The CONTEXT mechanism

The way A-code is currently implemented, has one substantial disadvantage. The two input directives (INPUT and QUERY) expect execution to proceed from the the point where they have been called, which is fine if the game executable takes care of its own main loop. However, in the single turn (CGI) and library modes, this is not the case. Input is obtained by other controlling software (e.g. by a PHP script, in the single turn mode) and there is no mechanism to return the game to the point at which an input directive is used. The problem is aggravated by the fact that it may make perfect sense to have multiple calls for input in a single pass through the main loop (e.g. get general command, if this command is SAVE without a name to save the game under, get the name; if there is already such a saved game, query whether it should be overwritten).

In Platt's original A-code implementation as a virtual machine this would not have been a problem. One simply saved the state of the VM before returning control to the external software. Once the game code is re-invoked and supplied with the next command, the VM state is restored and execution proceeds as normal. Things are nowhere near as simple when a game is simply an executable (or a library) built from C sources.

There are possible solutions, of course. The game could be split into two processes: one in charge of player communication and the other running the game's world. Indeed, that's how the browser mode build works, but this arrangement is not suitable for running in a third-party cloud.

The CONTEXT mechanism was my solution to this problem when implementing the original CGI-based implementation of Adv770 to be used by game's beta-testers. The idea is a simple one. The game can acquire query responses as a part of the main loop – it just needs to know that a question has been asked and specifically what question has been asked. Then having acquired player's input it can deal with it appropriately.

At first glance, no kernel involvement is required for this to work, but there are reasons why it does have to be aware of this mechanism. Otherwise anomalies would arise in e.g. orphan command word handling and the game persistence mechanism. Thus variable named CONTEXT is deemed to be special by the kernel. If it is set to zero, the next expected input is just a general game command. If set to non-zero, however, an answer to a particular query is expected by the game.

style 12
verb quit
init
repeat
# (Some house-keeping code)
repeat
  input               # No initialisation
  query "Really?_"    # The underscore is a forced space
    ifkey quit
      say "Bye..."
      stop
    else
      say "OK."
    fin
  else
    say "If not, not."
  fin
  say "_"             # Add a blank line

And here is one possible equivalent using the CONTEXT variable instead of the QUERY directive:

style 12
var context
verb quit
verb yes              # Any other query answer is deemed to be no
init                  # CONTEXT automatically initialised to zero
repeat
  ifturn
    and
  ifflag context, prompted
    proceed
  fin
# (Some house-keeping code here)
repeat
  ifeq context, 0     # A general command is required
    proceed
  fin
  save command        # Preserve player's original command
  input               # Get query response
  ifkey yes
    restore command   # Restore the original command
    ifkey quit        # And act on it if appropriate
      say "Bye..."
      stop
    fin
    say "OK."
  else
    say "If not, not."
  fin
  set context, 0      # Ensure next loop is just a general command one
  quip "_"            # Add a blank line
repeat
  input
  set context, 1      # Signal query processing is needed
  quip "f:Really?_"

Note the addition to the first, house-keeping REPEAT section. In single turn builds it ensures that the housekeeping code is not executed twice, whenever the player is prompted for input.

The CONTEXT mechanismor how to avoid use of QUERY

The CONTEXT mechanism
or how to avoid use of QUERY