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The International Obfuscated C Code Contest

2020/otterness - Most percussive

MIDI improver

← 2020/kurdyukov4 ↑ 2020 ↑ 2020/tsoj → C code Makefile Inventory ✓

Author:

To build:

    make

There is an unobfuscated version for those who wish to see the original final prior to obfuscation. See Alternate code below.

Bugs and (Mis)features:

The current status of this entry is:

STATUS: INABIAF - please DO NOT fix

For more detailed information see 2020/otterness in bugs.html.

To use:

    ./prog

Try:

    ./try.sh

Now play output.mid and output2.mid in an audio player that can play MIDI files.

Alternate code:

NOTICE to those who wish for a greater challenge:

If you want a greater challenge, don’t check this version: just try to understand the program via the entry source.

If you get stuck, come back and take a look at the alternate, unobfuscated code, described by the author as:

If you’re curious about the un-obfuscated program that I originally wrote, this was the final working version. I tried to keep the functions names and comments understandable, but it will make a lot more sense to readers familiar with the MIDI “SMF” file format.

… at their web page about the entry.

Alternate use:

Use prog.alt as you would prog.

Alternate try:

     ./try.alt.sh

Judges’ remarks:

In an entry that boasts not using literals, one would expect to see straightforward ways to produce small integers like !(x^x)<<!(x^x), etc. but this entry is trickier than that. Deciphering what it does, apart from reading stdin and writing to stdout, would take a while.

While doing that, you can be audibly entertained by a sample of its output.

Author’s remarks:

MIDI “boots and cats”

This program seeks to automatically “improve” standard MIDI files (.mid) by adding an extra track that provides a driving drum beat to bring some excitement into boring video-game music rips or classical music sequences that otherwise only see the light of day when modern musicians import them into FL Studio to copy a few chords.

Prerequisites

If you’re not using Windows Media Player or already familiar with MIDI files, you may need to install some additional software to be able to listen to them. For example, on Ubuntu, I was able to play them in VLC after installing vlc-plugin-fluidsynth and fluid-soundfont-gm.

You can obtain MIDIs in many places, but I mostly tested the program using files I found here. However, as discussed under the Limitations section below, not all of them work correctly. I will admit to having used http://www.kunstderfuge.com/-/midi.asp?file=beethoven/symphony_6_1_(c)cvikl.mid for most of my testing, see cvikl.mid.

Usage

Compile the program using the following command (I tested this using both clang and gcc):

    cc -pedantic -Wall -Werror -Wextra -O3 -o prog prog.c

The program expects a MIDI on stdin, and writes a modified MIDI to stdout:

    ./prog <input.mid >output.mid

If the program encounters an error, no output will generally be produced, and it will exit with a specific status code. Meanings of the various status codes are listed below, under the “Program error codes” heading. On success, the program exits with a code of 0.

Limitations

  1. The program does not support MIDI files over 4 MB (specifically, it will not take input files larger than 2^22 - 1 bytes). This is unlikely to be a big limitation in practice, since even lengthy MIDIs are rarely over a couple hundred KB.

  2. Not all MIDIs set (or correctly set) their timing information, which will lead to the tempo of the drum beat not matching the track. This will be especially true for “live performance” .mid files.

  3. There may be some types of MIDI messages that the program is unable to correctly parse, but I have rarely seen this in practice. The largest cause of program failures, in my experience, has been Limitation 2.

NOTICE to those who wish for a greater challenge:

If you want a greater challenge, don’t read any further: just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.

Obfuscation notes

I had one primary goal when writing this program: include no C literals whatsoever. The final C file includes no numerical, string, or character literals. Instead, all literals have been replaced with a handful of global variables, which are occasionally modified throughout the file to prevent simple uses of find-and-replace.

So, in a sense, this obfuscation is simply an intentional use of global variables’ classic problem: absolute spaghetti code. The absence of any sort of literals has the added benefit of forcing any would-be reader to trace the contents of many variables starting from main(), since, without literals, very little of the code gives any inherent visual cues as to its purpose.

I already took the liberty of running prog.c through clang-format, since it still fits in the size limit and the formatting of the code was never intended to be an obfuscation technique here.

Other remarks

MIDI files are generally divided up into one or more “tracks” that play concurrently. While it is easy to find the positions of each of these tracks in the file, figuring out the amount of time each track is supposed to play is more difficult, and requires parsing every track in its entirety. This means that generating a drum beat that covers the entire piece of music requires parsing every track (the entire file) in order to determine the longest track.

Once the program has determined the longest track in the file, it generates a new track, containing the simple 4-note percussion sequence, to match the length (in beats) of the longest-running track. It appends the new track onto the end of the file, taking care to update the file header, as well.

Program error codes

If the program encounters an error, it will exit with a nonzero code. The error codes can be interpreted as follows:

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