brainfuck.py

import readchar

tape = bytearray([0])
data_pointer = 0
loop_buffer = ''

def state():
    print ''.join(['{0:03} '.format(i) for i in tape])
    for i in range(data_pointer):
        print ' '*3,
    print '^'
    if loop_buffer:
        print loop_buffer

def lookahead(program, instruction_pointer):
    lookahead = instruction_pointer + 1
    depth = 1
    while True:
        if program[lookahead] == '[':
            depth += 1
        elif program[lookahead] == ']':
            depth -= 1
        if depth == 0:
            return lookahead

        lookahead += 1

        if lookahead == len(program) + 1:
            raise SyntaxError('Matching ] not found')

def lookback(program, instruction_pointer):
    lookback = instruction_pointer - 1
    depth = 1
    while True:
        if program[lookback] == '[':
            depth -= 1
        elif program[lookback] == ']':
            depth += 1
        if depth == 0:
            return lookback

        lookback -= 1

        if lookback < 0:
            raise SyntaxError('Matching [ not found')


def interpret(program):
    global data_pointer

    instruction_pointer = 0

    while True:
        char = program[instruction_pointer]

        if char == '>':
            data_pointer += 1
            if len(tape) == data_pointer:
                tape.append(0)
        if char == '<':
            if data_pointer > 0:
                data_pointer -= 1
        if char == '+':
            tape[data_pointer] += 1
        if char == '-':
            if tape[data_pointer] > 0:
                tape[data_pointer] -= 1
        if char == '.':
            print '{0:c}'.format(tape[data_pointer])
        if char == ',':
            tape[data_pointer] = readchar.readchar()
        if char == '[':
            if tape[data_pointer] == 0:
                instruction_pointer = lookahead(program, instruction_pointer)
        if char == ']':
            if tape[data_pointer] != 0:
                instruction_pointer = lookback(program, instruction_pointer)

        instruction_pointer += 1

        if len(program) == instruction_pointer:
            break

def main():
    global loop_buffer

    while True:
        state()
        char = readchar.readchar()
        if char == 'q':
            break
        elif loop_buffer:
            loop_buffer += char
            opens = loop_buffer.count('[')
            closes = loop_buffer.count(']')
            if opens == closes:
                print 'Loop:', loop_buffer
                interpret(loop_buffer)
                loop_buffer = ''
        elif char == '[':
            loop_buffer = char
        else:
            interpret(str(char))

if __name__ == '__main__':
    main()