[Python-Dev] switch-based programming in Python

[Greg Ewing]

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	   +--------------------------------------+