BrainFuck Interpreter and REPL in Python 3

README.md

PBFI: Pythonic BrainFuck Interpreter

Yet another educational interpreter for the BrainFuck Programming Language, written in Python 3. This might not be the shortest BrainFuck interpreter that you had come acorss, however the style of programming is for educational purposes only.

USAGE:

Help:

./brainfuck --help

REPL:

./brainfuck --repl

Interpreter:

./brainfuck --program ~/hello_world.bf
./brainfuck --code "++++++ [ > ++++++++++ < - ] > +++++ ."

LICENSE:

This gist and all its accompanying files are released under the MIT License.

  PBFI: Pythonic BrainFuck Inpterpreter
  
  Copyright (C) 2016  Ahmad Alhour

  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

  The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

brainfuck

#!/usr/bin/env python3

# -----------------------------------------------------------------------------------------
# PBFI: Pythonic BrainFuck Interpreter
#
# @author:          Ahmad Alhour (http://aalhour.com)
# @description:     BrainFuck Interpreter and REPL in less than 500 lines of Python 3
# @license:         MIT (https://opensource.org/licenses/MIT)
#
# @usage:
#   ./brainfuck --help                                  Prints help and exits
#   ./brainfuck --repl                                  Opens REPL
#   ./brainfuck --program ~/some_program.bf             Runs code in ~/some_program.bf
#   ./brainfuck --code "++++++[>++++++++++<-]>+++++."   Evalues a piece of code
# -----------------------------------------------------------------------------------------

# std
import sys
import subprocess
import argparse


# #########################################################################################
#                                                                                         #
#  TOKENS                                                                                 #
#                                                                                         #
# #########################################################################################
# Reads a single input character into the current cell.
READ_CMD = "READ_CMD"

# Prints the ASCII value at the current cell (i.e. 65 = 'A').
PRINT_CMD = "PRINT_CMD"

# Increments the value at the current cell by one.
INC_CELL_CMD = "INC_CELL_CMD"

# Decrements the value at the current cell by one.
DEC_CELL_CMD = "DEC_CELL_CMD"

# Moves the data pointer to the next cell (cell on the right).
MOV_PTR_NEXT_CMD = "MOV_PTR_NEXT_CMD"

# Moves the data pointer to the previous cell (cell on the left).
MOV_PTR_PREV_CMD = "MOV_PTR_PREV_CMD"

# If the value at the current cell is zero, skips to the corresponding ] .
# Otherwise, move to the next instruction.
BEGIN_LOOP_CMD = "BEGIN_LOOP_CMD"

# If the value at the current cell is zero, move to the next instruction.
# Otherwise, move backwards in the instructions to the corresponding [ .
END_LOOP_CMD = "END_LOOP_CMD"

# A matched [ and ]
LOOP_CMD = "LOOP"

# END OF FILE
EOF = "EOF"

# Fast lookup tables: Symbol to Token Type mapping, and its reversed
sym2tok = {
    ",": READ_CMD,
    ".": PRINT_CMD,
    "+": INC_CELL_CMD,
    "-": DEC_CELL_CMD,
    "<": MOV_PTR_PREV_CMD,
    ">": MOV_PTR_NEXT_CMD,
    "[": BEGIN_LOOP_CMD,
    "]": END_LOOP_CMD
}

tok2sym = {value: key for key, value in sym2tok.items()}


# #########################################################################################
#                                                                                         #
#  LAZY LEXER                                                                             #
#                                                                                         #
# #########################################################################################
class BrainFuckToken(object):
    """
    The BrainFuck Token class.
    """
    def __init__(self, ttype, tvalue):
        self.type = ttype
        self.value = tvalue

    def __str__(self):
        return "BrainFuckToken({0}, \"{1}\")".format(self.type, self.value)
    
    def __repr__(self):
        return self.__str__()


class Lexer(object):
    def __init__(self, input_text=None):
        """
        Initializer.
        """
        self.curr_pos = 0
        self.curr_char = None
        self.input_text = input_text if input_text and isinstance(input_text, str) else ""
    
    def scan(self, input_text):
        """
        Initilizes the lexer with an input text for scanning. 
        """
        self.curr_pos = 0
        self.curr_char = None
        self.input_text = input_text

        if not input_text or not isinstance(input_text, str):
            raise ValueError("Input text must be a non-empty string!")
        else:
            self.curr_char = input_text[0]

        return BrainFuckToken(EOF, None)

    def advance(self):
        """
        Advances the lexer pointer ahead and assigns the next character to self.curr_char.
        """
        if not self.input_text or not isinstance(self.input_text, str):
            raise("Please initialize the input text with the scan method first!")

        self.curr_pos += 1

        if self.curr_pos >= len(self.input_text):
            self.curr_char = None
        else:
            self.curr_char = self.input_text[self.curr_pos]
        
    def next_token(self):
        """
        Returns a Token representation of the next valid token in the tape or EOF.
        """
        if not self.input_text or not isinstance(self.input_text, str):
            raise("Please initialize the input text with the scan method first!")

