# NetHack 3.6  code_features.txt $NHDT-Date: 1524689669 2018/04/25 20:54:29 $  $NHDT-Branch: NetHack-3.6.0 $:$NHDT-Revision: 1.3 $
# Copyright (c) 2015 by Michael Allison
# NetHack may be freely redistributed.  See license for details.

Developer-useful info about code features, assumptions, purpose,
rationale, etc.

==============================================
FEATURE_NOTICE Alerts for a Release

There is a code mechanism for alerting players to a change in behavior
over prior versions of the game.

Here's how to do it:
   o Where the change in behavior needs to alert the player,
     - Add an 'if statement' to invoke the alert behavior
       if the condition is met, for example
          if (flags.suppress_alert < FEATURE_NOTICE_VER(3.6.0))
             pline("Note: and explain the change here.");
     - The example above will alert the users for a new feature
       added in 3.6.0 via a one-liner via pline(), but you
       could get more elaborate (just make sure it is all done
       in the 'if' code block..

Once the user finds the alert no longer useful, or becoming 
annoying, they can set the "suppress_alert" option.
      - The user can only set the suppress_alert to the current 
        version, not future versions. That restriction is done
        so that the feature can be used for new things in new
        releases.
      - The suppression can be done interactively mid game with
        the 'O' command, or via 
           OPTIONS=suppress_alert:3.6.0
        in the user's config file.

==============================================
PREFIXES_IN_USE and NOCWD_ASSUMPTIONS

Those provide a storage mechanism for holding the paths to various different
types of files. Those paths are stored in the fqn_prefix[] array. They are a
mechanism for enabling separation of the different files that NetHack needs. 

The prefixes are added to the beginning of file names  by various routines in
files.c immediately prior to opening one of the types of files that the game
uses.

They aren't about config file options (although config file options would be
one way to set non-default values for some of the paths in the fqn_prefix[]
array). Obviously the very first path needed (now sysconfdir, previously
configdir) isn't viable for setting via config file options, but the game
still needs to hunt it down "someplace."  When the "someplace" is figured
out, that place (path) would be stored in fqn_prefix[SYSCONPREFIX].  How it
gets stored in fqn_prefix[SYSCONPREFIX] is up to us as developers.

Any of the fqn_prefix[] entries can be set somehow. It could be done in port
startup code; in options processing; in config file processing; by 
translating a system environment variable such as USERPROFILE; whatever 
you/we want.  The point is that NOCWD_ASSUMPTIONS and PREFIXES_IN_USE are
there to ensure that there is a place to store that path information.  The
code to *utilize* the information is already in files.c (see fqname()).

There is a fqn_prefix[] entry for holding the path to each of the following:
    PREFIX        NAME
0 HACKPREFIX     hackdir
1 LEVELPREFIX    leveldir    location to create level files
2 SAVEPREFIX     savedir location to create/read saved games
3 BONESPREFIX    bonesir location to create/read bones
4 DATAPREFIX     datadir location to read data.base etc.
5 SCOREPREFIX    scoredir    location to read/write scorefile
6 LOCKPREFIX     lockdir     location to create/read lock files
7 SYSCONFPREFIX  sysconfdir  location to read SYSCF_FILE
8 CONFIGPREFIX   configdir   location to read user configuration file
9 TROUBLEPREFIX  troubledir  location to place panic files etc.

To recap, they are about enabling "different paths for different things", and
separation of:
- read-only stuff from read-write stuff.
- sysadmin stuff from user-writeable stuff.
etc.

==============================================
REGULAR EXPRESSIONS

There are multiple regular expression libraries supported. Currently, one (and
only one) of the following files should be linked into a built:
  sys/share/cppregex.cpp
  sys/share/posixregex.c
  sys/share/pmatchregex.c

This provides a way to access different system regular expression libraries,
or fall back onto pmatch() if none is available. cppregex.cpp uses the regular
expression library in the C++11 standard, and is the default on Windows.
posixregex.c uses the POSIX regular expression library, and is the default on
POSIX. pmatchregex.c is the fallback.

Configuration files written using either of the two true regex engines are
compatible with one another, as the regular expressions are both POSIX
extended regular expressions. Configuration files written using the fallback
engine are incompatible.

Additional regular expression implementations can be written. The full
interface documentation is in sys/share/posixregex.c

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