2 files changed, 182 insertions, 0 deletions
diff --git a/internal/world/room_migrate.go b/internal/world/room_migrate.go
new file mode 100644
index 0000000..6d79499
--- /dev/null
+++ b/internal/world/room_migrate.go
@@ -0,0 +1,71 @@
+package world
+
+// RewriteRoomIDs remaps every genuine room-ID reference in this room per idMap
+// (oldID -> newID): exit targets, trigger Room/Teleport/DespawnRooms, on-enter
+// Teleport/DespawnRooms, and mob WanderRooms. It does NOT touch author-chosen
+// set_flags/set_player_flags values or condition `value:` fields — those are not
+// structural room references (and historically the swapid regex rewrote them
+// incorrectly as a side effect of matching any bare integer).
+//
+// idMap is assumed to be a bijection (each old ID maps to a distinct new ID);
+// this holds for both swap (A<->B) and rename-to-unused (A->B). Returns whether
+// any field was changed.
+func (r *Room) RewriteRoomIDs(idMap map[int]int) bool {
+	if r == nil || len(idMap) == 0 {
+		return false
+	}
+	changed := false
+	for dir, exit := range r.Exits {
+		if remapInt(idMap, &exit.Room) {
+			r.Exits[dir] = exit
+			changed = true
+		}
+	}
+	for i := range r.Triggers {
+		if remapInt(idMap, &r.Triggers[i].Room) {
+			changed = true
+		}
+		for j := range r.Triggers[i].Steps {
+			s := &r.Triggers[i].Steps[j]
+			if remapInt(idMap, &s.Teleport) {
+				changed = true
+			}
+			if s.SpawnMob != nil && remapIntSlice(idMap, s.SpawnMob.DespawnRooms) {
+				changed = true
+			}
+		}
+	}
+	for i := range r.OnEnter {
+		s := &r.OnEnter[i]
+		if remapInt(idMap, &s.Teleport) {
+			changed = true
+		}
+		if s.SpawnMob != nil && remapIntSlice(idMap, s.SpawnMob.DespawnRooms) {
+			changed = true
+		}
+	}
+	for i := range r.Mobs {
+		if remapIntSlice(idMap, r.Mobs[i].WanderRooms) {
+			changed = true
+		}
+	}
+	return changed
+}
+
+func remapInt(idMap map[int]int, n *int) bool {
+	if newID, ok := idMap[*n]; ok && newID != *n {
+		*n = newID
+		return true
+	}
+	return false
+}
+
+func remapIntSlice(idMap map[int]int, slice []int) bool {
+	changed := false
+	for i := range slice {
+		if remapInt(idMap, &slice[i]) {
+			changed = true
+		}
+	}
+	return changed
+}
diff --git a/internal/world/room_migrate_test.go b/internal/world/room_migrate_test.go
new file mode 100644
index 0000000..b035172
--- /dev/null
+++ b/internal/world/room_migrate_test.go
@@ -0,0 +1,111 @@
+package world
+
+import "testing"
+
+func TestRoomRewriteRoomIDs(t *testing.T) {
+	r := &Room{
+		Exits: map[ExitDir]ExitDef{
+			North: {Room: 10, SetFlags: map[string]any{"last": 10}},
+			South: {Room: 99},
+		},
+		Triggers: []TriggerDef{
+			{
+				Room: 30,
+				Steps: []TriggerStep{
+					{Teleport: 40, SpawnMob: &SpawnMobConfig{DespawnRooms: []int{50, 60}}},
+				},
+			},
+		},
+		OnEnter: []EnterStep{
+			{Teleport: 70, SpawnMob: &SpawnMobConfig{DespawnRooms: []int{80}}},
+		},
+		Mobs: []RoomMob{
+			{ID: "guard", WanderRooms: []int{90, 100}},
+		},
+	}
+
+	idMap := map[int]int{
+		10: 11, 20: 21, 30: 31, 40: 41, 50: 51, 60: 61,
+		70: 71, 80: 81, 90: 91, 100: 101,
+	}
+	if !r.RewriteRoomIDs(idMap) {
+		t.Fatal("expected changed=true")
+	}
+
+	checkExit := func(dir ExitDir, want int) {
+		t.Helper()
+		if got := r.Exits[dir].Room; got != want {
+			t.Errorf("exit %s room = %d, want %d", dir, got, want)
+		}
+	}
+	checkExit(North, 11)
+	checkExit(South, 99) // unchanged (not in idMap)
+
+	// Author set_flags values are NOT structural room references and must not
+	// be remapped (the old swapid regex wrongly rewrote these).
