[Python-Dev] switch-based programming in Python (original) (raw)

Greg Ewing greg@cosc.canterbury.ac.nz
Fri, 09 Nov 2001 15:50:32 +1300 (NZDT)

As an example of a real-life application which might benefit from a fast switch-statement in Python, I'd like to offer the following excerpt from Plex, which runs as part of a tight loop once per input character.

    if input_state == 1:
      cur_pos = next_pos
      # Begin inlined: c = self.read_char()
      buf_index = next_pos - buf_start_pos
      if buf_index < buf_len:
        c = buffer[buf_index]
        next_pos = next_pos + 1
      else:
        discard = self.start_pos - buf_start_pos
        data = self.stream.read(0x1000)
        buffer = self.buffer[discard:] + data
        self.buffer = buffer
        buf_start_pos = buf_start_pos + discard
        self.buf_start_pos = buf_start_pos
        buf_len = len(buffer)
        buf_index = buf_index - discard
        if data:
          c = buffer[buf_index]
          next_pos = next_pos + 1
        else:
          c = ''
      # End inlined: c = self.read_char()
      if c == '\n':
        cur_char = EOL
        input_state = 2
      elif not c:
        cur_char = EOL
        input_state = 4
      else:
        cur_char = c
    elif input_state == 2:
      cur_char = '\n'
      input_state = 3
    elif input_state == 3:
      cur_line = cur_line + 1
      cur_line_start = cur_pos = next_pos
      cur_char = BOL
      input_state = 1
    elif input_state == 4:
      cur_char = EOF
      input_state = 5
    else: # input_state = 5
      cur_char = ''
    # End inlined self.next_char()
  else: # not new_state
    if trace: #TRACE#
      print "blocked"  #TRACE#
    # Begin inlined: action = self.back_up()
    if backup_state:
      (action, cur_pos, cur_line, cur_line_start, 
        cur_char, input_state, next_pos) = backup_state
    else:
      action = None
    break # while 1
    # End inlined: action = self.back_up()

Greg Ewing, Computer Science Dept, +--------------------------------------+ University of Canterbury, | A citizen of NewZealandCorp, a | Christchurch, New Zealand | wholly-owned subsidiary of USA Inc. | greg@cosc.canterbury.ac.nz +--------------------------------------+