Add support for up to 6 players.

internal/game/packs.go

@@ -1,5 +1,10 @@
 package game
 
+import (
+	"fmt"
+	"strings"
+)
+
 // Card packs are the selectable sets of pets and food a game is played with.
 // Turtle, Golden, and Unicorn all ship with full six-tier card data.
 
@@ -31,3 +36,73 @@ func packByID(id string) (PackInfo, bool) {
 	}
 	return PackInfo{}, false
 }
+
+// PacksNeeded is how many packs a game must combine to seat n players: the
+// rulebook asks for at least 2 packs at 4 players and 3 at 6, i.e. one per
+// pair. More packs than the minimum are always allowed — a deeper shop just
+// means fewer repeated pets.
+func PacksNeeded(players int) int {
+	if players < MinPlayers {
+		return 1
+	}
+	return players / 2
+}
+
+// sortPacks puts a pack selection into catalog order, so the same choice
+// always reads the same way in views and logs.
+func sortPacks(ids []string) []string {
+	out := make([]string, 0, len(ids))
+	for _, p := range Packs {
+		for _, id := range ids {
+			if id == p.ID {
+				out = append(out, p.ID)
+				break
+			}
+		}
+	}
+	return out
+}
+
+// validatePacks checks a pack selection: non-empty, known, playable, and free
+// of duplicates. It returns the selection in catalog order.
+func validatePacks(ids []string) ([]string, error) {
+	if len(ids) == 0 {
+		return nil, fmt.Errorf("%w: pick at least one pack", ErrInvalidAction)
+	}
+	seen := map[string]bool{}
+	for _, id := range ids {
+		pack, ok := packByID(id)
+		if !ok {
+			return nil, fmt.Errorf("%w: unknown pack %q", ErrInvalidAction, id)
+		}
+		if !pack.Playable {
+			return nil, fmt.Errorf("%w: the %s isn't available yet", ErrInvalidAction, pack.Name)
+		}
+		if seen[id] {
+			return nil, fmt.Errorf("%w: %s is selected twice", ErrInvalidAction, pack.Name)
+		}
+		seen[id] = true
+	}
+	return sortPacks(ids), nil
+}
+
+// PackNames renders a pack selection as a readable list ("Turtle Pack and
+// Golden Pack") for log lines.
+func PackNames(ids []string) string {
+	names := make([]string, 0, len(ids))
+	for _, id := range ids {
+		if p, ok := packByID(id); ok {
+			names = append(names, p.Name)
+		}
+	}
+	switch len(names) {
+	case 0:
+		return "no packs"
+	case 1:
+		return names[0]
+	case 2:
+		return names[0] + " and " + names[1]
+	default:
+		return strings.Join(names[:len(names)-1], ", ") + ", and " + names[len(names)-1]
+	}
+}