ioccc/2019/adamovsky/README.md

## To build:

``` <!---sh-->
    make
```


### Bugs and (Mis)features:

The current status of this entry is:

> **STATUS: INABIAF - please DO NOT fix**

For more detailed information see [2019/adamovsky in bugs.html](../../bugs.html#2019_adamovsky).


## To use:

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


## Try:

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


## Judges' remarks:

Even after pre-processing and indenting, the C code of this entry it
is about as understandable as the Unlambda code.

Pretending that we don't know a bit of functional programming :) and lacking a
better way to understand the entry but to fuzz it, we stumbled on a string of
bytes which crashed it:

```
    ````.0`.0`.0`c.0``sssss
```

Functional programming is not a panacea against core dumps, after all.

This can be demonstrated by:

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


## Author's remarks:

### A highly structured state state machine

### Purpose:

The purpose of this program is to allow you to play Colossal Cave Adventure as
implemented by [Kunihiko Sakamoto][1]. As that was written in [Unlambda programming
language][2] designed by David Alexander Madore it has to be an Unlambda interpreter.


### Aesthetics:

The program grew a bit too long so I had to use several macros to downsize it.
That unfortunately reduced my options for program layout. I decided to separate
it into two portions: the initial macro section where I had limited resources
which you should disregard its shape, unless you like it, and the rest, which is a
separator in the form of a single line followed by a section signifying the
function of the program, a block with removed shape of lambda character. Even
the official IOCCC size tool acknowledges this: When you call `./iocccsize -i
<prog.c`, using the [2019 version of
iocccsize.c](%%REPO_URL%%/2019/iocccsize.c), iocccsize returns **955**,
which is the Unicode codepoint of the **&#955;** character.

It might be a surprise for you that according to the IOCCC size tool the program is
actually a `one-liner`. It certainly was for me. I like to think the tool is overwhelmed
by the sheer length of the dividing line and totally overlooks the rest of the program.

### Usage:

When built directly using

``` <!---sh-->
    make
```

the program (`prog`) accepts a single parameter with the name of an Unlambda program. You can
download the [Colossal Cave Adventure][1] (the [advent.unl](%%REPO_URL%%/2019/adamovsky/advent.unl) file)
and run it like this:

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

**NOTE**: the file was added to the entry so there is no need to download it.

You can also try other programs from the Web. Many example programs are in the official
[Unlambda distribution][3] on the Unlambda homepage. The most complex programs there are
entries from [quine contest][4].

There is an alternative build path that requires the 2018 IOCCC size tool to complete. Put the tool
source [iocccsize_2018.c](%%REPO_URL%%/2019/adamovsky/iocccsize_2018.c) in the project directory and call

``` <!---sh-->
    make identify
```

It will build the tool and use it to build an alternative program `prog2`, which in turn will
produce an Unlambda program `identify.unl`. `make identify` runs this program to identify
itself.

The base program (`prog`) implements an interpreter of Unlambda 2.0. It
recognises all its builtins (`k`, `s`, `i`, `v`, `c`, `d`, `.x`, `e`, `@`, `?x`
and `|`) including the syntactic sugar function `r` and formatting properties in
the form of ignoring whitespace and comments to the end of line introduced by
the `#` character.

In case of wrong usage (missing Unlambda program file or missing parameter) or
use of an unseekable (`fseek(3)`) Unlambda program source if it uses the `c`
function, the program returns the message `fail`. It also informs you about
syntactic errors in the Unlambda program should there be any (but only when the
execution reaches them).

The Unlambda source file may contain several Unlambda programs. The interpreter
will run them all in sequence.


### Unlambda

If you want to familiarise yourself with the language, visit its [Wikipedia page][5] for
a brief introduction or its [homepage][2] for an in depth discussion. For further reading you
can visit Wikipedia pages on [Lambda Calculus][6] and [Combinatory Logic][7].

You can also play with an [online interpreter][8] by GitHub user inazz. There you can
debug one of the example programs provided or try your wits in writing your own.


### Hints:

Here are some pointers where to start when trying to understand the interpreter:

- Where are syntax errors detected?
- How is the memory managed?
- How does a program character turn into a function?
- How does exit function `e` work?
- Can `fD0` function be called?
- Are the function names familiar? (warning: some are misleading)


### Obfuscation sources

- The elephant in the room is the Unlambda language. As we all know, functional
programming defies sanity. Furthermore, Unlambda was specifically designed to be
unintelligible and hard to code in. Most of its functions are quite simple and
straightforward to implement, but two make it a mess: the promise operator `d`
and the call with current continuation `c`.
- Most of the additional obfuscation is derived from basic design choices. That
is, how the state of the program is stored and how the Unlambda functions are
implemented.

More on this in the [Obfuscation](#obfuscation) section.


### Build notes:

The program does not require any special treatment by compilers. Some warnings
had to be suppressed when compiling with `-Wall -Wextra -pedantic`. Those were
produced because of the formatting of the source code, not any programming
technique. Compilation with `clang -Weverything` produced many more warnings,
but those are only relevant for multi-source-file projects and some are really
questionable (e.g. `-Wdisabled-macro-expansion`).


