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日期:2020-05-01 06:08

COMP2017/COMP9017 Assignment 2

Assignment due: 1 May 2020, 11:59 PM AEST (Week 9 Friday)

This assignment is worth 10 % of your final assessment

1 Introduction

Version control systems are crucial in any software project. In this assignment, you will design

and implement the storage method, as well as some functions for Simple Version Control (SVC),

a (very) simplified system derived from the Git version control system.

2 Description of SVC

Projects that are to be placed under SVC must be initialised with svc_init. This allows the

system to create necessary data structures containing information about the state of the project.

Projects are comprised of files, and SVC computes a hash of each file to figure out if a change

has occurred. Only files which are explicitly added to the version control system are tracked.

Commits are like a snapshot in time of the state of the project. In SVC, they contain details

about which files have been added, removed, or modified, and sufficient information to restore a

file to this state. The currently active commit is often referred to as the HEAD. Each commit has

a “commit id” which uniquely identifies a given commit. Commit ids in SVC are represented in

hexadecimal numbers and are exactly 6 characters long.

cb410d Initial commit

+ README.md

ec8856 Added tests

+ tests/test1

+ tests/test2

ce9bd8 Updated readme

/ README.md

Figure 2.1: Examples of commits

Branches are useful to keep working copies of the project while working on new features or fixing

bugs. When a project is created, a master branch is created by default. For example, this branch

may be used to keep release versions of projects, and a branch features/something_special

might contain additional code with a new feature that is not yet complete and would not be

suitable for releasing.

A branch can be “checked out” to indicate that we want to switch our project to a specific

branch and work from there. For example, after we create the features/something_special

branch, it should be checked out before working on that feature.

Merging is the process of integrating changes from one branch into another. Once a new feature

is complete, or a bug is fixed and testing is complete, we may wish to merge these changes back

into the master branch to release. Note that merges can happen between any two branches, not

necessarily with the master branch.

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COMP2017/COMP9017 Assignment 2

cb410d

master

HEAD

(a) After the initial commit

cb410d ec8856

master

HEAD

(b) After another commit to the master branch

cb410d ec8856

master

feature

HEAD

(c) New branch created

cb410d ec8856

master

feature

HEAD

(d) After checking out the feature branch

cb410d ec8856 90ae30 b96919

master

feature

HEAD

(e) Two commits to the feature branch

cb410d ec8856

90ae30 b96919

ce9bd8

master

feature

HEAD

(f) master branch checked out and one commit made

cb410d ec8856

90ae30 b96919

ce9bd8 e8ca9d0

master

feature

HEAD

(g) feature branch merged into master

Figure 2.2: Examples of branches and merging

Sometimes, we may want to reset to a specific commit. In this case, the files are reverted to the

state they were in at that commit, and any new commits continue from that commit. This may

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COMP2017/COMP9017 Assignment 2

result in some commits being detached from the rest of the SVC system. For example, if the

master branch is reset to ec8856 and a new commit is made, ce9bd8 would not be reachable.

3 Functions to Implement

void * svc_init ();

This function will only be called once, before any other functions are called. In this function,

you may choose to perform preparation steps. If you require some data structure to help with

other operations, return a pointer to the area of memory containing this structure. In all further

function calls, this will be passed in as void *helper.

void cleanup ( void * helper );

This function will only be called once, after all other functions have been called. Ensure all

dynamically allocated memory has been freed when this function returns.

int hash_file ( void * helper , char * file_path );

Given the path to a file, compute and return the hash value using the algorithm described in

Section 3.1. If file_path is NULL, return -1. If no file exists at the given path, return -2. This

function should work even for files that are not being tracked.

char * svc_commit ( void * helper , char * message );

Create a commit with the message given, and all changes to the files being tracked. This should

return the commit id, which can be calculated by implementing the algorithm described in Section

3.2. If there are no changes since the last commit, or message is NULL, return NULL.

void * get_commit ( void * helper , char * commit_id );

