bf.py

"""
Taken from: https://learnxinyminutes.com/docs/brainfuck/

There are eight commands:
+ : Increments the value at the current cell by one.
- : Decrements the value at the current cell by one.
> : Moves the data pointer to the next cell (cell on the right).
< : Moves the data pointer to the previous cell (cell on the left).
. : Prints the ASCII value at the current cell (i.e. 65 = 'A').
, : Reads a single input character into the current cell.
[ : If the value at the current cell is zero, skips to the corresponding ] .
    Otherwise, move to the next instruction.
] : If the value at the current cell is zero, move to the next instruction.
    Otherwise, move backwards in the instructions to the corresponding [ .

[ and ] form a while loop. Obviously, they must be balanced.
"""

import sys


INCREMENT = '+'
DECREMENT = '-'
MOVER = '>'
MOVEL = '<'
PRINT = '.'
READ = ','
WHILE_START = '['
WHILE_END = ']'

OPERATIONS = (
    INCREMENT,
    DECREMENT,
    MOVER,
    MOVEL,
    PRINT,
    READ,
    WHILE_START,
    WHILE_END)

TAPE = [0]*30000


def lex(stream):
    return [char for char in stream if char in OPERATIONS]


def parse(tokens):
    index = 0
    # A stack of executed instructions.
    stack = []
    while True:

        try:
            next_ = tokens.pop(0)
        except IndexError:
            break
        else:
            stack.append(next_)

        if next_ == INCREMENT:
            TAPE[index] += 1

        elif next_ == DECREMENT:
            TAPE[index] -= 1

        elif next_ == MOVER:
            index += 1

        elif next_ == MOVEL:
            if index == 0:
                raise RuntimeError()
            index -= 1

        elif next_ == PRINT:
            print(chr(TAPE[index]), end='')

        elif next_ == READ:
            TAPE[index] = ord(sys.stdin.read(1))

        elif next_ == WHILE_START:
            if TAPE[index] == 0:
                # Skip to matching ']'.
                while True:
                    n = tokens.pop(0)
                    if n == WHILE_END:
                        tokens.insert(0, n)
                        break
                    stack.append(n)

        elif next_ == WHILE_END:
            if TAPE[index] != 0:
                # Go back to opening '['.
                while True:
                    n = stack.pop()
                    tokens.insert(0, n)
                    if n == WHILE_START:
                        break


def read_file(path):
    with open(path) as fh:
        return fh.read()


if __name__ == "__main__":
    path = sys.argv[1]
    parse(lex(read_file(path)))