HOWTO-Engine Inclusion

Introduction about the engine merge procedure

This page describes the procedure for merging a new engine into the main ScummVM development line. It intends to provide guidance for both the engine developers and the ScummVM team on the procedure for merging new engines into ScummVM.

Preparing your engine

What needs to be done before you can ask for your engine to be merged into ScummVM?

ScummVM is a project which focuses on reimplementation of adventure game engines and RPGs. We support a wide range of platforms. Due to these facts, there are a few requirements your engine has to fulfill before it has a chance of being included in the ScummVM mainline.

We cover the really basic requirements here first:

• Your engine must target an adventure game or an RPG
• All your code must follow our Code Formatting Conventions. It does not have to be perfect, but it needs to follow the overall style. We will happily point out issues and help you with the remaining issues.
• Your code should reflect the fact that you know our Coding Conventions and have tried to implement them in your code. If you have questions about specifics, you are free to ask us.
• Your engine needs to be on github.
• The git history of your engine is linear and based on tip of the ScummVM master.
• The engine development history should not change/add common code and/or modify backend related code, without discussing it with us first.
• Your code compiles with g++, C++11.

Furthermore, we require a bit more from you, which isn't basic anymore:

• You will be becoming the maintainer for the engine. That gives you the possibility to work on your engine in our master development branch. But it also comes with responsibilities. You should be able to spend some time handling incoming bugs for your engine, especially in release phases. Furthermore, we need you to be contactable, so we can discuss issues in your engine or backend/middleware changes with you, so either we or you can adapt your engine to these.
• Your code needs to be in a maintainable state. It should be written in such a way that we (as C++ coders) can maintain/port/fix the input/graphics/sound code and that we can at least understand the game logic (i.e. not too many unknown fields, comments where necessary, not depending on asm files or an IDB or etc to be readable)
• The game supported by your engine needs to be completable or with a clear path to completion on at least x86/amd64. You need to have time to adapt your code so it will work on as many platforms as we support (and is feasible) OR help our porters or other developers with it.
• Your engine needs to be in a state that we can easily include in the next release - or, at the latest, in the one afterwards.

How you ask for inclusion in ScummVM

It is always a good idea to first come to our Discord server and point people to your engine. Even when you do not want it to be merged yet! That will help you to get feedback on what you need to take care of when you eventually want your engine to be merged. There is always a developer - or even a few - who will happily help you with your engine and answer general questions.

Please, also look at our Wiki or ask around on Discord, perhaps we already have someone working on this engine, so you could rather join your efforts.

Processing out-of-tree engines

The modern way of engine development is keeping it in a separate repo with contents only of the `engines/<engine>` directory. This, however, leads to history starting from the root commit and in wrong directory. To fix the history, use the following commands:

 git filter-branch --index-filter \
       'git ls-files -s | sed "s-\t\"*-&engines/bagel/-" |
               GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
                       git update-index --index-info &&
        mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"' HEAD

Replace `bagel` with name of your engine. This will rewrite history and make it look like it was always developed in `engines/bagel` directory.

Then do:

  git format-patch -o a --root

Then, in your scummvm fork:

  git checkout -b bagel
  git am <path-to-directory-where-you-have-the-repo/a/*>

This will create a set of commits of the engine as it was developed right in-tree. Then push and create a PR.

Starting the pull request

This is really easy, just open a pull request for your engine on github. Also write a mail to our -devel mailing list.

The merge discussion

This is the part were you and the ScummVM team discuss about the chances of your engine being merged, and eventually, even discuss how it will be merged. You should have time to reply to the questions we have about your engine and its code. In this phase we usually try to provide code feedback too, so it would be nice if you could show us that you can work with that.

There are minimal requirements before you engine will be merged:

• There is a delay of at least one week between your initial pull request and -devel mail and the merge. In that time we will give all ScummVM members the chance to check your engine and its state.
• All serious issues which arise in that time have to be taken care of before the engine will be merged.
• The majority ScummVM core team members have to *explicitly* state that they are fine with the merge. All other core team members have at least told their stance on the merge.

How we proceeded from then on will be decided on a per case basis.