Given a commit_id, return a pointer to the area of memory you stored this commit. If a commit

with the given id does not exist, or commit_id is NULL, this function should return NULL. Note

only the NULL return values for this function will be tested. The commit you return here will be

passed to some of the other functions to implement.

char ** get_prev_commits ( void *helper , void * commit , int * n_prev );

Given a pointer to a commit, return a dynamically allocated array. Each element in the array

should point to the id of a parent commit. The number of parent commits should be stored in

the area of memory pointed to by n_prev. If n_prev is NULL, return NULL. If commit is NULL,

or it is the very first commit, this function should set the contents of n_prev to 0 and return

NULL. Note: only the allocated array will be freed for you by the tester.

void print_commit ( void * helper , char * commit_id );

Given a commit_id, print the details of the commit as detailed below. If no commit with this

id exists, or commit_id is NULL, you should print Invalid commit id

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COMP2017/COMP9017 Assignment 2

For a valid commit, you should print the commit id, the branch it was committed to, a list of

the added, removed and changed files in any order, and a list of the tracked files at this point in

commit history on that branch along with their hash values, also in any order. Hashes are left

padded with spaces to be exactly 10 characters wide.

commit id [branch name]: commit message

+ added file(s)

- removed file(s)

/ changed file(s) [previous hash --> new hash]

Tracked files (number of tracked files):

[hash] file name

int svc_branch ( void * helper , char * branch_name );

Create a new branch with the given name. In this SVC, valid branch names are restricted to

those that contain only alphanumeric characters, underscores, slashes and dashes: a-z, A-Z,

0-9, _, /, -. If the given branch name is invalid or NULL, return -1. If the branch name

already exists, return -2. If there are uncommitted changes, return -3. If the branching is

successful, return 0. Note: creating a branch does not check it out.

int svc_checkout ( void * helper , char * branch_name );

Make this branch the active one. If branch_name is NULL or no such branch exists, return -1. If

there are uncommitted changes, return -2 and do not make this the active branch. Otherwise,

return 0 and make it the active branch. Note in SVC, the branch is not created if it does not exist.

char ** list_branches ( void * helper , int * n_branches );

Print all the branches in the order they were created. In addition, return a dynamically allocated

array of the branch names in the same order, and store the number of branches in the memory

area pointed to by n_branches. If n_branches is NULL, return NULL and do not print anything.

Note: only the allocated array will be freed for you by the tester.

int svc_add ( void * helper , char * file_name );

This is a notification that a file with the name file_name should be added to version control.

If file_name is NULL, return -1 and do not add it to version control. If a file with this name is

already under version control, return -2. If this file does not exist, return -3. Otherwise, add

the file to the SVC system and return the file’s hash value.

int svc_rm ( void * helper , char * file_name );

This is a notification that a file with the name file_name should be removed from the version

control system. If file_name is NULL, return -1. If the file with the given name is not being

tracked, return -2. Otherwise, remove the file from SVC and return its last known hash value

(from adding or committing).

int svc_reset ( void * helper , char * commit_id );

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COMP2017/COMP9017 Assignment 2

Reset the current branch to the commit with the id given, discarding any uncommitted changes.

If commit_id is NULL, return -1. If no commit with the given id exists, return -2. It is guaranteed

that if a commit with this id exists, there will be one simple path from the HEAD of the

current branch. That is, all commits from HEAD to the commit will have exactly one previous

commit. Reset the branch to this commit and return 0. Note that this function means that

some commits may be detached from the rest of the SVC system.

char * svc_merge ( void * helper , char * branch_name , resolution *

resolutions , int n_resolutions );

This function will be called to merge the branch with the name branch_name into the current

branch. If branch_name is NULL, print Invalid branch name and return NULL. If no such

branch exists, print Branch not found and return NULL. If the given name is the currently

checked out branch, print Cannot merge a branch with itself and return NULL. If there are

uncommitted changes, print Changes must be committed and return NULL. In all other cases,

the merge procedure begins. Note that the way branches are merged in SVC is different to Git.

