Add support for up to 6 players.

internal/ai/eval.go

@@ -36,9 +36,11 @@ func winScore(res *game.BattleResult, mySeat int) float64 {
 	default:
 		base = 0
 	}
+	// Survivors is indexed by battle side, not by seat.
+	mySide := res.Side(mySeat)
 	margin := 0
-	for seat, s := range res.Survivors {
-		if seat == mySeat {
+	for side, s := range res.Survivors {
+		if side == mySide {
 			margin += s
 		} else {
 			margin -= s
@@ -58,19 +60,30 @@ func (cx *ctx) winProb(myDeck []game.Card, oppDecks [][]game.Card, simsPer int)
 	total, n := 0.0, 0
 	for _, opp := range oppDecks {
 		for range simsPer {
-			var res *game.BattleResult
-			if cx.me.Seat == 0 {
-				res = game.SimulateBattle(cx.v.Round, cx.v.PrioritySeat, myDeck, opp, nil)
-			} else {
-				res = game.SimulateBattle(cx.v.Round, cx.v.PrioritySeat, opp, myDeck, nil)
-			}
-			total += winScore(res, cx.me.Seat)
+			// The bot's own deck always takes scratch seat 0, so a rollout reads
+			// the same however the real table happens to be seated.
+			res := game.SimulateBattle(cx.v.Round, cx.simFirstSeat(), myDeck, opp, nil)
+			total += winScore(res, 0)
 			n++
 		}
 	}
 	return total / float64(n)
 }
 
+// simFirstSeat picks who acts first in a rollout, with the bot at seat 0. Two
+// players pass a priority token the bot can see, so it plans against the real
+// one; bigger tables flip a coin for each battle, which the bot can't know in
+// advance — it rolls too, and averages over both possibilities.
+func (cx *ctx) simFirstSeat() int {
+	if len(cx.v.Players) == 2 {
+		if cx.v.PrioritySeat == cx.me.Seat {
+			return 0
+		}
+		return 1
+	}
+	return rand.IntN(2)
+}
+
 // keepValue ranks a single card's worth to the bot's future: what it loses
 // by selling or trading it away. Temporary cards (apples) are nearly free to
 // lose — they vanish after the next battle anyway.