# Return to [%%DESCRIPTION%%](index.html)

A "**fun challenge**" for a winning IOCCC entry is a puzzle or task
that an [IOCCC judge](../judges.html) 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](../faq.html#what_fun_challenge)".


<div id="ayu">
## Fun challenge for [2025/ayu](ayu/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="cable">
## Fun challenge for [2025/cable](cable/index.html)
</div>


<div id="cable_0">
### Fun challenge 0
</div>

Use the [toolchain](https://github.com/adriancable/eternal) 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](../faq.html#solve_fun_challenge)".


<div id="cable_1">
### Fun challenge 1
</div>

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](../faq.html#solve_fun_challenge)".


<div id="cesmoak">
## Fun challenge for [2025/cesmoak](cesmoak/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="diels-grabsch">
## Fun challenge for [2025/diels-grabsch](diels-grabsch/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="dogon">
## Fun challenge for [2025/dogon](dogon/index.html)
</div>


<div id="dogon_0">
### Fun challenge 0
</div>

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

This fun challenge for **2025/dogon** is still **OPEN**.


<div id="dogon_1">
### Fun challenge 1
</div>

Given the "*constant*" that is printed, explain the last line of output:

``` <!---sh-->
    ./prog | tail -1
```

This fun challenge for **2025/dogon** is still **OPEN**.


<div id="dogon_2">
### Fun challenge 2
</div>

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**.


<div id="dogon_3">
### Fun challenge 3
</div>

Additionally, observe the [try.sh](%%REPO_URL%%/2025/dogon/try.sh) script uses the
following display the program's output:

``` <!---sh-->
    ./prog | cat
```

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

``` <!---sh-->
    ./prog
```

Explain why this command:

``` <!---sh-->
    ./prog | cat > foo
```

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

``` <!---sh-->
    ./prog > bar
```

This fun challenge for **2025/dogon** is still **OPEN**.


<div id="endoh1">
## Fun challenge for [2025/endoh1](endoh1/index.html)
</div>

Modify `prog.c` so that the code that is also works with UPPER CASE LETTERS.
You would certainly have to
[embiggen](https://en.wiktionary.org/wiki/embiggen#English)
the tube size to better distinguish individual letters and digits.
Such code would be a [cromulent](https://en.wiktionary.org/wiki/cromulent#English)
addition to [2025/endoh1](%%REPO_URL%%/2025/endoh1/index.html).

This fun challenge for **2025/endoh1** is still **OPEN**.

See also the
FAQ on "[submit a fun challenge solution](../faq.html#solve_fun_challenge)".


<div id="endoh2">
## Fun challenge for [2025/endoh2](endoh2/index.html)
</div>

Modify `prog.c` so that the code prints a line instead of varying brightness of a yellow line,
using [256 ANSI Color Codes](https://color-palette.hexdocs.pm/ansi_color_codes.html).
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](../faq.html#solve_fun_challenge)".


<div id="endoh3">
## Fun challenge for [2025/endoh3](endoh3/index.html)
</div>

Explain why the `git-am` patch generator must be used about 25 times (see
[try.sh](%%REPO_URL%%/2025/endoh3/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](../faq.html#solve_fun_challenge)".


<div id="ferguson">
## Fun challenge for [2025/ferguson](ferguson/index.html)
</div>


<div id="ferguson_0">
### Fun challenge 0
</div>

Modify `prog.c` so that the code
allows for different [map projections](https://en.wikipedia.org/wiki/List_of_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](../faq.html#solve_fun_challenge)".


<div id="ferguson_1">
### Fun challenge 1
</div>

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](../faq.html#solve_fun_challenge)".


<div id="ferguson_2">
### Fun challenge 2
</div>

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](../faq.html#solve_fun_challenge)".


<div id="ferguson_3">
### Fun challenge 3
</div>

Modify `prog.c` so that the code it solves [challenge 2](#ferguson_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](../faq.html#solve_fun_challenge)".


<div id="ferguson_4">
### Fun challenge 4
</div>

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:

``` <!---c-->
    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](../faq.html#solve_fun_challenge)".


<div id="howe">
## Fun challenge for [2025/howe](howe/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="jhshrvdp">
## Fun challenge for [2025/jhshrvdp](jhshrvdp/index.html)
</div>

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](https://github.com/lcn2/rogue5.4) 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](../faq.html#solve_fun_challenge)".


<div id="jingp49">
## Fun challenge for [2025/jingp49](jingp49/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="kurdyukov">
## Fun challenge for [2025/kurdyukov](kurdyukov/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="mattpep">
## Fun challenge for [2025/mattpep](mattpep/index.html)
</div>
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](../faq.html#solve_fun_challenge)".


<div id="ncw1">
## Fun challenge for [2025/ncw1](ncw1/index.html)
</div>


<div id="ncw1_0">
### Fun challenge 0
</div>

Create a 32K ROM file that tests if **2^p-1**, is a Mersenne prime using the
[Lucas–Lehmer primality test](https://en.wikipedia.org/wiki/Lucas–Lehmer_primality_test).

This fun challenge for **2025/ncw1** is still **OPEN**.

See also the
FAQ on "[submit a fun challenge solution](../faq.html#solve_fun_challenge)".


<div id="ncw1_1">
### Fun challenge 1
</div>

Create 32K ROM file that tests if **h\*2^n-1** is prime, using the
[Lucas–Lehmer–Riesel primality test](https://en.wikipedia.org/wiki/Lucas–Lehmer–Riesel_test).
You may find the pages related to the **Riesel Test** in the
[A Grand Coding Challenge](http://www.isthe.com/chongo/tech/math/prime/prime-tutorial.pdf)
helpful.

This fun challenge for **2025/ncw1** is still **OPEN**.

See also the
FAQ on "[submit a fun challenge solution](../faq.html#solve_fun_challenge)".


<div id="ncw2">
## Fun challenge for [2025/ncw2](ncw2/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="ncw3">
## Fun challenge for [2025/ncw3](ncw3/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="tompng">
## Fun challenge for [2025/tompng](tompng/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="uellenberg">
## Fun challenge for [2025/uellenberg](uellenberg/index.html)
</div>


<div id="uellenberg_0">
### Fun challenge 0
</div>

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](../faq.html#solve_fun_challenge)".


<div id="uellenberg_1">
### Fun challenge 1
</div>

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](../faq.html#solve_fun_challenge)".


<div id="uellenberg_2">
### Fun challenge 2
</div>

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](../faq.html#solve_fun_challenge)".


<div id="yang1">
## Fun challenge for [2025/yang1](yang1/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="yang2">
## Fun challenge for [2025/yang2](yang2/index.html)
</div>

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](../faq.html#solve_fun_challenge)".


<div id="yang3">
## Fun challenge for [2025/yang3](yang3/index.html)
</div>

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](../faq.html#solve_fun_challenge)".