit it, and we expect you to make use of that harness. We will make no allowances for special treatment if you fail to do this.

Pledge

Each of your program submissions must be pledged to conform to the Honor Code requirements for this course. Specifically, you must include the following pledge statement in the submitted file:

// On my honor:
//
// - I have not discussed the C language code in my program with
// anyone other than my instructor or the teaching assistants
// assigned to this course.
//
// - I have not used C language code obtained from another student,
// the Internet, or any other unauthorized source, either modified
// or unmodified.
//
// - If any C language code or documentation used in my program
// was obtained from an authorized source, such as a text book or
// course notes, that has been clearly noted with a proper citation
// in the comments of my program.
//
// - I have not designed this program in such a way as to defeat or
// interfere with the normal operation of the grading code.
//
//
//

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