ioccc/2015/muth

Most Complete Use of CPP

Dominik Muth 
<muth.ioccc@freenet.de>  

To build:

make

To run:

make -B [MACHINE=your_machine.h] [TAPE=your_tape.h] [X=[0|1|2|3|4|5|6|7|8|9]] [V=[0|1|2]] run

Try:

make -B V=2 run

strings prog | grep '(,)'

make -B MACHINE=machine_times2.h TAPE=tape_five.h V=1 run

make -B MACHINE=machine_chaos_5_2.h X=5 run

Judges' comments:

This is something we were fearing all along: that the C preprocessor is Turing-complete even without conditionals and recursive file inclusion as some kind of a rewriting system. Now we have the proof!

Author's comments:

Preprocessor Turing Engine

Features

• Turing machine runs in preprocessor
• universal (unlimited configurable states, symbols, and tape)
• no #ifs or other conditional directives, only #defines
• no recursive #include
• single compiler run (no repeated preprocessing)
• no variadic macros
• verbosity levels:
  • V=0 (default) only prints the final tape
  • V=1 additionally prints one tilde character per machine step (slow)
  • V=2 additionally prints details on each machine step (slow; generates large files)
• make target run runs ./prog and filters a little statistics out of the verbose output
• X (defaults to 3) sets the maximum number of steps: M(X) ~ 8^X

Limitations

If the machine does not halt after M(X) steps, you will see unexpanded macros in the output.

Compiler warnings

If V=1 or V=2 is used, you may see "warning: string length is greater than the length '4095' ISO C99 compilers are required to support".

Programming

Implementing your own Turing machine is easy. Take a look at the supplied header files for examples.

Tape Symbols

Every symbol used (except "_") must be explicitly declared using

#define sym_SYMBOL(sym, _SYMBOL) sym

Example:

#define sym_1(sym, _1) sym

Tape

The initial content of the tape must be defined in the tape header using a triple like:

#define tape ((((,),...l2), l1), c, (r1, (r2..., (,))))

• l1, l2, ... are the symbols to the left
• c is the symbol at the current position
• r1, r2, ... are the symbols to the right

The empty pairs "(,)" represent the continuations of the tape, filled with underscore symbols ("_"). They are expanded on demand.

State Machine

All transitions are defined in the machine header using the syntax

#define CURRENT_SCANNED (WRITE, MOVEMENT, NEXT)

where

• CURRENT is the current state
• SCANNED is the symbol at the current position
• the whitespace before the parentheses is significant
• WRITE is the symbol to be written to the tape
• MOVEMENT is "L" for left, "R" for right, or nothing
• NEXT is the next state

There must be a state "A" defined, which becomes the initial state of the machine.

To halt the machine, use the keyword "break" as in

#define A_1 (2, break)

Copyright:

(c) Copyright 1984-2016, Leo Broukhis, Simon Cooper, Landon Curt Noll - All rights reserved This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.