Fix bugs around AI opponents.

internal/ai/ai_test.go

@@ -1,6 +1,8 @@
 package ai
 
 import (
+	"fmt"
+	"slices"
 	"testing"
 
 	"github.com/greyson/super-auto-pets-board-game/internal/game"
@@ -251,3 +253,56 @@ func TestSimulateBattleIsPure(t *testing.T) {
 		t.Error("SimulateBattle mutated its input decks")
 	}
 }
+
+// TestDecideShopNeverSellsLastPet guards the invariant that the bot never
+// voluntarily turns its whole deck into food. In a hopeless late-game spot
+// (final round, a strong modeled opponent, so every rollout is a loss) the
+// candidate scores all collapse toward zero and the softmax degenerates to a
+// near-uniform pick — the exact situation that once let the bot sell every pet
+// across a few shop turns and hand over an automatic loss. However the dice
+// fall, a returned sell must leave at least one pet standing.
+func TestDecideShopNeverSellsLastPet(t *testing.T) {
+	pet := func(id, name string, power int) game.Card {
+		return game.Card{ID: id, Kind: game.KindPet, Name: name, Tier: 1, Power: power, Suit: game.SuitRed}
+	}
+	v := &game.View{
+		Phase:        game.PhaseShop,
+		Round:        game.MaxRounds, // alpha == 1: score is win-now only
+		MaxRounds:    game.MaxRounds,
+		MaxPets:      game.MaxPets,
+		Pack:         game.DefaultPack,
+		YouSeat:      0,
+		Turn:         0,
+		PrioritySeat: 0,
+		DeckCounts:   make([]int, game.MaxRounds+1),
+		Players: []game.PlayerView{
+			{Seat: 0, Coins: 0, PetCount: 2, DeckSize: 2, // no coins: buying is off the table
+				Deck: []game.Card{pet("p1", "Ant", 1), pet("p2", "Cricket", 1)}},
+			{Seat: 1, PetCount: 5, DeckSize: 5},
+		},
+	}
+	// Model a crushing opponent so every simulated battle is a loss.
+	mem := &Memory{}
+	mem.Opp.Seat = 1
+	for i := range 5 {
+		mem.Opp.Known = append(mem.Opp.Known,
+			game.Card{ID: fmt.Sprintf("o%d", i), Kind: game.KindPet, Name: "Wall", Tier: 1, Power: 50})
+	}
+
+	bot := New(0.5) // medium difficulty — the level from the bug report
+	for i := range 400 {
+		act := bot.decideShop(v, mem)
+		if act.Type != "sell" {
+			continue
+		}
+		remaining := 0
+		for _, c := range v.Players[0].Deck {
+			if c.IsPet() && !slices.Contains(act.Cards, c.ID) {
+				remaining++
+			}
+		}
+		if remaining == 0 {
+			t.Fatalf("iter %d: bot sold its last pet(s) %v, leaving an all-food deck", i, act.Cards)
+		}
+	}
+}