To merge two branches together, all tracked files in both branches are used. If there are conflicting

files, it will appear in the resolutions array. Each resolution struct contains the name

of the conflicting file, and a path to a resolution file. This file contains the contents that the file

should contain after the merge. However, if the path given is NULL, the file should be deleted.

A commit with the message Merged branch [branch_name] replacing [branch_name] with

branch_name is created with the necessary changes for the current branch to reflect changes

made in the other branch. The previous commits order should be the current branch’s HEAD and

then the other branch’s HEAD. The function should then print the message Merge successful

and return the new commit id.

3.1 File Hash Algorithm

Below is the pseudocode to determine the hash value of a file. Note: this is not the same

algorithm used in real world version control systems.

function file_hash(file_name):

file_contents = read(file_name)

file_length = num_bytes(file_contents)

hash = 0

for unsigned byte in file_name:

hash = (hash + byte) % 1000

for unsigned byte in file_contents:

hash = (hash + byte) % 2000000000

return hash

3.2 Commit ID Algorithm

Below is the pseudocode to determine the commit id of a file. Note: this is not the same

algorithm used in real world version control systems.

function get_commit_id(commit):

id = 0

for unsigned byte in commit.message:

id = (id + byte) % 1000

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COMP2017/COMP9017 Assignment 2

for change in commit.changes in increasing alphabetical order of file_name:

if change is addition, id = id + 376591

if change is deletion, id = id + 85973

if change is modification, id = id + 9573681

for unsigned byte in change.file_name:

id = (id * (byte % 37)) % 15485863 + 1

return id as hexadecimal string

4 Examples

4.1 File Hashing

File Name File Contents Hash Value

Tests/diff.txt Empty file (0 bytes) 385

Tests/diff.txt This is some text in a file\n 2832

sample.txt Hello, world\n 1178

sample.txt Hello, world!\n 1211

Tests/test1.in 5 3 2\n 564

hello.py print("Hello")\n 2027

Taking the last example, hello.py corresponds to ASCII values [104, 101, 108, 108, 111, 46,

112, 121]. Following the algorithm gives a hash of 811 % 1000 = 811. The contents of the file

correspond to [112, 114, 105, 110, 116, 40, 34, 72, 101, 108, 108, 111, 34, 41, 10] giving (811 +

1216) % 2000000000 = 2027.

4.2 SVC Example 1

void *helper = svc_init();

Return value: helper

hash_file(helper, "hello.py");

Return value: 2027 (from example above)

hash_file(helper, "fake.c");

Return value: -2 (non-existent file)

svc_commit(helper, "No changes");

Return value: NULL

svc_add(helper, "hello.py");

Return value: 2027

svc_add(helper, "Tests/test1.in");

Return value: 564 (from example above)

svc_add(helper, "Tests/test1.in");

Return value: -2

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COMP2017/COMP9017 Assignment 2

svc_commit(helper, "Initial commit");

Return value: "74cde7"

The ASCII values for the commit message Initial commit are [73, 110, 105, 116, 105, 97, 108,

32, 99, 111, 109, 109, 105, 116]. After the first stage, this gives an id of 1395 % 1000 = 395. The

two changes are additions of hello.py [104, 101, 108, 108, 111, 46, 112, 121] and Tests/test1

.in [84, 101, 115, 116, 115, 47, 116, 101, 115, 116, 49, 46, 105, 110]. Following the algorithm,

the id is then 7654887. Converting this to a 6 character hexadecimal string gives 74cde7.

void *commit = get_commit(helper, "74cde7");

Return value: Pointer to area of memory containing the commit created above

int n_prev;

char **prev_commits = get_prev_commits(helper, commit, &n_prev);

Return value: NULL

Afterwards, n_prev = 0

print_commit(helper, "74cde7");

Output:

74cde7 [master]: Initial commit

+ hello.py

+ Tests/test1.in

Tracked files (2):

