Return to IOCCC Year 2025

A “fun challenge” for a winning IOCCC entry is a puzzle or task that an IOCCC judge suggested people consider solving. After you figure out what a given winning entry does, we encourage you to attempt the fun challenge.

Some fun challenges ask you to explain something about a winning IOCCC entry. Other fun challenges ask you to produce a variant of the prog.c program that has a different or enhanced behaviour. And still, other types of fun challenges may be posed.

NOTE: Some of these challenges are easier than others.

See also the FAQ on “fun challenges”.

Modify prog.c so that the code that allows for N to be defined for values larger than 11? I.e., that works with -DN=12, and up to some reasonable limit?

This fun challenge for 2025/ayu is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Use the toolchain to compile your own software to run on this VM.

This fun challenge for 2025/cable is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Produce variant of the vmlinux bootimage that contains a C compiler for this VM.

This fun challenge for 2025/cable is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code that supports the FORTRAN READ statement.

This fun challenge for 2025/cesmoak is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain the purpose of the constant 9018655 as used in prog.c.

This fun challenge for 2025/diels-grabsch is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain why prog.c prints the number of digits of the **constant* that it does?

This fun challenge for 2025/dogon is still OPEN.

Given the “constant” that is printed, explain the last line of output:

    ./prog | tail -1

This fun challenge for 2025/dogon is still OPEN.

Modify prog.c so that the code that os that prints IOCCC 2025 at the very end, in honor of the contest year in which it won, instead of IOCCC 2026. Of course, the digits printed beforehand should remain the same.

This fun challenge for 2025/dogon is still OPEN.

Additionally, observe the try.sh script uses the following display the program’s output:

    ./prog | cat

However, the following may, for some environments, will print what seems like garbage:

    ./prog

Explain why this command:

    ./prog | cat > foo

may produce, for some environments, different output than the following command:

    ./prog > bar

This fun challenge for 2025/dogon is still OPEN.

Modify prog.c so that the code that is also works with UPPER CASE LETTERS. You would certainly have to embiggen the tube size to better distinguish individual letters and digits. Such code would be a cromulent addition to 2025/endoh1.

This fun challenge for 2025/endoh1 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code prints a line instead of varying brightness of a yellow line, using 256 ANSI Color Codes. Choose a colors/brightness palette to represent the charge intensity.

This fun challenge for 2025/endoh2 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain why the git-am patch generator must be used about 25 times (see try.sh for details) to produce a unified diff tool.

This fun challenge for 2025/endoh3 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code allows for different map projections. Add -p projection as a command line option to change the default projection. Add this to a debug.c as well for extra points.

This fun challenge for 2025/ferguson is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain why there are four args (in one invocation) but yet the value of argc, g, is only checked once, besides g!= 3 && g < 5 (which should tell you the two forms of invocation).

This fun challenge for 2025/ferguson is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code so that it renders smaller (or larger) sized arrows, making sure that coordinates at the edge of the map are placed correctly. Add a debug.alt3.c with the same updates.

This fun challenge for 2025/ferguson is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code it solves challenge 2, with a -p projection option.

This fun challenge for 2025/ferguson is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain why d has to start at -1 (bonus: experiment with different values and explain what happens and specifically why) and why you can’t change the !=1 in the code:

    if  (d=(c!=e==!d!=d!=d==1&&e!=c)!=1||/* NOW YOU KNOW d IS 1! */fread(c, -*(S[F])-d*3 ,
        b*j,q)^j *j *2)X;

Hint: the above are related to each other.

This fun challenge for 2025/ferguson is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code so that it slightly harder to play by allowing the closest invader to be killed. Compensate slightly for that difficulty increase by allowing the shift TAB key to cycle in the opposite direction of the TAB key.

This fun challenge for 2025/howe is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code that has a “Amulet of Yendor”.

In particular, add an “Amulet of Yendor” (similar to the “,” comma character in the rogue 5.4 game) to floor 26. If the player descends below floor 26 without the amulet, place it on that lower floor until the player eventually acquires it. Block the player’s ascent (to a lower floor number) unless they possess the amulet. If the player returns to the top floor 0 with the amulet, display a winning message, and exit.

This fun challenge for 2025/jhshrvdp is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code displays IOCCC 2025 instead of DOCTOR WHO.

This fun challenge for 2025/jingp49 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code allows for the use of numbers in the interval [1, 2^31), or even the interval [1, 2^63).

This fun challenge for 2025/kurdyukov is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain how the argument to ./prog determines if input is encoded or decoded.

This fun challenge for 2025/mattpep is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Create a 32K ROM file that tests if 2^p-1, is a Mersenne prime using the Lucas–Lehmer primality test.

This fun challenge for 2025/ncw1 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Create 32K ROM file that tests if h*2^n-1 is prime, using the Lucas–Lehmer–Riesel primality test. You may find the pages related to the Riesel Test in the A Grand Coding Challenge helpful.

This fun challenge for 2025/ncw1 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code with a modified p[] array that prints the string IOCCC 2025 followed by a newline.

This fun challenge for 2025/ncw2 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code that draws a Julia set.

This fun challenge for 2025/ncw3 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain the purpose of the constant .5, used several times in prog.c. How does changing .5 to a different value affect the generated sound?

This fun challenge for 2025/tompng is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Write alternate version of try.sh that detects when the quasi-quine, left to its own iterative devices, starts to repeat.

If you do not attempt to move the paddle, and let the game run on its own, the game will “evolve” to other “modes”, and eventually cycle back to the original style of game play.

This fun challenge for 2025/uellenberg is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Implement an additional game mode using only the original game objects (the ball and two paddles). For example, you could implement a “flappy bird” game using the ball as the bird and the paddles as the pipes.

As an extra challenge, try to keep the original behavior of the program the same, with this new game mode added on top.

This fun challenge for 2025/uellenberg is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Create a version of the game that “plays itself”, running through all the gameplay without user input and returning to the exact source code it started with.

This fun challenge for 2025/uellenberg is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code so that is allows for integers, somewhat larger than 9999, to be tested for primality.

This fun challenge for 2025/yang1 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Modify prog.c so that the code implements the compression algorithm used in the entry as a standalone program outputting a bitstream, and compare its performance with gzip -1.

This fun challenge for 2025/yang2 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.

Explain the purpose of the constant 114 prog.c. Are there any alternative values that could replace 114 without affecting the program’s functionality?

This fun challenge for 2025/yang3 is still OPEN.

See also the FAQ on “submit a fun challenge solution”.