Retreats and the winter

Movement is the part everybody thinks about and it is not the part that
decides the game. A beaten unit that finds somewhere to stand is a nuisance
for years; one that does not is gone. And the winter is where a good spring
turns into a bigger army, or does not.

Four things are closed to a retreat: anywhere unreachable, anywhere occupied,
the province the attacker came from, and any province two units bounced out
of -- that last is the one people forget, since a bounce leaves a province
empty and closed rather than empty and open. The exception is an attacker who
arrived by convoy: it never marched through the ground between, so there is
nothing to stop the loser falling back that way. Two units retreating to the
same province destroy each other, there being no support in this phase and
nothing to break the tie with.

Ownership changes only in the autumn and only by standing on a centre. A
centre you walked through in the spring is not yours; one you walked away
from stays yours until somebody else stands on it, which is why a power with
no units can still hold centres.

Civil disorder removes what is furthest from home, fleets first, then
alphabetically -- arbitrary in the last step on purpose, so that two
adjudicators never disagree about a game nobody is playing.

src/game/adjudicate.ts

@@ -28,6 +28,12 @@ export interface Dislodgement {
   unit: Unit
   /** Where the attacker came from. A unit may not retreat back into it. */
   attackedFrom: string
+  /**
+   * The attacker came over water. That lifts the restriction above: a unit
+   * put ashore by a convoy never passed through the ground between, so there
+   * is nothing to say the loser cannot fall back that way.
+   */
+  byConvoy: boolean
 }
 
 export interface Outcome {
@@ -323,7 +329,11 @@ export function adjudicate(board: Board, orderList: readonly Order[]): Outcome {
   for (const p of board.keys()) {
     if (!isDislodged(p)) continue
     const attacker = movesInto(p).find((q) => success.get(q))!
-    dislodged.set(p, { unit: unitAt(p)!, attackedFrom: attacker })
+    dislodged.set(p, {
+      unit: unitAt(p)!,
+      attackedFrom: attacker,
+      byConvoy: isConvoyed(attacker),
+    })
   }
 
   /*