[ 2027] hello.py

[ 564] Tests/test1.in

int n;

char **branches = list_branches(helper, &n);

Output: master

Return value: Array with pointer to area of memory containing the string master

Afterwards, n = 1

4.3 Commit ID

In the above example, the commit ID for the valid commit “Initial commit” is calculated by:

1. The commit message has ascii values [73, 110, 105, 116, 105, 97, 108, 32, 99, 111, 109, 109,

105, 116] so after the first step, the ID is 1395 % 1000 = 395

2. There are two changes, both of which are additions

3. The first change adds 376591 to the ID, and then for each byte, the calculation in Section

3.2 is followed, giving 111

4. The second change also adds 376591 to the ID, and the above is repeated giving 7654887

5. Converting this to a 6 digit hexadecimal number gives 74cde7

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COMP2017/COMP9017 Assignment 2

4.4 SVC Example 2

Starting from a blank project (no files), we create two files:

COMP2017/svc.h has the contents

# ifndef svc_h \n

# define svc_h \n

void * svc_init ( void );\n

# endif \n

COMP2017/svc.c has the contents

# include " svc .h"\n

void * svc_init ( void ) {\n

// TODO : implement \n

}\n

The hashes of the two files are 5007 and 5217 (all ‘tabs’ are four spaces and new line characters

have been explicitly shown above).

svc_add(helper, "COMP2017/svc.h");

Return value: 5007

svc_add(helper, "COMP2017/svc.c");

Return value: 5217

svc_commit(helper, "Initial commit");

Return value: "7b3e30"

svc_branch(helper, "random_branch");

Return value: 0

svc_checkout(helper, "random_branch");

Return value: 0

Next, the file COMP2017/svc.c is changed to have the following contents

# include " svc .h"\n

void * svc_init ( void ) {\n

return NULL ;\n

}\n

which has the hash 4798.

svc_rm(helper, "COMP2017/svc.h");

Return value: 5007

svc_commit(helper, "Implemented svc_init");

Return value: "73eacd"

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COMP2017/COMP9017 Assignment 2

You realise you accidentally deleted COMP2017/svc.h and want to revert to the initial commit

svc_reset(helper, "7b3e30");

Return value: 0

Then, the file COMP2017/svc.c is changed again to have the contents shown above.

svc_commit(helper, "Implemented svc_init");

Return value: "24829b"

void *commit = get_commit(helper, "24829b");

Return value: Pointer to area of memory containing the commit created above

int n_prev;

char **prev_commits = get_prev_commits(helper, commit, &n_prev);

Return value: Pointer to an array of length one, containing "7b3e30"

Afterwards, n_prev = 1

svc_checkout(helper, "master");

Return value: 0

The test framework creates a file resolutions/svc.c with the contents

# include " svc .h"\n

void * svc_init ( void ) {\n

return NULL ;\n

}\n

The following code is then executed to perform a merge:

// Resolution (s) are created by the test framework

resolution * resolutions = malloc ( sizeof ( resolution ));

resolutions [0]. file_name = " COMP2017 / svc.c";

resolutions [0]. resolved_file = " resolutions / svc .c";

// Call to merge function

svc_merge ( helper , " random_branch ", resolutions , 1);

// The test framework will free the memory

free ( resolutions );

Return value: "48eac3"

The commit message is Merged branch random_branch. The conflicts array indicates that only

the change is to be kept, following the merge rules described above. This means that from the

perspective of the master branch, the only change to be committed is this modification.

int n_prev;

void *commit = get_commit(helper, "48eac3");

char **prev_commits = get_prev_commits(helper, commit, &n_prev);

Return value: Pointer to an array of length two, containing "7b3e30" and "24829b" in that

order. Afterwards, n_prev = 2

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COMP2017/COMP9017 Assignment 2

5 Notes and Hints

? In SVC, branches are not deleted. After merging a branch, it remains in the list of branches.