+	if v := r.Exits[North].SetFlags["last"]; v != 10 {
+		t.Errorf("set_flags value remapped: got %v, want 10", v)
+	}
+
+	if r.Triggers[0].Room != 31 {
+		t.Errorf("trigger room = %d, want 31", r.Triggers[0].Room)
+	}
+	if r.Triggers[0].Steps[0].Teleport != 41 {
+		t.Errorf("trigger teleport = %d, want 41", r.Triggers[0].Steps[0].Teleport)
+	}
+	if got := r.Triggers[0].Steps[0].SpawnMob.DespawnRooms; len(got) != 2 || got[0] != 51 || got[1] != 61 {
+		t.Errorf("trigger despawn_rooms = %v, want [51 61]", got)
+	}
+	if r.OnEnter[0].Teleport != 71 {
+		t.Errorf("on_enter teleport = %d, want 71", r.OnEnter[0].Teleport)
+	}
+	if got := r.OnEnter[0].SpawnMob.DespawnRooms; len(got) != 1 || got[0] != 81 {
+		t.Errorf("on_enter despawn_rooms = %v, want [81]", got)
+	}
+	if got := r.Mobs[0].WanderRooms; len(got) != 2 || got[0] != 91 || got[1] != 101 {
+		t.Errorf("mob wander_rooms = %v, want [91 101]", got)
+	}
+}
+
+func TestRoomRewriteRoomIDsNoChange(t *testing.T) {
+	r := &Room{
+		Exits: map[ExitDir]ExitDef{North: {Room: 5}},
+	}
+	// idMap does not mention 5 -> nothing changes.
+	if r.RewriteRoomIDs(map[int]int{99: 100}) {
+		t.Error("expected changed=false when no references match")
+	}
+	if r.Exits[North].Room != 5 {
+		t.Errorf("exit room = %d, want 5 (untouched)", r.Exits[North].Room)
+	}
+}
+
+func TestRoomRewriteRoomIDsSwapRoundTrips(t *testing.T) {
+	r := &Room{Exits: map[ExitDir]ExitDef{
+		North: {Room: 10},
+		South: {Room: 20},
+	}}
+	swap := map[int]int{10: 20, 20: 10}
+	r.RewriteRoomIDs(swap)
+	if r.Exits[North].Room != 20 || r.Exits[South].Room != 10 {
+		t.Errorf("after swap: north=%d south=%d, want north=20 south=10", r.Exits[North].Room, r.Exits[South].Room)
+	}
+	// Applying the same swap again reverts (a swap is its own inverse).
+	r.RewriteRoomIDs(swap)
+	if r.Exits[North].Room != 10 || r.Exits[South].Room != 20 {
+		t.Errorf("after double swap: north=%d south=%d, want north=10 south=20", r.Exits[North].Room, r.Exits[South].Room)
+	}
+}
+
+func TestRoomRewriteRoomIDsIdempotentRename(t *testing.T) {
+	r := &Room{Exits: map[ExitDir]ExitDef{North: {Room: 10}}}
+	rename := map[int]int{10: 11}
+	r.RewriteRoomIDs(rename)
+	if r.Exits[North].Room != 11 {
+		t.Fatalf("after rename: room=%d, want 11", r.Exits[North].Room)
+	}
+	// Second pass: 11 is not a key in idMap, so nothing changes.
+	if r.RewriteRoomIDs(rename) {
+		t.Error("expected changed=false on second pass of a rename idMap")
+	}
+}