bf.py

# Brainfuck interpreter in python
#
# @author: Maxence 'pepibumur' De Saint <contact@pepibumur.me>
# @license: Free for any use
# @since: 2012-04-04
# @version: 0.1

# A few things to note :
#   - To make it more readable, I've chosen to add a line break before each '[' or ']'.
#     This way, it's easier to see the program structure without needing to scroll.
#   - I could have used the '^' (caret) to mark the position of the pointer in the code,
#     but I feel like it's overkill, and not really what I wanted to do.
#   - The '<' and '>' characters are replaced by '-' and '+' for easier coding.
#     The original characters can be accessed with the 'bf_inc' and 'bf_dec' functions.
#   - The ',' character is replaced by 'bf_getchar', and the '.' character by 'bf_putchar'.
#     The original characters can be accessed with the 'bf_getc' and 'bf_putc' functions.
#   - The 'bf_getchar' and 'bf_putchar' functions return the original characters,
#     but they can also be overriden to return whatever you want.
#   - Printing a character on stdout will use the 'bf_putc' function by default,
#     but it can be overriden to use whatever you want.
#   - All non-ASCII characters are replaced by their ASCII equivalents.
#     This includes characters from the Unicode Supplementary Plane (U+10000 to U+10FFFF).
#     This is done because the font I've used to display the program (Inconsolata)
#     doesn't support Unicode characters higher than U+FFFF.
#   - The most important reason why I've chosen to write a Brainfuck interpreter, is
#     because I wanted to learn Python.
#     The original project I've found online was in PHP, and I didn't like it,
#     so I've decided to rewrite it in Python.
#     It's been a good way to learn the language.
#   - If you want to contribute (and correct my French if you speak it),
#     feel free to fork the project at https://github.com/pepibumur/brainfuck.py
#     and to send me a pull request.
#     I'm pretty bad at coding, so just be nice and help me make it better.
#     You will also learn a lot if you do so.


import sys

from time import sleep

# The '<', '>', '+' and '-' characters
bf_inc = '+'
bf_dec = '-'
bf_next = '>'
bf_prev = '<'

# The ',' and '.' characters
bf_getc = ','
bf_putc = '.'

# The character used to print the ASCII character corresponding to an integer
bf_printchar = '$'


# The program itself
prog = ""


# The input buffer and pointer
input_buffer = [0]
input_ptr = 0


# The output buffer
output_buffer = []

# The function used to print the ASCII character corresponding to an integer
def bf_printascii(char):
    sys.stdout.write(chr(char))


# The function used to print an ASCII character
bf_printc = bf_printascii


# The function used to get a character from stdin
def bf_getchar():
    return ord(sys.stdin.read(1))


# The function used to put a character on stdout
def bf_putchar(char):
    sys.stdout.write(chr(char))


# The tape
tape = [0]
tape_ptr = 0


# The program pointer
prog_ptr = 0


# The execution step counter
step_cnt = 0


# The main execution loop
while prog_ptr < len(prog):
    if prog[prog_ptr] == bf_inc:
        # Increase cell value
        tape[tape_ptr] = (tape[tape_ptr] + 1) % 256
    elif prog[prog_ptr] == bf_dec:
        # Decrease cell value
        tape[tape_ptr] = (tape[tape_ptr] - 1) % 256
    elif prog[prog_ptr] == bf_next:
        # Move the tape pointer to the next cell
        tape_ptr += 1
        if tape_ptr >= len(tape):
            # If the tape pointer is out of bounds, add a new cell to the tape
            tape.append(0)
    elif prog[prog_ptr] == bf_prev:
        # Move the tape pointer to the previous cell
        if tape_ptr > 0:
            tape_ptr -= 1
    elif prog[prog_ptr] == bf_getc:
        # Get a character from stdin
        input_buffer[input_ptr] = bf_getchar()
    elif prog[prog_ptr] == bf_putc:
        # Put an ASCII character on stdout
        bf_printc(input_buffer[input_ptr])
    elif prog[prog_ptr] == bf_printchar:
        # Put an integer on stdout
        bf_printc(tape[tape_ptr])
    elif prog[prog_ptr] == '[':
        # If the current cell value is null, jump to the instruction after the ']'
        if tape[tape_ptr] == 0:
            prog_ptr = prog.find(']', prog_ptr)
    elif prog[prog_ptr] == ']':
        # If the current cell value is different from null, jump to the instruction after the '['
        if tape[tape_ptr] != 0:
            prog_ptr = prog.rfind('[', 0, prog_ptr)
    else:
        # The current instruction is an empty cell
        pass
    prog_ptr += 1
    step_cnt += 1

# Print the execution step counter
print("\nExecuted {0} steps".format(step_cnt))