? When a reset to a commit occurs, don’t forget to undo all types of changes (addition,

deletion, modification) to the files too!

? Useful tools and functions: address sanitizer, valgrind, strdup

? The hashing algorithm may produce the same hash, even if the contents of the file have

changed. Test cases are designed with this in mind and it is guaranteed this will not occur.

? Error codes should be checked in order described in the function descriptions.

? A file may be deleted from the file system without svc_rm being called Your code should

detect that this has happened and remove the file from SVC accordingly. Note, however,

that the function svc_rm should not delete the file from the file system!

? You should write your own test cases and draw out diagrams to help visualise the problem.

? The maximum file path is 260 characters long (excluding NULL terminator)

? Branch names are at most 50 characters long (excluding NULL terminator)

? All other strings can be of any length

? The two examples given above correspond to the example test cases on Ed

5.1 Staging

In version control systems, there is a concept of “staging”. Adding/removing/changing files are

kept in the “staging” phase until a call to commit actually occurs. This means that multiple

changes can occur in the staging phase that should only be represented as a single change when

a commit occurs. For example, modifying a file twice should appear only as a single change in

the commit. Adding a file and then removing it before committing will mean neither of these

appear in the commit’s changes. Think about all the possible combinations of file operations

that could occur and what the effective change actually is (hint: there are 5 pairs you should

consider).

5.2 Resolution Struct

The resolution struct is provided in the scaffold code and is also provided below for reference.

Do not modify this struct

typedef struct resolution {

char * file_name ;

char * resolved_file ;

} resolution ;

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COMP2017/COMP9017 Assignment 2

6 Submission

You must submit your code via git to Ed for automatic marking. It must not produce any

compilation errors, and it must free any dynamically allocated memory unless otherwise specified

in Section 3.

Your code will be compiled with the following options:

gcc -O0 -std=gnu11 -lm -Wextra -Wall -Werror -g -fsanitize=address

Warning: Any attempts to deceive or disrupt the marking system will result in an immediate

zero for the entire assignment. Negative marks can be assigned if you do not follow the

assignment problem description or if your code is unnecessarily or deliberately obfuscated.

7 Mark Breakdown

This assignment is worth 10 % of your final mark. There are 10 points available (1 % each):

Component Points Description

Correctness 6 Proportion of test cases passed on Ed

Performance 2 How much memory and time does your program take?

Code structure and style 2 Manual marking by your tutor. Marks depend on

style, layout and readability of your code

Note on performance: memory and time usage are each worth 1 point and will be proportionately

allocated based on other students’ submissions. The memory usage is based on the maximum

memory allocated at any given time in the execution of your code. You should consider different

data structures and algorithms before implementing your solution.

8 Academic Declaration

By submitting this assignment you declare the following:

I declare that I have read and understood the University of Sydney Student Plagiarism: Coursework Policy

and Procedure, and except where specifically acknowledged, the work contained in this assignment/project

is my own work, and has not been copied from other sources or been previously submitted for award or

assessment.

I understand that failure to comply with the Student Plagiarism: Coursework Policy and Procedure

can lead to severe penalties as outlined under Chapter 8 of the University of Sydney By-Law 1999 (as

amended). These penalties may be imposed in cases where any significant portion of my submitted work

has been copied without proper acknowledgment from other sources, including published works, the Internet,

existing programs, the work of other students, or work previously submitted for other awards or

assessments.

I realise that I may be asked to identify those portions of the work contributed by me and required to

demonstrate my knowledge of the relevant material by answering oral questions or by undertaking supplementary

work, either written or in the laboratory, in order to arrive at the final assessment mark.

I acknowledge that the School of Computer Science, in assessing this assignment, may reproduce it entirely,

may provide a copy to another member of faculty, and/or communicate a copy of this assignment

to a plagiarism checking service or in-house computer program, and that a copy of the assignment may

be maintained by the service or the School of Computer Science for the purpose of future plagiarism

checking.

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