h/plugins/bf.py

'''brainfuck interpreter adapted from (public domain) code at
http://brainfuck.sourceforge.net/brain.py'''

import re
import random

from util import hook


BUFFER_SIZE = 5000
MAX_STEPS = 1000000


@hook.command
def bf(inp):
    ".bf <prog> -- executes brainfuck program <prog>"""

    program = re.sub('[^][<>+-.,]', '', inp)

    # create a dict of brackets pairs, for speed later on
    brackets = {}
    open_brackets = []
    for pos in range(len(program)):
        if program[pos] == '[':
            open_brackets.append(pos)
        elif program[pos] == ']':
            if len(open_brackets) > 0:
                brackets[pos] = open_brackets[-1]
                brackets[open_brackets[-1]] = pos
                open_brackets.pop()
            else:
                return 'unbalanced brackets'
    if len(open_brackets) != 0:
        return 'unbalanced brackets'

    # now we can start interpreting
    ip = 0        # instruction pointer
    mp = 0        # memory pointer
    steps = 0
    memory = [0] * BUFFER_SIZE  # initial memory area
    rightmost = 0
    output = ""   # we'll save the output here

    # the main program loop:
    while ip < len(program):
        c = program[ip]
        if c == '+':
            memory[mp] = memory[mp] + 1 % 256
        elif c == '-':
            memory[mp] = memory[mp] - 1 % 256
        elif c == '>':
            mp += 1
            if mp > rightmost:
                rightmost = mp
                if mp >= len(memory):
                    # no restriction on memory growth!
                    memory.extend([0] * BUFFER_SIZE)
        elif c == '<':
            mp = mp - 1 % len(memory)
        elif c == '.':
            output += chr(memory[mp])
            if len(output) > 500:
                break
        elif c == ',':
            memory[mp] = random.randint(1, 255)
        elif c == '[':
            if memory[mp] == 0:
                ip = brackets[ip]
        elif c == ']':
            if memory[mp] != 0:
                ip = brackets[ip]

        ip += 1
        steps += 1
        if steps > MAX_STEPS:
            if output == '':
                output = '(no output)'
            output += '[exceeded %d iterations]' % MAX_STEPS
            break

    stripped_output = re.sub(r'[\x00-\x1F]', '', output)

    if stripped_output == '':
        if output != '':
            return 'no printable output'
        return 'no output'

    return stripped_output[:430].decode('utf8', 'ignore')