Brainfuck

My implementations of the Brainfuck Interpreter in a number of Turing-complete scripting languages. Complete with comments throughout the code to explain what is happening in the interpreter. Guaranteed to work properly for all valid Brainfuck programs, with or without comments. MIT Licensed

Brainfuck - The Language

Brainfuck is an extremely minimal Turing-complete programming language with just 8 commands. [1] It is represented by an array of 30,000 cells, each initialized to 0. Initially, the data pointer points at the first cell (index 0).

The 8 commands are as follows:

• . - Prints the ASCII value of the current cell, e.g. 65 -> A
• , - Stores one byte of input from the user into the current cell, e.g. A -> 65
• + - Increments the value of the current cell by 1. If the value of the current cell overflows (i.e. exceeds 255), the value is looped back to 0.
• - - Decrements the value of the current cell by 1. If the value of the current cell is less than 0, the value is looped back to 255.
• > - Moves the data pointer to the next cell
• < - Moves the data pointer to the previous cell
• [ - If the value at the current cell is zero, skip to the corresponding ]. Otherwise, move to the next instruction. [1]
• ] - If the value at the current cell is nonzero, backtrack to the corresponding [. Otherwise, move to the next instruction.

Common sense dictates that the [ and ] characters form a sort of (primitive) while loop when balanced. Obviously, they must be balanced. All other characters that do not belong to either of the 8 commands are simply ignored (and therefore can serve as comments).

A Simple Example - Hello World

Below is a simple example of a Brainfuck program that prints out the string "Hello World!" to the screen.

++++++++++ Initialize cell #0 to 10
[
  "while" loop begins
  >+++ Go to cell #1 and add 3
  >+++++++ Go to cell #2 and add 7
  >+++++++++ Go to cell #3 and add 9
  >++++++++++ Go to cell #4 and add 10
  >+++++++++++ Go to cell #5 and add 11
  <<<<<- Return to cell #0 and decrement its value
  "while" loop ends
]
[
  Now cell #0 has value 0,
  cell #1 has value 70,
  cell #2 has value 100,
  cell #3 has value 110,
  and cell #4 has value 30:
  0 | 70 | 100 | 110 | 30 | 0 | ...
  Note that this is what is known as a "comment loop".
  In a comment loop, all special characters in Brainfuck are ignored
  PROVIDED THAT: the value of the current cell is 0
  AND: all opening and closing square brackets "[]" are balanced
]
>>++. Print "H"
>>+. Print "e"
>--. Print "l"
. Print "l"
+++. Print "o"
<<<<++. Print " " (spacebar character)
>>---. Print "W"
>>. Print "o"
+++. Print "r"
------. Print "l"
<-. Print "d"
<<<+. Print "!"

Note that there are countless ways to print out "Hello World!" in a Brainfuck program. The example shown above is fairly optimised in that it makes use of a simple while loop to simplify some of the calculations but there are probably shorter Brainfuck programs out there that does the same job.

Interpreters

Brainfuck interpreters in each scripting language is placed in a folder of its own complete with test cases / examples to prove that it works.

PHP

• Folder: php
• File containing interpreter: function.brainfuck.php
• Full suite of test cases (powered by PHPTester): test_cases.php

The Interpreter

NOTE: The interpreter function in PHP requires PHP 7+ as it uses PHP 7 syntax features such as type declarations and return type declarations. In order to adapt the interpreter for PHP 5+ (or older), simply remove the type declarations and return type declarations in the first line of the interpreter function and the code should work properly for older versions of PHP.

string brainfuck(string $code[, string $input = ""])

The function receives 1 required and 1 optional argument, the first (required) argument being the Brainfuck program passed in as a string. Comments (i.e. characters other than the 8 command characters) are supported. The second (optional) argument is the list of bytes of data (in the form of a string not an array) passed into the Brainfuck program if the program contains the , character. For example, if your program contains two ,s in total and your $input string is "BF", then the value of the character "B" will be passed into the program upon the first , followed by the character value of "F" upon the second ,. The interpreter is not guaranteed to behave properly if the number of requests for user input , exceeds the number of bytes (characters) in the $input string provided.

JavaScript

• Folder: js
• File containing interpreter: function.brainfuck.js
• File containing proof of functioning interpreter (no testing framework employed): proof.html

The Interpreter

var stringOutput = brainfuck(code[, input = ""]);

The interpreter accepts 1 required argument and 1 optional argument. The first (required) argument is the Brainfuck program passed in as a string. Comments (i.e. characters other than the 8 command characters in Brainfuck) are supported. The second (optional) argument is the list of bytes of character data passed into the Brainfuck program if the program requires user input (i.e. ,) in the form of a string not an array. For example, if the Brainfuck program contains two ,s and the input string is "BF" then "B" will be passed in for the first , and "F" for the next ,. Note that the interpreter is not guaranteed to function properly if the number of requests for use input (,) exceeds the number of characters in the input string. The return value of the interpreter is always a string even if only digits are printed out.

Credits

1. Learn X in Y minutes (where X = brainfuck)