fix(server): support dynamic hidden event pack purchase and entry

why the f^^kkkkkk does sh^t AI love writing useless tests so much just to clutter up my repo, looks like I’ll have to find some time to clean this s^^t up later.

go/internal/server/world/event_field_pack.go

@@ -0,0 +1,100 @@
+package world
+
+import (
+	"fmt"
+	"slices"
+
+	"bd2server/internal/server/gamedata"
+	"bd2server/internal/server/wire"
+)
+
+// EventFieldPackSource binds installed hidden packs to the server's current
+// calendars. A purchase receipt alone never authorizes an expired event.
+type EventFieldPackSource interface {
+	ResolveEventFieldPack(int) (gamedata.EventFieldPack, bool, error)
+	ListEventFieldPacks() ([]gamedata.EventFieldPack, error)
+}
+
+func (s *Service) AttachEventFieldPacks(source EventFieldPackSource) error {
+	if source == nil {
+		return fmt.Errorf("world: nil event field pack source")
+	}
+	s.eventFieldPacks = source
+	return nil
+}
+
+func (s *Service) resolveEventFieldPack(id int) (gamedata.EventFieldPack, bool, error) {
+	if s.eventFieldPacks == nil {
+		return gamedata.EventFieldPack{}, false, nil
+	}
+	return s.eventFieldPacks.ResolveEventFieldPack(id)
+}
+
+func (s *Service) eventPackPurchased(id int) bool {
+	if s.collection == nil {
+		return false
+	}
+	_, found := s.collection.Grant(packPurchaseIdentity(id))
+	return found
+}
+
+func (s *Service) eventPackDBInfo(pack gamedata.EventFieldPack) []byte {
+	row := wire.AppendVarint(nil, 1, uint64(pack.ID))
+	if s.eventPackPurchased(pack.ID) {
+		row = wire.AppendVarint(row, 8, 1)
+	}
+	return row
+}
+
+func (s *Service) eventPackInfoRows() ([][]byte, error) {
+	if s.eventFieldPacks == nil {
+		return nil, nil
+	}
+	packs, err := s.eventFieldPacks.ListEventFieldPacks()
+	if err != nil {
+		return nil, err
+	}
+	var rows [][]byte
+	for _, pack := range packs {
+		// PackManager buys a hidden pack only when PackInfo has no row for it.
+		if s.eventPackPurchased(pack.ID) {
+			rows = append(rows, s.eventPackDBInfo(pack))
+		}
+	}
+	return rows, nil
+}
+
+func (s *Service) enterEventFieldPack(pack gamedata.EventFieldPack) (int, []byte, bool, error) {
+	if !s.eventPackPurchased(pack.ID) {
+		return 0, nil, true, fmt.Errorf("%w: event pack %d is not purchased", ErrInvalidRequest, pack.ID)
+	}
+	position := pack.InitialPosition
+	if position == "" {
+		return 0, nil, true, fmt.Errorf("world: missing event pack initial position")
+	}
+	if saved, found := s.state.Position(); found && saved.PackID == pack.ID {
+		if !slices.Contains(pack.MapIDs, saved.Position.MapID) {
+			return 0, nil, true, fmt.Errorf("%w: map outside event pack", ErrInvalidRequest)
+		}
+		position = saved.RawJSON
+	}
+	response := wire.AppendString(nil, 4, position)
+	// The common callback dereferences HuntingGroundInfo even in hidden packs.
+	// Use the domain-generated empty/current snapshot; never borrow the outside
+	// map's monsters or story progress.
+	var hunting []byte
+	if s.huntingGround != nil {
+		var err error
+		hunting, err = s.huntingGround.EnsureForPack(pack.ID)
+		if err != nil {
+			return 0, nil, true, err
+		}
+	}
+	response = wire.AppendBytes(response, 12, hunting)
+	// Hidden-pack entry must retain the outside field position: the client
+	// intentionally suppresses SaveUserPosition while playing these packs. The
+	// persistent active pack also stays outside, so relogin cannot be stranded
+	// in a hidden scene after its calendar closes.
+	s.setCurrentPack(pack.ID)
+	return 5, response, true, nil
+}