===========================================================================
 Modpack installer tool
===========================================================================

Installing modpacks
-------------------

The modpack installer is a simple tool to download custom Freeciv
content called "modpacks" from the Internet, and automatically install
them into the correct location to be used by the Freeciv client and
server.

A "modpack" can consist of one or more of:
 - 'rulesets' for game rules
 - 'scenarios' for game maps (with or without players/cities/etc)
 - 'tilesets' for game graphics
 - 'soundsets' or 'musicsets' for sound effects and in-game music

Depending which client you installed, you will probably have one modpack
installer tool called something like 'freeciv-mp-gtk3' or
'freeciv-mp-qt' -- these different versions have the same features.
There may also be a command-line-only tool called 'freeciv-mp-cli',
which is mainly useful for headless game servers.

In standard Freeciv builds, when you start the graphical modpack
installer, it will show a list of modpacks from modpack.freeciv.org,
curated by the Freeciv developers. You can select one and press
"Install modpack"; the tool will download the files for the selected
modpack, and any other modpacks it depends on, and install them ready
for the main Freeciv programs to use.

You can also point the installer at modpacks or lists on other servers:

 * If someone has given you an individual modpack URL (ending in
   '.mpdl'), you can paste it into the "Modpack URL" box, and when
   you press "Install modpack", the modpack will immediately be
   downloaded and installed.
   (If you have been given a URL ending up '.modpack', that is
   probably for an older version of Freeciv.)

 * If someone has given you a URL for a list of modpacks (probably
   ending in '.list') and you want to browse them with the standard
   Freeciv modpack installer build, you need to start the tool with
   that URL on its command line, for instance:

     freeciv-mp-gtk3.exe --List http://example.com/3.1/my-modpacks.list

   (note the capital letter in '--List')

(The tool has some other command-line options, but most users will not
need to use them. Use --help for a list of them.)

Once you have installed a modpack, how you use it depends on the modpack
type:

 - Scenarios (maps) should be listed under 'Start scenario game' from
   the client start page, or from the game server prompt via
   '/list scenarios'.
   (For network play, scenarios need only be installed on the game
   server.)

 - Rulesets should appear on the 'Ruleset' drop-down from the client's
   'Start new game' page. On the game server, you can load a ruleset
   with '/read <name>' or failing that perhaps '/set rulesetdir <name>'.
   (For network play, rulesets need only be installed on the game
   server.)

 - Tilesets should appear for selection in the local client options, in
   the appropriate topology-specific 'Tileset' drop-down under 'Graphics'.
   (Tilesets should be installed on the game client machine.)

 - Soundsets and musicsets should appear in the dropdowns on the 'Sound'
   page of the local client options.
   (These should be installed on the game client machine.)

With standard Freeciv builds, modpacks get installed into a per-user
area, not into the main Freeciv installation. So you shouldn't need any
special permissions to download them, and if you uninstall the Freeciv
game, any modpacks you downloaded are likely to remain on your system.
Conversely, if you delete downloaded modpacks by hand, the standard
rulesets, tilesets etc supplied with Freeciv won't be touched.

The precise location where files are downloaded to depends on your build
and platform. For Unix systems it is likely to be under the hidden
'.freeciv' directory in your home directory. It is likely to be near
where the Freeciv client stores its saved games.

Most modpacks are specific to a particular major version of Freeciv; for
instance, while a 3.0 ruleset or tileset can be used with all Freeciv
3.0.x releases, it cannot be used as-is with any 3.1.x release. So, most
modpacks are installed in a specific directory for the major version,
such as ~/.freeciv/3.1/ on Unix. (The modpack installer displays which
version it will install for at the top of its window.)

An exception to this is scenario maps; scenarios created for one version
of Freeciv can usually be loaded in later versions, so they are
installed in a version-independent location (typically
~/.freeciv/scenarios/ on Unix).

Once a modpack is installed, there is no uninstall action, and if you
remove the files by hand, the installer will still consider the modpack
to be installed; the installer maintains its own database
(.control/modpacks.db) listing which modpack versions are installed, but
does not keep track of which files were installed by which modpack. If
the database gets of of sync with reality (or is deleted), it's
harmless for already installed modpacks and the main Freeciv programs
(which do not consult the database), but can confuse the modpack
installer's dependency tracking later.

Modpacks consist mostly of data files read by the Freeciv engine; they
do not contain compiled binary code (and are thus platform-independent).
Rulesets can contain code in the form of Lua scripts, but this is
executed in a sandbox to prevent obvious security exploits. Modpacks are
installed to a specific area and cannot overwrite arbitrary files on
your system. Nevertheless, you should only install modpacks from sources
you trust.


Serving modpacks for the installer
----------------------------------

The rest of this document discusses how to set up a web server so that
users can download modpacks you publish. (If you just want to install
modpacks, you need not read on.)

