#!/usr/bin/env python3

from string import whitespace

class TuringParser:

  def parse_string(self, string):
    return build_transition_table(string.split('\n'))

  def parse_file(self, path):
    with open(path) as insts:
      return self.build_transition_table(insts)

  def build_transition_table(self, insts):
    transitions = {}
    for inst in insts:
      condition, result = self.parse_inst(inst)
      transitions[condition] = result
    return transitions

  def parse_inst(self, inst):
    direcs = { 'l' : -1, 'r' : 1, 's' : 0 }
    inst = inst.strip(whitespace)
    
    from_state, tape_character, to_state, new_char, direc = \
      inst.split(',')
      
    if not direc in direcs.keys():
      raise Exception('Expected: l, r, or s instead of {0}'.format(direc))
    return (from_state, tape_character), (to_state, new_char, direcs[direc])

class TuringMachine:

  def __init__(self, string, transitions, state='1'):
    self.initial = state
    self.string = string
    self.transitions = transitions
    
    self.reset()
  
  def reset(self):
    self.tape = list(self.string)
    self.state = self.initial
    self.pointer = 0
  
  def step(self, steps=1):
    for i in range(steps):
      next(self)
  
  def __iter__(self):
    return self
  
  def __next__(self):
    if self.pointer >= len(self.tape):
      self.tape.append('_')
    
    try:
      to_state, new_char, direc = \
        self.transitions[(self.state, self.tape[self.pointer])]
    except KeyError:
      raise StopIteration()
    
    if to_state == 'H':
      raise StopIteration()
    
    self.tape[self.pointer] = new_char
    self.state = to_state
    self.pointer += direc
    
    return (self.pointer, self.state, ''.join(self.tape))