Author:

• Name: Edward Giles
  Location: AU - Commonwealth of Australia (Australia)

To build:

This entry requires SDL2 to be installed. See the FAQ on “SDL1 and SDL2” if you don’t have it installed and don’t know how to do so.

    make

To use:

    ./prog

Try:

    ./prog ioccc.txt

    ./prog prog.c

    ./prog sand.txt

    ./prog sand.alt.txt

NOTE: these invocations will take quite some time to finish.

Judges’ remarks:

Can you explain the behavior of the program when ioccc.txt is given to it as input?

How would the description of the state machine look like when translated from Polynomish to English?

Author’s remarks:

Remarks

This entry graphically displays a simulation of sand falling. It uses SDL to draw the interface.

The code makes the following assumptions about the underlying system:

• SDL2 is installed from https://libsdl.org or your package manager.
• double is an IEEE 754 floating-point type and sizeof(double)==8.
• The compiler supports the C99 standard.

In order to simulate the motion of the sand, a cellular automaton is used. The state transition table is not stored directly. Instead, the program stores the coefficients of a polynomial that map each state to the correct one. These coefficients are stored as raw hexadecimal in the V[] array, and they are reinterpreted as doubles when used. This allows more numeric precision in fewer characters (base 16 > base 10), and makes the purpose of the array less clear.

The code is laid out graphically as a bucket pouring sand, and it is far easier to observe this high-level layout than the code’s actual function.

The preprocessor is used to both obfuscate the code as a whole and abbreviate the calls to SDL. Additionally, throughout the code, the variables were named so that they are short and easily confused with each other.

Primary files

• prog.c - entry source code
• Makefile - entry Makefile
• prog.orig.c - original source code
• ioccc.txt - sample input
• sand.alt.txt - sample input
• sand.txt - sample input

Secondary files

• 2018_giles.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• index.html - this web page

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