        while self.curr_char is not None:
            value = self.curr_char
            if value in sym2tok:
                self.advance()
                return BrainFuckToken(ttype=sym2tok[value], tvalue=value)
            else:
                self.advance()
        # EOF
        return BrainFuckToken(EOF, None)



# #########################################################################################
#                                                                                         #
#  PARSER                                                                                 #
#                                                                                         #
# #########################################################################################
class Parser(object):
    """
    BrainFuck Grammar Parser.
    """
    def __init__(self, lexer=None):
        """
        Initializer.
        """
        if lexer is None:
            self._lexer = Lexer()
        elif isinstance(lexer, Lexer):
            self._lexer = lexer
        else:
            raise("Lexer should be of type `Lexer` or None. If it's None, it will be created automatically.")
        self.curr_token = None
    
    def set_lexer(self, new_lexer):
        """
        Sets internal lexer to a new instance.
        """
        if not isinstance(new_lexer, Lexer):
            raise("Lexer can only be of type `Lexer`.")
        self._lexer = new_lexer
    
    def parse(self, input_text):
        """
        Parser entry point. Parses the complete BrainFuck Language.

        program             : while_loop (program)*
                            | terminal_command (program)*
        
        while_loop          : BEGIN_LOOP_CMD program END_LOOP_CMD

        terminal_command    : READ_CMD
                            | PRINT_CMD
                            | INC_CELL_CMD
                            | DEC_CELL_CMD
                            | MOV_PTR_NEXT_CMD
                            | MOV_PTR_PREV_CMD
        """
        # Scan the input text
        self._lexer.scan(input_text)
        self.curr_token = self._lexer.next_token()

        # Begin parsing
        cmd_node = self._program()
        if self.curr_token.type != EOF:
            self.error(EOF)
        
        # Return AST
        return cmd_node

    def error(self, expected_token_type=None):
        """
        Error reporting.
        """
        if expected_token_type:
            msg = 'Invalid syntax at {!r}. Expected: {!r}'.format(
                self.curr_token, expected_token_type)
        else:
            msg = 'Invalid syntax at {!r}.'.format(self.curr_token)
        
        raise Exception(msg)

    def consume(self, token_type):
        """
        Checked tokens consumption.
        """
        if self.curr_token is None:
            raise ValueError("Please run parse method with an input program text first!")

        if self.curr_token.type == token_type:
            self.curr_token = self._lexer.next_token()
        else:
            self.error(token_type)

    # ############################## PRIVATE ###########################
    
    def _program(self):
        """
        program             : while_loop (program)*
                            | terminal_command (program)*
        """
        if self.curr_token is None:
            raise ValueError("Please run parse method with an input program text first!")
        
        ast = []
        token_types = tok2sym.keys()

        while self.curr_token.type in token_types:
            token = self.curr_token
            
            # Parse while_loop
            if token.type == BEGIN_LOOP_CMD:
                cmd_node = self._while_loop()
            elif token.type == END_LOOP_CMD:
                return ast
            # Parse terminal_command
            else:
                cmd_node = self._terminal_command()
            
            # Append to AST (list of commands)
            ast.append(cmd_node)

        return ast
    
    def _while_loop(self):
        """
        while_loop          : BEGIN_LOOP_CMD program END_LOOP_CMD
        """
        if self.curr_token is None:
            raise ValueError("Please run parse method with an input program text first!")

        self.consume(BEGIN_LOOP_CMD)
        cmd_node = (LOOP_CMD, self._program())
        self.consume(END_LOOP_CMD)
        return cmd_node
    
    def _terminal_command(self):
        """
        terminal_command    : READ_CMD
                            | PRINT_CMD
                            | INC_CELL_CMD
                            | DEC_CELL_CMD
                            | MOV_PTR_NEXT_CMD
                            | MOV_PTR_PREV_CMD
        """
        if self.curr_token is None:
            raise ValueError("Please run parse method with an input program text first!")

        token = self.curr_token
        cmd_node = (token.type,)
        self.consume(token.type)
        return cmd_node



# #########################################################################################
#                                                                                         #
#  INTERPRETER and TAPE                                                                   #
#                                                                                         #
# #########################################################################################
class BrainFuckTape(object):
    """
    The BrainFuck Tape Data Structure.
    Implemented internally as a dictionary for on-demand allocation of values 
      at specific indices.
    """
    @property
    def MAX_INDEX(self):
        return 30000
    
    def __init__(self):
        self.map = {}
    
    def __getitem__(self, index):
        # return 0 if index was not in self.map
        return self.map.get(index, 0)
    
    def __setitem__(self, index, value):
        self.map[index] = value


###
# GLOBAL TAPE STATES
global_tape_index = 0
global_tape = BrainFuckTape()


def bf_eval(commands_ast):
    """
    BrainFuck AST Nodes Evaluator.

