Awesome

sbfi

Simple BrainFuck Interpreter written in C. The aim of this interpreter is to be as fast as possible while remaining easy to understand.

Explanations about how my program works can be found in the comments !

Compilation

I compile with gcc -Wall -Wextra -Ofast -s, which doesn't output any warning.

-pedantic complains about label addresses / computed gotos, C++ comments, and declarations mixed with code.

Note that Clang supports the computed gotos extension as well and compiles fine.

Implementation details

In the original Brainfuck specification by Urban Müller in 1993, a lot of details were left unspecified or unclear, which means they are implementation-defined. As such, any Brainfuck interpreter or compiler is free to do whatever it wants with them, as long as the choices are documented.

The following is a list of implementation-defined behaviors in sbfi.

Cell size

In the reference implementation, cells can hold an integer between 0 and 255. In sbfi, the default setting is that each cell is of the type unsigned char, which has almost always the same range. You can change this setting by modifying the CELL macro and defining with any other integer type.

Cell bounds

The reference implementation has wrapping cells : if a cell that contains the maximal value for its type is incremented, the value is set to the minimal value for its type, and vice versa.

With a C example, it means :

unsigned char c = 255;

++c; // c = 0
--c; // c = 255

Cells in sbfi have this behavior too, since it's a natural way of dealing with overflows in many languages.

Array size

The cell array contains 30000 cells in the reference implementation, and so is the default in sbfi. However, you can change this setting by modifying the INITIAL_ARRAY_SIZE macro, as long as you define it with at least 1 cell.

Array bounds

The reference implementation raises an error and stops the execution if a Brainfuck program tries to access a cell outside of the array. sbfi implements five different behaviors regarding the bound checking ; you can enable them by setting the MEMORY_BEHAVIOR macro to one of the following macros :

• NONE : the default setting. No bound checking is performed at all, to improve performance. It is assumed that the program will not try to access a cell outside of the array. It it does, the behavior is undefined.
• EXTEND : if needed, the array is extended during runtime to be able to run the program correctly.
• ABORT : raises an error and stops the execution.
• WRAP : the array wraps around, like the cells. If a bound is reached, the array pointer goes to the other bound.
• BLOCK : if the pointer is on a bound and tries to go further, it is blocked and stays here. The program execution doesn't stop and the pointer can still move in the other direction.

End-Of-Line in input / output

ASCII defines 10 as a line-feed (LF), and 13 as a carriage-return (CR). Different conventions for representing the line break are in use today, among them LF (Unix systems), CR (Mac OS), and even CR LF (Windows), which hinders portability. However, the general consensus (and the reference implementation) favors LF, and that's what sbfi implements too. In other words, inputting a line break will set the current cell to 10, and outputting a 10 will display a line break.

End-Of-File in input

When EOF is encountered in the input, there is no definite decision about what to do. Three behaviors are generally considered :

• Set the current cell to -1
• Set the current cell to 0
• Leave the current cell unchanged

The last one is often considered more portable, that's why it's the default setting in sbfi. However, you can chose the behavior by changing the EOF_INPUT_BEHAVIOR macro to either NO_CHANGE or the value you want to set the current cell to (0, -1 or any other integer value).