Brainfuck Interpreter

brainfuck.py

# -*- coding: utf-8 -*-
import os
import sys
import time

IP = 0     # Instruction pointer
DP = 0     # Data pointer
DATA = [0] * 20
OUTPUT = []


def search_matching_bracket(text, direction):

    brackets = ("[", "]")
    if direction == "backwards":
        brackets = brackets[::-1]  # use ("]", "[") instead
        text = text[::-1]  # revert text
    same_bracket, matching_bracket = brackets

    matching_found = 0
    for index, char in enumerate(text):
        if char == same_bracket:
            matching_found -= 1
        elif char == matching_bracket:
            matching_found += 1
            if matching_found == 1:
                return index
    raise Exception("Unbalanced brackets in program")


def jump_to_matching_bracket(program, current_position, direction):
    if direction == 1:  # search forward
        index = search_matching_bracket(program[current_position:],
                                        "forwards")
        return current_position + index
    else:
        index = search_matching_bracket(program[:current_position],
                                        "backwards")
        return current_position - (index + 1)


def show_state(program):
    os.system("clear")
    state = (
        "OUT: {output}\n"
        "{program}\n{instruction_pointer}\n"
        "{data}\n{data_pointer}^"
    ).format(
        output="".join(OUTPUT),
        program=program,
        instruction_pointer=(" " * IP) + "^",
        data="|" + "|".join(map(lambda x: str(x).rjust(3), DATA)) + "|",
        data_pointer=("    " * DP) + "  "
    )
    print state


if __name__ == "__main__":

    # This BF program prints "Hello World!"
    program = ("++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++"
               "++..+++.>>.<-.<.+++.------.--------.>>+.>++.")

    DEBUG = '--debug' in sys.argv
    while 1:
        try:
            next_instruction = program[IP]
        except IndexError:
            # we've reached the end of the program
            show_state(program)
            break

        if DEBUG:
            show_state(program)
            time.sleep(0.01)

        if next_instruction == ">":
            DP += 1
        elif next_instruction == "<":
            DP -= 1
        elif next_instruction == "+":
            DATA[DP] += 1
            if DATA[DP] > 255:  # rollover to 0 when overflowing
                DATA[DP] = 0
        elif next_instruction == "-":
            DATA[DP] -= 1
            if DATA[DP] < 0:  # rollover to 255 when underflowing
                DATA[DP] = 255
        elif next_instruction == ".":
            OUTPUT.append(chr(DATA[DP]))
        elif next_instruction == ",":
            done = False
            while not done:
                try:
                    val = input("Enter a value (0-255):")
                    if 0 <= val <= 255:
                        done = True
                        DATA[DP] = val
                except NameError:
                    pass
        elif next_instruction == "[":
            if DATA[DP] == 0:
                IP = jump_to_matching_bracket(program, IP, 1)
        elif next_instruction == "]":
            if DATA[DP] != 0:
                IP = jump_to_matching_bracket(program, IP, -1)
        IP += 1