    Evaluates every node in a given AST representation of a BrainFuck program.
    The AST is expected to be a linear collection of tuples. Every tuple's first
      item is the type of the command. Only Loop Commands contain two items, where
      the second item is another linear collection of commands.  
    """
    global global_tape
    global global_tape_index
    
    for cmd_node in commands_ast:
        if not cmd_node:
            return None
        # Move pointer to previous cell
        elif cmd_node[0] == MOV_PTR_PREV_CMD:
            if global_tape_index > 0:
                global_tape_index -= 1
            else:
                raise Exception("Error: index out of bound! Min allowed index value is 0.")
        # Move pointer to next cell
        elif cmd_node[0] == MOV_PTR_NEXT_CMD:
            if global_tape_index < global_tape.MAX_INDEX:
                global_tape_index += 1
            else:
                raise Exception("Error: index out of bound! Max allowed index value is {}".format(
                    global_tape.MAX_INDEX))
        # Print the current cell's contents
        elif cmd_node[0] == PRINT_CMD:
            cell_val = global_tape[global_tape_index]
            print(chr(cell_val), sep='', end='')
        # Read input to current cell
        elif cmd_node[0] == READ_CMD:
            str_input = str(input())
            if len(str_input) > 0:
                global_tape[global_tape_index] = ord(str_input[0])
        # Increment the contents of current cell
        elif cmd_node[0] == INC_CELL_CMD:
            global_tape[global_tape_index] += 1
        # Decrement the contents of current cell
        elif cmd_node[0] == DEC_CELL_CMD:
            global_tape[global_tape_index] -= 1
        # Loop until the current cells contents are equal to 0
        elif cmd_node[0] == LOOP_CMD:
            while global_tape[global_tape_index] != 0:
                bf_eval(cmd_node[1])


def bf_interpret(program_text):
    """
    BrainFuck Program Text Interpreter.
    """
    try:
        brainfuck_parser = Parser()
        bfcode = bfcode = brainfuck_parser.parse(program_text)
        bf_eval(bfcode)
    except (KeyboardInterrupt, SystemExit):
        print("\r\nReceived KeyboardInterrupt, stopping...")
        print("Goodbye!")
        sys.exit(0)
    except Exception as err:
        raise


def bf_repl():
    """
    BrainFuck REPL.
    """
    print("BrainFuck on Python!")
    print("Type :clear to clear the screen. Type :quit to exit REPL.")

    # Initialize the parser
    brainfuck_parser = Parser()

    while True:
        try:
            program = input('BrainFuck> ')
        except (EOFError, KeyboardInterrupt, SystemExit):
            break

        if not program:
            continue
        
        if program == ":clear":
            subprocess.call("clear")
        elif program == ":quit":
            break
        else:
            try:
                bf_code = brainfuck_parser.parse(program_text)
                bf_eval(bf_code)
                print()
            except (KeyboardInterrupt, SystemExit):
                print("\r\nReceived KeyboardInterrupt, stopping...")
                break
            except Exception as err:
                print("An error has occurred: {!r}".format(err))
                
    print("\r\nGoodbye!")



# #########################################################################################
#                                                                                         #
#  MAIN PROGRAM ENTRY                                                                     #
#                                                                                         #
# #########################################################################################
def create_argparser():
    arg_parser = argparse.ArgumentParser(prog="brainfuck")

    # --repl argument
    arg_parser.add_argument(
        "--repl",
        action="store_true", default=False,
        help="BrainFuck REPL: interactive mode.")
    
    # --program argument
    arg_parser.add_argument(
        "--program",
        type=str, nargs=1, default=None,
        help="Path to a BrainFuck program file, for example: ./brainfuck --program ~/hello_world.bf")

    # --code argument
    arg_parser.add_argument(
        "--code", 
        type=str, nargs=1, default=None,
        help="A double-quoted BrainFuck source code program, for example: ./brainfuck --code \"++++ ++++\"")
    
    return arg_parser


def main():
    arg_parser = create_argparser()
    args = arg_parser.parse_args()
    
    # First, --repl has the highest priority
    if args.repl is True:
        bf_repl()
    # Second, --program has the second highest priority.
    elif args.program is not None and len(args.program) > 0:
        program_code = ""
        try:
            with open(args.program[0], mode="r", encoding="utf-8") as file:
                program_code = file.read()
        except (IOError, FileNotFoundError):
            print("Error: file was not found!")
            sys.exit(1)
        bf_interpret(program_code)
        print("")
    # Third, --code has the lowest priority.
    elif args.code is not None and len(args.code) > 0:
        bf_interpret(args.code[0])
        print("")
    # Nothing was specified, print help.
    else:
        arg_parser.print_help()


if __name__ == "__main__":
    main()

Hey! Cool example. It was a while back, but if you'd like to see another implementation of Brainfuck in Python 3, you can see mine and compare 😄

Hey @zombiehippie, thanks for passing by, I will give your BF implementation a go and will definitely compare it up!

Hi @zombiehippie, I like your implementation, it is small and concise. I'd still favor a dictionary over a list for the Tape implementation as I'd consider any non-existent key (Tape cell that is not set) as None and then interpret it as a Zero value.

Cheers!