Make the AI a little smarter.

internal/ai/eval.go

@@ -9,16 +9,34 @@ import (
 )
 
 // winScore converts a simulated battle outcome to a utility for mySeat:
-// win 1, draw 0.5 (nobody gains ground), loss 0.
+// win 1, draw 0.5 (nobody gains ground), loss 0, plus a small margin term.
+//
+// The margin — your surviving pets minus the enemy's — breaks ties among
+// outcomes that share a verdict: a loss where you took most of the enemy down
+// beats a wipe, and a decisive win beats a squeaker. It is capped well under
+// 0.25 so it can never reorder win above draw above loss; it only decides
+// between moves the coarse win/draw/loss signal rates identically. That
+// gradient is what makes the bot play on sensibly when every option looks
+// hopeless — fielding its strongest force instead of picking at random.
 func winScore(res *game.BattleResult, mySeat int) float64 {
+	var base float64
 	switch res.WinnerSeat {
 	case mySeat:
-		return 1
+		base = 1
 	case -1:
-		return 0.5
+		base = 0.5
 	default:
-		return 0
+		base = 0
 	}
+	margin := 0
+	for seat, s := range res.Survivors {
+		if seat == mySeat {
+			margin += s
+		} else {
+			margin -= s
+		}
+	}
+	return base + 0.1*math.Tanh(float64(margin)/3)
 }
 
 // winProb estimates the chance the arranged deck wins the upcoming battle by

internal/ai/eval_test.go

@@ -0,0 +1,45 @@
+package ai
+
+import (
+	"testing"
+
+	"github.com/greyson/super-auto-pets-board-game/internal/game"
+)
+
+// TestWinScoreMarginBreaksTiesNotVerdicts pins down the margin tie-breaker:
+// within a single verdict it prefers the more decisive result (a win with pets
+// to spare, a loss that took most of the enemy down), but no margin, however
+// lopsided, may ever raise a loss above a draw or a draw above a win.
+func TestWinScoreMarginBreaksTiesNotVerdicts(t *testing.T) {
+	mk := func(winner, mine, theirs int) *game.BattleResult {
+		return &game.BattleResult{WinnerSeat: winner, Survivors: []int{mine, theirs}}
+	}
+
+	decisiveWin := winScore(mk(0, 5, 0), 0)
+	narrowWin := winScore(mk(0, 1, 0), 0)
+	drawAhead := winScore(mk(-1, 3, 1), 0)
+	evenDraw := winScore(mk(-1, 0, 0), 0)
+	drawBehind := winScore(mk(-1, 1, 3), 0)
+	narrowLoss := winScore(mk(1, 0, 1), 0)
+	blowoutLoss := winScore(mk(1, 0, 6), 0)
+
+	// Ties within a verdict resolve toward the stronger finish.
+	if !(decisiveWin > narrowWin) {
+		t.Errorf("a win with survivors should beat a squeaker: %.4f !> %.4f", decisiveWin, narrowWin)
+	}
+	if !(drawAhead > evenDraw && evenDraw > drawBehind) {
+		t.Errorf("draws should order by margin: ahead %.4f, even %.4f, behind %.4f", drawAhead, evenDraw, drawBehind)
+	}
+	if !(narrowLoss > blowoutLoss) {
+		t.Errorf("a narrow loss should beat a blowout: %.4f !> %.4f", narrowLoss, blowoutLoss)
+	}
+
+	// Verdict ordering is inviolable: the best-possible loss still ranks below
+	// the worst-possible draw, and the best draw below the worst win.
+	if !(narrowWin > drawAhead) {
+		t.Errorf("the worst win must outrank the best draw: %.4f !> %.4f", narrowWin, drawAhead)
+	}
+	if !(drawBehind > narrowLoss) {
+		t.Errorf("the worst draw must outrank the best loss: %.4f !> %.4f", drawBehind, narrowLoss)
+	}
+}