Reverse

Implement a program that reverses a WAV file, per the below.

./reverse input.wav output.wav

Background

In Electric Light Orchestra’s “Fire on High”, there’s something a little off about the first minute or so of the music. If you take a listen, it sounds almost like the audio is playing backwards. As it turns out, if you play the beginning section of the song in reverse, you’ll hear the following:

“The music is reversible. Time is not. Turn back, turn back!”

Creepy, right? This is a technique called “backmasking,” or hiding messages in music that can only be heard when the song is played backwards. Many artists have used (or been suspected of using) this technique in their songs. To be able to do our own investigation into backmasking, we’ve asked you to write a program that can reverse WAV files for us!

Unlike MP3 audio files, WAV files are not compressed. This makes the files much easier to edit and manipulate, which is useful for the task at hand. To learn a little more about WAV files, we need to take a closer look at the WAV file format.

Getting Started

Open VS Code.

Start by clicking inside your terminal window, then execute cd by itself. You should find that its “prompt” resembles the below.

Click inside of that terminal window and then execute

wget https://cdn.cs50.net/2022/fall/psets/4/reverse.zip

followed by Enter in order to download a ZIP called reverse.zip in your codespace. Take care not to overlook the space between wget and the following URL, or any other character for that matter!

Now execute

unzip reverse.zip

to create a folder called reverse. You no longer need the ZIP file, so you can execute

rm reverse.zip

and respond with “y” followed by Enter at the prompt to remove the ZIP file you downloaded.

Now type

cd reverse

followed by Enter to move yourself into (i.e., open) that directory. Your prompt should now resemble the below.

reverse/ $

If all was successful, you should execute

ls

and see a file named reverse.c. Executing code reverse.c should open the file where you will type your code for this problem set. If not, retrace your steps and see if you can determine where you went wrong!

The WAV File Format

Notice that, in the visual below, a WAV file is broken into three chunks. Each chunk has a few blocks of data inside of it.

The first chunk contains information about the file’s type. In particular, see how the “File Format” block in the first chunk spells out ‘W’ ‘A’ ‘V’ ‘E’ in bytes 8–11, to indicate the file is a WAV file.

The second chunk contains information about the upcoming audio data, including how many “channels” of audio are present and how many bits are in each audio “sample”. Audio files have 1 channel when they’re “monophonic”: if you were to wear headphones, you’d hear the same audio in your left and right ear. Audio files have 2 channels when they’re “stereophonic”: wearing headphones, you’d hear slightly different audio in your left and right ear, creating a sense of spaciousness. Samples are the individual chunks of bits which make up the audio you hear. With more bits per sample, an audio file can have greater clarity (at the cost of more memory used!).

Finally, the third chunk contains the audio data itself—those samples we mentioned just above.

Everything before the audio data is considered part of the WAV “header”. Recall that a file header is simply some metadata about the file. In this case, the header is 44 bytes long.

WAV Header

A more technical explanation of WAV headers can be found here, which is the resource by which this visual was inspired. Notice that we’ve included a file, wav.h, which implements all these details for you in a struct called WAVHEADER.

Specification

Let’s write a program called called reverse that enables us to reverse a WAV file given by the user and create a new WAV file that contains the resulting reversed audio. For simplicity’s sake, we’ll limit the files we deal with to the WAV format. At the time the user executes the program, they should provide, using two command-line arguments, the name of the input file to be read and reversed, and the name of the output file they would like to save the resulting audio in. A successfully executed program should not output any text, and should create a WAV file with the user-specified name that plays the audio of the input WAV file in reverse. For example:

$ ./reverse input.wav output.wav

In reverse.c, you’ll notice that a few helpful libraries have been included, as well as a header file, wav.h. You’ll likely find these to be useful when implementing your program. We’ve left eight TODOs and two helper functions for you to fill in, and we recommend you tackle them in order from 1 to 8.

