Freeciv

The way this patch works to allow access to all the previous behaviors (claim nothing, claim adjacent, claim continent / lakes and bays surrounded by the continent, claim anything), without half a dozen separate boolean effects, is to have thresholds (like the Casus_Belli_* effects).

Since an effect/requirement context can only hold a single tile, I've realized this using two separate effects (Tile_Claimable_From_Same and Tile_Claimable_From_Other) to evaluate against the target tile. The decision which tile to evaluate against was mostly made on a conceptual basis; the old behavior is phrased more in terms of "claim ocean (from wherever)" than "claim (whatever) from land".
Evaluating against the target tile does open the door to future changes, like making tiles with improvements (e.g. ocean tiles with fishing boats, bridges, burrow tubes etc.) claimable sooner.
There are also good arguments for instead evaluating against the border source, e.g. to let cities claim more than bases, or some bases claim more than others, or make it so coastal cities only claim sea tiles once they have a harbor. Hm. The more I think about it, the less sure I am that evaluating against the target is actually the more reasonable option. I'd appreciate any and all comments on this.

Alien ruleset: I decided to ditch the Claim_Ocean flag and directly use a tech requirement. If there were plans to give that flag to more than one tech, better to let me know now so we don't have to change it one way now and back later.

Stub ruleset: not sure about these changes. This preserves the old behavior, but that isn't really necessary for a minimal ruleset to be functional.

Finally, if there are any fundamental changes we want to make to what the rules can do before codifying them in a way ruleset authors depend upon, we should do them soon; possibly even before merging this, possibly after; definitely before S3_3-d3f, which is still a ways off, but the sooner, the better. (For instance, claiming parts another continent across a strait, or tiny islands in a city's coastal waters, or territorial waters along the coast in general; all of which would complicate the model, but some of which might be worthwhile.)

Rebased patch on top of #590.

Updated patch based on Feature #616: MaxDistanceSq requirement. Though I am officially pushing this back (unforeseen circumstances notwithstanding) until I've at least gotten it to the point where it can all be done via requirements; switching from effects to an internal action later is less of a hassle than switching away from a threshold-based solution again.

Updated patch: All requirements are now unhardcoded. I've also gotten rid of the added effects to the stub ruleset, given that they're not necessary and that ruleset doesn't have any border sources with positive radius anyway.

I'm still deliberating whether to merge it like this, or add a "Same Terrain Class" TileRel requirement to need just one effect type (and streamline the implementation in some of the rulesets), or wait for non-unit-actor internal actions.

Alina Lenk wrote in #note-10:

I'm still deliberating whether to merge it like this, or add a "Same Terrain Class" TileRel requirement to need just one effect type (and streamline the implementation in some of the rulesets)

Did that. Updated patch is going to be the final one (I think); this might be migrated to an internal action later, but I'm not doing that now.