Support Baron von Powell's 1900 variant

[johnpooch]

Goal

Add Baron M. Powell's 1900 as a playable variant. 1900 is a Classical-style redraw of Europe and the North African coast for seven powers, but it carries three custom rules the current adjudicator cannot express: the Suez Canal half-strength crossing, the Russian Emergency Measures Rule, and Gibraltar acting as a convoy-capable yet army-occupiable space. Supporting it means adding the variant data plus a small set of generic, reusable adjudicator mechanisms so these rules — and similar mechanics in future variants — are declarative schema features rather than one-off hacks.

Definition of done

1900 is published and plays to Baron Powell's official rules. Concretely:

• The full 1900 map is in place: 39 supply centers, all renamed/split/added provinces (Moscow/Siberia, Trieste/Bosnia, Cologne, Milan, Alsace, Palestine, Arabia, Cyrenaica, Southern Algeria, Macedonia's two coasts, neutral Morocco/Switzerland/Tripolitania, etc.), Spain's three coasts, Iceland/Ireland/Switzerland passability and the crossing arrows, and the documented seven-power starting position.
• Solo victory requires 18 supply centers and strictly more than any other power; a tie at 18+ continues play.
• Britain cannot build in Egypt and France cannot build in Algeria, and both are treated as home centers when selecting default disbands under civil disorder.
• Russian Emergency Measures Rule: while Russia holds at least one but not all four of its home supply centers, it may maintain one unit beyond its supply-center count and may build in an unoccupied Siberia; Siberia never becomes a supply center, and the allowance is lost when Russia holds zero or all four home centers.
• Gibraltar: an army may occupy it (and block a fleet), and a fleet in Gibraltar may serve as a link in a convoy chain.
• The Suez Canal rules hold in full: movement between the Mid-Atlantic and Egypt/Hejaz is at half strength in both directions; the Mid cannot support to or from Egypt/Hejaz and vice versa; a unit crossing the cape does not cut support in the destination unless it dislodges the supporter; an army convoyed from Egypt/Hejaz attacks at full strength while one convoyed to Egypt/Hejaz attacks at half; and when two units contend for the same destination and one must travel around the cape, the cape-bound unit disbands while the other retreats.
• The new adjudicator mechanics are generic, not keyed to 1900: reduced-strength crossings are an edge-level attribute on adjacencies (defaulting to full) that the movement strength solver reads, and convoy-node capability is a province-level flag; either is usable by any variant.
• Each custom rule has adjudicator-level test coverage, and the variant round-trips through the existing DVAR upload/download path.

Context

Most of 1900 is ordinary variant data and needs no adjudicator change — the map, starting units, the victory condition (already handled by check_for_solo_winner in service/victory/utils.py, including the strict-majority tiebreak), and the Egypt/Algeria non-build rule (automatic once neither is declared a home center). Three rules are the real work, and they resolve to two reusable schema loci plus one contained solver change:

Suez half-strength lives on the edge, not the province. The same province (Egypt, Mid) is entered at full strength from its normal neighbours and at half strength only across the cape, so the reduced strength attaches to the specific adjacency, not the destination. The generic form is an optional strength attribute on the Adjacency schema object (default full), read by the movement strength solver. The solver has no fractional strength today; the intended encoding scales strengths (normal base 2, a half-strength crossing seeds base 1) so every comparison stays integer. This is the one change to service/adjudicator/resolution.py — the most tightly-guarded module in the engine, whose architectural guide asks for maintainer sign-off before structural changes — so the solver work should be reviewed before it is implemented. The sibling Suez rules (no support across the cape, and the cut-exception) are also edge-level and ride on the same adjacency; the cut-exception reuses machinery the solver already has for DATC 6.D.4.

Gibraltar convoy capability lives on the province. Every convoy path-finding step and the convoy-issue check gate on a province being sea (service/adjudicator/convoy.py, ConvoyerIsInSeaCheck in service/adjudicator/types.py), so a coastal Gibraltar cannot be a convoy link while a sea Gibraltar cannot be occupied by an army — neither existing type fits. The generic form is a province-level convoy-node flag consulted by the convoy gates alongside the sea check, leaving occupancy behavior untouched.

Russian EMR is expressible as input/output rewriting. When the EMR condition holds, injecting Siberia as a Russia-owned home supply center into the adjudication input produces all three effects — one extra build allowed, Siberia buildable, one fewer required disband — by reusing the existing build, disband, and limit logic unchanged (NationView.allowed_builds/required_disbands, EnforceBuildLimits), then stripping the phantom center from the output so it never persists or counts toward victory. The rewrite must be conditional on live home-supply-center ownership rather than static variant data, and must not leak the phantom center into stored ownership.

The adjudication-modifier system is currently a set of bare string tags with no parameters. The EMR and the edge/province features are parameterized (which nation, which province, which edges), so carrying that metadata will require extending how modifiers or variant data express parameters.

[johnpooch]

There will also need to be some work on the variant creator to support these options.

[jeanhenribernard89]

Hello,

If needed, I have a SVG version of the map, with layers etc. So this can allow to skip the PNG-to-SVG steps with Claude.

Also if it can help, on vDiplomacy you can find the DATC with tests of edge cases coming from the special rules:
https://www.vdiplomacy.com/datc.php
(For 1900 specific tests, you need to scroll down to the bottom.)