Modes backwards compatibility.

[benblack769]

Modes in ALE have been changing with this commit, and soon afterwards PR #365.

Me and @justinkterry are hoping to document interesting game modes for our Multi-Agent ALE project and were wondering how these modes might change moving forward. In particular:

1. Are modes in old games (like surround) untouchable for backwards compatibility reasons, even when they are hacked to start at 0?
2. If so, when will modes for new games (like othello) be locked in place?

[jkterry1]

@tkoeppe is this something you have any insight into?

[tkoeppe]

Re 2 I think we want the modes to be exactly as they are in the ROMs, no hacks.

I have not thought about 1. Can you maybe enumerate the games that have hacks? @kieranhj, what do you think?

We also have a document of modes internally. Maybe we can share that if that would be useful?

[kieranhj]

One challenge is that for many ROMs the supported game modes are enumerated from 0, whereas the actual modes start at 1, at least from the user's point-of-view / as documented in the ROM manuals. This pattern was generally carried forward when the newer games were added. We could document the discrepancies to get an idea of how big a change that might be.

Having the game modes be consistent with the documentation certainly reduces confusion when attempting to articulate the differences in game play.

[JesseFarebro]

@kieranhj do you have an idea of how much work it would be to get every mode to be consistent with their respective manual?

[benblack769]

@tkoeppe I also feel it would be far easier to use the library when the modes are consistent with their manual than if they start from 0 (as the mode byte defined by the ROM often does). The reasons are:

1. This way, the Atari manuals can be used as documentation for ALE.
2. People familiar with the Atari games, or who own consoles can use the modes much more easily.

In terms of our particular vested interests in Multi-player ALE, the relevant games are Pong (Video Olympics) and Surround, both of which have several interesting multi-player modes we wish to document but the mode numberings for these games are currently not aligned with the manual. Surround maps manual mode 2 to ALE mode 0 and Pong maps manual mode 1 to ALE mode 0.