bfi

A Brainfuck interpreter written in C++. Hobby project — C++ isn't my main language, just messing around.

What is Brainfuck?

Brainfuck is an esoteric programming language created by Urban Müller in 1993. The entire language has only 8 instructions, yet it's Turing-complete — meaning anything computable can theoretically be written in it. In practice, even simple programs like Hello World require dozens of characters and a lot of mental arithmetic to write by hand.

The point isn't utility. It's a thought experiment: what's the absolute minimum a programming language needs to be "real"?

Build

g++ -std=c++17 -O2 -Wall -o bfi main.cpp

Usage

./bfi <program.bf>

Demo programs

$ ./bfi programs/hello.bf
Hello World!

$ echo "hello" | ./bfi programs/cat.bf
hello

$ ./bfi programs/count.bf
0123456789

File	Description
programs/hello.bf	Classic Hello World — exercises nested loops
programs/cat.bf	Echo stdin to stdout — the one-liner ,[.,]
programs/count.bf	Print digits 0–9 — shows arithmetic via loops

How it works

BF's memory model is a 30,000-cell array of bytes called the tape. Two pointers drive execution:

• data pointer — which cell on the tape we're reading/writing
• instruction pointer — which character in the source we're executing

The 8 instructions:

Instruction	Effect
> / <	Move the data pointer right / left
+ / -	Increment / decrement the current cell (wraps 0–255)
.	Output the current cell as an ASCII character
,	Read one byte of input into the current cell
[	Jump past the matching ] if the current cell is 0
]	Jump back to the matching [ if the current cell is nonzero

Any character that isn't one of the eight above is silently ignored — which means you can freely add comments inline with the code.

Bracket pairs are resolved into a jump table before execution starts, so [ / ] jumps are O(1) rather than scanning the source each time.

Reading a BF program

cat.bf is just three characters: ,[.,]

,      read one byte of input into cell[0]
[      while cell[0] != 0:
  .      print cell[0]
  ,      read next byte into cell[0]
]      end loop (exits when input is exhausted / EOF sets cell to 0)

count.bf shows how BF does arithmetic — there are no number literals, so you build values up with loops:

++++++++          cell[0] = 8
[                 repeat 8 times:
  >++++++           cell[1] += 6
  <-                cell[0] -= 1
]                 result: cell[1] = 48 (ASCII '0'), cell[0] = 0
>                 move to cell[1]
.+.+.+.+.+.+.+.+.+.   print 48..57 ('0'..'9')
>++++++++++.      cell[2] = 10 (newline), print it