- Added ACS function IsTIDUsed(tid): It returns whether any actors using the given TID exist. This

is more efficient than ThingCount(tid, T_NONE), because it only needs to check for one actor
  with the TID and not all of them. It also makes no distinction between dead things and live
  things like ThingCount does.
- Added ACS function UniqueTID(tid, limit): It returns a new TID that is not currently used by
  any actors. It has two modes of operation. If tid is non-zero, then it checks TIDs one-by-one
  starting at the given tid until if finds a free one. If tid is zero, then it returns a completely
  random TID. If limit is non-zero, then it will only check that many times for a free TID, so
  it might not find a free one. If no free TID is found, 0 is returned. If limit is zero, then
  the search is effectively unlimited.

SVN r3798 (trunk)

src/p_acs.cpp

@@ -3363,6 +3363,8 @@ enum EACSFunctions
 	ACSF_ACS_NamedLockedExecuteDoor,
 	ACSF_ACS_NamedExecuteWithResult,
 	ACSF_ACS_NamedExecuteAlways,
+	ACSF_UniqueTID,
+	ACSF_IsTIDUsed,
 
 	// ZDaemon
 	ACSF_GetTeamScore = 19620,
@@ -3879,6 +3881,14 @@ int DLevelScript::CallFunction(int argCount, int funcIndex, SDWORD *args)
 			}
 			break;
 
+		case ACSF_UniqueTID:
+			return P_FindUniqueTID(argCount > 0 ? args[0] : 0, argCount > 1 ? args[1] : 0);
+			break;
+
+		case ACSF_IsTIDUsed:
+			return P_IsTIDUsed(args[0]);
+			break;
+
 		default:
 			break;
 	}