In the first TODO, you should ensure the program accepts two command-line arguments: the name of the input WAV file and the name of the output WAV file. If the program does not meet these conditions, you should print an appropriate error message and return 1, ending the program.
- Hint
  - Keep in mind, the number of command-line arguments can be found in the argc variables passed to the main function when the program is executed.
  - Remember that argv[0] holds the name of the program as the first command-line argument.
In the second TODO, you should open your input file. We’ll need to open the input file in “read-only” mode, since we’ll only read data from the input file. It may be wise to check that the file has been opened successfully. Otherwise, you should print an appropriate error message and return 1, exiting the program. We should hold off on opening the output file, though, lest we create a new WAV file before knowing the input file is valid!
- Hint
  - If the first TODO has been implemented properly, it is safe to assume we can reference the name of the input file using argv[1].
  - Keep in mind, any file that we open, we must also close when we are finished using it. This may mean adding code elsewhere in the program.
In the third TODO, you should read the header from the input file. Recall that, in wav.h, we’ve already implemented a struct that can store a WAV file’s header. Since we’ve written #include "wav.h" at the top of reverse.c, you, too, can use the WAVHEADER struct.
In the fourth TODO, you should complete the check_format function. check_format takes a single argument, a WAVHEADER called header, representing a struct containing the input file’s header. If header indicates the file is indeed a WAV file, the check_format function should return true. If not, check_format should return false. To check if a file is of the WAV format, we can compare the elements from the input file header to those we would expect from a WAV file. It suffices to show the “WAVE” marker characters are found in the format member of the WAVHEADER struct (see Background for more detail on WAV file headers).
In the fifth TODO, you can now safely open the output file for writing. It would still be wise to check that the file has been opened successfully.
- Hints
  - If the first TODO has been implemented properly, it is safe to assume we can reference the name of the output file using argv[2].
  - Keep in mind, any file that we open, we must also close when we are finished using it. This may mean adding code elsewhere in the program.

This may be a good place to stop and test that your program behaves as expected. If implemented properly, your program should open a new file when executed with the proper command-line arguments.

If at any point you find it necessary to delete a file, execute the following command in your current working directory.

$ rm file_name.wav

If you’d rather not be prompted to confirm each deletion, execute the command below instead.

$ rm -f file_name.wav

Just be careful with that -f switch, as it “forces” deletion without prompting you.

Next, now that the file type has been verified, the sixth TODO tells us to write the header to the output file. The reversed WAV file will still have the same underlying file structure as the input file (same size, number of channels, bits per sample, etc.), so it suffices to copy the header we read in from the input file in the third TODO to the output file.
In the seventh TODO, you should implement the get_block_size function. get_block_size, like check_format, takes a single argument: this is a WAVHEADER called header, representing the struct containing the input file’s header. get_block_size should return an integer representing the block size of the given WAV file, in bytes. We can think of a block as a unit of auditory data. For audio, we calculate the size of each block with the following calculation: number of channels multiplied by bytes per sample. Luckily, the header contains all the information we need to compute these values. Be sure to reference the Background section for a more in-depth explanation as to what these values mean and how they are stored. See too wav.h, to determine which members of WAVHEADER might be useful.
Hints
- Notice that one of the members of WAVHEADER is bitsPerSample. But to calculate block size, you’ll need bytes per sample!
The eighth and final TODO is where the actual reversing of the audio takes place. To do this, we need to read in each block of auditory data starting from the very end of the input file and moving backwards, simultaneously writing each block to the output file so they are written in reverse order. First, we should declare an array to store each block we read in. Then, it’s up to you to iterate through the input file audio data. You’ll want to be sure you read through all of the audio, but don’t erroneously copy any of the data from the header! Additionally, for testing purposes, we would like to maintain the order of the channels for each audio block. For example, in a WAV file with two channels (stereophonic sound), we want to make sure that the first channel of the last audio block in the input becomes the first channel of the first audio block in the output.
Hints
- A few functions (and a thorough understanding of their usage) may be especially helpful when completing this section - the CS50 manual pages may prove especially useful here:
  - fread: reads from a file to a buffer. The output of the get_block_size helper function may be useful here when deciding which values to use for the size and number of data to be read at a time.
  - fwrite: writes from a buffer to a file.
  - fseek: sets a file pointer to a given offset. It may be useful to experiment with negative offset values to move a file pointer backwards.
  - ftell: returns the current position of a file pointer. It may be useful to inspect what value ftell returns after the input header is read in the third TODO in addition to what it returns while the audio data is being read.
- Keep in mind that after you use fread to load in a block of data, the input pointer will be pointing at the location where the read concluded. In other words, the input pointer may need to be moved back two block sizes after each fread, one to move back to where the fread began, and the second to move to the previous, unread block.
Finally, be sure to close any files you’ve opened!

Usage

Here are a few examples of how the program should work. For example, if the user omits one of the command-line arguments:

$ ./reverse input.wav
Usage: ./reverse input.wav output.wav

Or if the user omits both of the command-line arugments:

$ ./reverse
Usage: ./reverse input.wav output.wav

Here’s how the program should work if the user provides an input file that is not an actual WAV file:

$ ./reverse image.jpg output.wav
Input is not a WAV file.

You may assume the user enters a valid output filename, such as output.wav.

A successfully executed program should not output any text, and should create a WAV file with the user-specified name that plays the audio of the input WAV file in reverse. For example:

$ ./reverse input.wav output.wav

Testing

Execute the below to evaluate the correctness of your code using check50. But be sure to compile and test it yourself as well!

check50 cs50/problems/2023/x/reverse

Execute the below to evaluate the style of your code using style50.

style50 reverse.c

How to Submit

In your terminal, execute the below to submit your work.

submit50 cs50/problems/2023/x/reverse