To host modpacks, you just need a web server that can host plain static
files; you do not need to run any custom code or frameworks on that web
server, just to publish files with a specific layout, detailed below.
(You can browse https://modpack.freeciv.org/ for some working examples of
modpack trees.)

Unlike some previous versions, all the standard Windows Freeciv 3.1.x
builds should be able to access modpacks hosted at https (secure)
URLs, as well as plain http.

On the modpack server, there are up to three layers of files required:
  1. modpack.list: A list of available modpacks (optional, advanced).
  2. *.mpdl: A control file for an individual modpack.
  3. The individual files comprising the modpack.

Each of these layers is described in detail below.

Each layer refers to files in the next layer down. References can be
with relative URLs (so that a modpack or set of modpacks can be moved
without changing any file contents), or with absolute URLs (so that the
different layers can be hosted on different web servers).

Almost all of these file formats are specific to one major version of
Freeciv; this document only describes the formats for the major version
of Freeciv it is shipped with. If you wanted to publish modpacks for
both 3.0.x and 3.1.x, you would publish two sets of modpack control
files, and perhaps two different modpack.list files.


1. modpack.list, a list of modpacks

(Advanced usage only -- most publishers can skip to the next section)

This is only needed if you want to let users browse a list of available
modpacks before choosing one to install. To look at your modpack.list
instead of the standard one, users will usually have to start the
modpack installer with non-standard arguments (see above).

If you build and distribute your own Freeciv binaries, you can customise
the default modpack list URL with MODPACK_LIST_URL in the project
definition file; see doc/README.packaging for details about that file.

(You could even build and distribute just the modpack installer -- once
installed, in principle there's nothing stopping the standard Freeciv
client/server using modpacks installed by it -- but you would need to
take care that the data paths used by your modpack installer build were
the same as those used by the standard builds, since getting the paths
right is most of the point of the modpack installer.)

modpack.list is a file in Freeciv's "spec-file" format, the same as is
used for rulespecs, tilespecs, etc; see those for examples of the
general syntax.

Here's an example:

[info]
options = "+Freeciv-3.1-modlist"
message = "2019-12-31: updated Mappamundi tileset. Happy new year!"

[modpacks]
list =
{ "name", "version", "type", "subtype", "license", "URL", "notes"
  "Ancients", "3.1-0", "Modpack", "-", "GPLv2+", "./ancients.mpdl"
  "Survivors", "3.1r6", "Scenario", "iso", "GPLv2+", "./survivors-3.1r6.mpdl", "Installs custom ruleset and two world maps"
  "Mappamundi", "20191231", "Tileset", "iso", "MIT", "http://otherserver.example.com/mappamundi-20191231.mpdl"
}

Here's what's in the [info] section:
 - "options" should be exactly as shown here.
 - "message" is displayed on the status line when the modpack installer
   starts up. Should be kept to one line.

The [modpacks] section is a header line, followed by a list of modpack
lines.
The header line ("name, "version", etc) should usually be left as shown
here, and followed by one line for each modpack:
 - "name", "version", "type":
     These three fields should match those in the .mpdl file which
     "URL" links to.
 - "subtype":
     Optional free text. For tilesets or scenarios, conventionally
     indicates the map topology with one of "overhead", "iso", "hex",
     or "hex & iso" (and these will be localised). Otherwise "-".
 - "license":
     Free text summarising the distribution terms for the modpack
     content (by naming a well-known license, not quoting the full
     license text!)
 - "URL":
     The URL to a .mpdl file for the individual modpack.
     Either a relative URL starting "./" (in which case it's relative to
     the URL of modpack.list) or an absolute URL (which can be on some
     other web server).
 - "notes":
     Optional free text; usually shown as a tooltip.


2. *.mpdl control file, defining an individual modpack

This is the core control file for a modpack, specifying what files it
contains, where to download them from, and where they are installed.

Most modpack authors will just publish the URL of an .mpdl file
directly, for users to give to the modpack installer tool. (There
doesn't have to be a modpack.list file anywhere that refers to the
.mpdl file.)

Again, this is a file in Freeciv's "spec-file" format, the same as is
used for modpacks like rulespecs, tilespecs, etc; see those for examples
of the general syntax. Its filename must end in ".mpdl".