### Portability:

Development was done on Debian 9 on x86-64 architecture using mainly Clang 3.8.1-24
and GCC 6.3.0. The source is C11 compliant and does not use any special properties
except the following:

- The interpreter sort of relies on tail call optimization. It works well even
without it, but the stack runs out quickly with some Unlambda programs (e.g.
`trivial.unl` example from the [reference Unlambda distribution][3] produces
segfault).  Clang with `-O3` option does this optimization well and said example
does not segfault with it. I was not able to force GCC to optimize it out
(probably an operator fault).
- The only data structure of the program (the `union`) had much more
members originally. I joined all integers to a single `long long` member in the
name of obfuscation. It is used for reference counting (only 1 for every
reference) among other uses. `size_t` should be used for that, but practically,
I think the memory would run out sooner than the reference counter in any real
world situation even when `size_t` is bigger than `long long`. It is also used
for storing `char`s for later printing. This might produce some warnings, but
since it was developed using `char`s it should be always used safely.


### Obfuscation:

- You can examine the [deobfuscated.c](%%REPO_URL%%/2019/adamovsky/deobfuscated.c) file. It is the original
final version of the interpreter, before I started the downsizing. It accepts
one more optional parameter that specifies a filename for the output Unlambda
function (it's the Unlambda function that remains after all function
applications are done).  When no filename is provided, `stderr` is used.

    The function *output* is used for this additional functionality. You can use
    it while debugging to visualise current variables.

- The Unlambda functions are represented by a record consisting of an array of
five multipurpose arguments. The first one refers to the function implementing
its application, the second and the third ones are for arguments of the
function, the fourth one is for stack building, and the fifth one is the
reference counter.

    The function argument is used not only for evaluation of the functions, but also
    as a state variable to decide whether we want to call it, or do something else.

- The interpreter reads the Unlambda source as it interprets it, translating the
incoming character to a function using a constructor stored in translation array
`T`.

    This means the interpreter must reposition in the source file whenever current
    continuation is called.

- The state of the interpreter is represented by trees representing evaluated
Unlambda functions stored on the C stack as local variables, plus the source
file position. When a current continuation is created, the stack is unwound
using the fourth argument of the nodes, stored in the continuation, and rebuilt
back resuming the evaluation. When its called, the stack is unwound and thrown
away, and the stack stored in the continuation is restored.

    This makes implementation of a complete debugger a bit more challenging.

    For this to work, the functions that do the work (i.e. make a series of applications
    and/or evaluations each of which may be a call to current continuation), the `s`
    combinator and the `` ` `` application operation had to be broken down to pieces, one
    for each application/evaluation and corresponding helper functions for stack
    rebuilding. (the helper functions are named beginning with `r` instead of `f`)

- The `` ` `` application operation does the reading of the Unlambda source when
needed. It also implements the promise function. Whenever it would apply it, it
overwrites the Apply constructor in translation array `T`, changing its function
to build an unevaluated sub-tree that is later evaluated whenever the promise is
applied.

- To further the obfuscation, I shortened the function names while downsizing,
but preserved their meaning (with the exception of Apply functions, which I
renamed to `Y` to possibly confuse with `Y` combinator). Simple functions retained
their names to misguide the reader to believe to know what is what. The names of
continuation and exit functions were swapped. The names of `S` and Apply functions
were shuffled both for `S` functions and Apply functions in the same way to hide
the meaning of `f` and `r` prefixes.


### 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.


### Identification

The alternative build path `make identify` output commemorates a [previous IOCCC
winning entry][9] which uses similar methods to mess with the size tool.


### How this entry works:

It is not a bug, it is a feature. :)

The crash is caused by a stack overflow. Conventional Unlabmda interpreters
implement the Unlambda execution stack using data structures on the heap. This
interpreter uses the C call stack for this purpose. In a conventional
interpreter, the judges' program would consume memory until the user (or the
operating system) would lose patience. The C call stack is usually of a limited
size and protected, so this interpreter crashes, when the judges' program
depletes the stack. You can always increase the maximal stack size through the
system settings (on Linux) or compiler options (on Windows) if your Unlambda
program needs it. Nevertheless, it will not help the judges' program which
quickly depletes any limit you define.

The judges, pretending to not know what they are doing, created a program that
grows Unlambda execution stack indefinitely. You may consider it an Unlambda
anti-pattern.  Normally, infinite cycles in Unlambda work by creating two
functions and applying one to the other which creates further functions to
apply. The application of the function frees its position on the stack. This
program builds one ever growing function that will never be complete and be able to
be applied.

[1]: https://github.com/irori/advent-unlambda                "Adventure"
[2]: http://www.madore.org/~david/programs/unlambda/         "Unlambda programming language"
[3]: http://www.madore.org/~david/programs/unlambda/#distrib "Reference Unlambda distribution"
[4]: http://www.madore.org/~david/programs/unlambda/#quine   "Unlambda quine contest"
[5]: https://en.wikipedia.org/wiki/Unlambda                  "Unlambda on Wikipedia"
[6]: https://en.wikipedia.org/wiki/Lambda_calculus           "Lambda Calculus on Wikipedia"
[7]: https://en.wikipedia.org/wiki/Combinatory_logic         "Combinatory Logic on Wikipedia"
[8]: https://inazz.jp/unlambda/                              "Online Unlambda interpreter"
[9]: ../../2014/birken/index.html                            "Best use of port 1701"

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