Here is an example of a .mpdl file:

[info]
options = "+Freeciv-3.1-mpdl"
baseURL = "./ancients-6f88ee23de"
name    = "Ancients"
type    = "Modpack"
version = "3.1-0"

[dependencies]
list =
{ "modpack", "URL", "type", "version"
  "Ancients-tiles", "./ancients-tiles.mpdl", "Tileset", "3.1-0"
}

[files]
list =
{ "src", "dest"
  "ancients/ancients.serv", "ancients.serv"
  "ancients/game.ruleset"
  ; ...
}

The [info] section has overall control information:
 - "options": must be exactly as shown in the example.
 - "name":
     A short name for the modpack. This is used for version and
     dependency tracking, so should not contain minor version
     information, and should not change once a modpack has been
     released for a given major version of Freeciv. Case-insensitive.
 - "version":
     Textual version information. If another modpack uses this one as a
     dependency, this string is subject to version number comparison
     (using the rules of Freeciv's 'cvercmp' library, which should give
     sensible results for most version numbering schemes).
 - "type":
     This must be one of the following:
      - "Ruleset" (foo.serv, foo/*.ruleset, foo/*.lua, etc)
      - "Tileset" (foo.tilespec, foo/*.png, etc)
      - "Soundset" (foo.soundspec, foo/*.ogg, etc)
      - "Musicset" (foo.musicspec, foo/*.ogg, etc)
      - "Scenario" (foo.sav; installed to a version-independent location)
      - "Modpack": conventionally used for modpacks that contain more
        than one of the above kinds of material
      - "Group": contains no files but only depends on other modpacks
     At the moment, only "Scenario" causes special behavior.
 - "baseURL":
     URL to prepend to the "src" filenames in the [files] section.
     May be relative to the .mpdl file (starting with "./"), or
     absolute (in which case the files can be on some web server
     different to where the .mpdl file lives).

The [files] section lists the individual files comprising your modpack.
(It must list every file individually; any files in the same directory
on the webserver that are not listed will not be downloaded.)
After the header, each line can contain two strings:
 - "src"
     Mandatory. Path of file on webserver, relative to "baseURL",
     and also by default installation path.
 - "dest"
     Optional. Normally, "src" is also used as the path (relative to the
     installation root) for the file to be installed on the user's
     system. If for some reason you need the installation path for some
     file to be different, specify it here.
     Usually, the path under "src" will be fine for this, in which case
     there is no need to have a "dest" string on the line.
     Path components should be separated with forward slashes ('/').

In the example, the first file would be downloaded from
  ./ancients-6f88ee23de/ancients/ancients.serv
(relative to the URL of the .mpdl file), and installed in
  ancients.serv
in the relevant path of the user's system.

Some advice on the structure of files in modpacks:

 * You should generally install files in a directory named after the
   modpack, with a few exceptions (.serv, .tilespec, .soundspec, and
   .musicspec files must be installed to the top level, and should
   reference files in your subdirectory).

   Individual files' and directories' install names should usually not
   embed version numbers, dates, etc, so that when a new version of
   modpack X is installed, it cleanly overwrites the old version, rather
   than leaving both cluttering up the user's installation.
 
 * The modpack installer does not stop different modpacks overwriting
   each other's files, so published modpacks should be disciplined about
   namespace usage. If you've derived from someone else's modpack, you
   should probably give your derivative new filenames, so that both can
   be installed simultaneously.

 * There is no 'white-out' facility to delete files from a user's
   installation -- if a newer version of a modpack has fewer files than
   an old one, the old file will persist in some users' installations,
   so your modpacks should be designed to be tolerant of that.

 * At the moment, there is no restriction on what kind of files a given
   "type" can install, but modpacks should stick to installing the
   advertised kinds of content. It's OK to install extra files such as
   documentation in any case (LICENSE/COPYING, README.txt, etc).

 * If your modpack contains a ruleset, you should usually install a
   .serv file at the top level (which can be a one-line file consisting
   of "rulesetdir <name>", as this is needed for the server to enumerate
   the available rulesets.

The [dependencies] section is optional, and not normally needed
(although "Group" type modpacks should have a [dependencies] section
and no [files] section). If your modpack requires another one that can
also be installed separately (for instance, a scenario that needs a
specific ruleset), you can specify it here. After the header, each
line consists of:
 - "modpack":
     What the dependency modpack calls itself when installed (that is,
     "name" from its .mpdl file).
 - "URL":
     URL to download modpack if needed. Can be relative or absolute.
 - "type":
     Must match "type" from dependency's .mpdl file.
 - "version":
     Minimum version of dependency (as declared in its .mpdl file).
     Subject to version number comparison algorithm.
If the modpack installer thinks the required version, or a newer
version, of the dependency is already installed, it will do nothing,
otherwise it will download the dependency modpack (and any of its own
dependencies, recursively).


3. Individual modpack files

These are the files comprising the modpack (*.ruleset, *.png, etc), that
will be copied verbatim to the user's Freeciv data directory and read by
the Freeciv client and server. The modpack installer does not modify the
files in any way.

The files must be hosted individually on the web server; the modpack
installer tool cannot unpack any archives such as .zip files.
(Individual scenarios can be compressed, e.g. .sav.gz, as the Freeciv
engine can uncompress these files.)

Because the *.mpdl file can change the file paths / names on download,
the layout on the modpack server doesn't have to correspond with the
installed layout. An individual file can be shared between multiple
modpacks, if you want.

How to write modpacks is beyond the scope of this document -- see
README.rulesets, README.graphics, etc for some clues.
