Don't immediately delete unused extradata entries

Instead, count how many consecutive times the entry has been found to be
unused and delete it once that count exceeds a limit. This will prevent
excessive reallocating of extradata ID's in applications like folding.

src/ass_file.cpp

@@ -236,7 +236,7 @@ uint32_t AssFile::AddExtradata(std::string const& key, std::string const& value)
 			return data.id;
 		}
 	}
-	Extradata.push_back(ExtradataEntry{next_extradata_id, key, value});
+	Extradata.push_back(ExtradataEntry{next_extradata_id, 0, key, value});
 	return next_extradata_id++; // return old value, then post-increment
 }
 
@@ -334,10 +334,16 @@ void AssFile::CleanExtradata() {
 		}
 	}
 
+	for (ExtradataEntry &e : Extradata) {
+		if (ids_used.count(e.id))
+			e.expiration_counter = 0;
+		else
+			e.expiration_counter++;
+	}
 	if (ids_used.size() != Extradata.size()) {
 		// Erase all no-longer-used extradata entries
 		Extradata.erase(std::remove_if(begin(Extradata), end(Extradata), [&](ExtradataEntry const& e) {
-			return !ids_used.count(e.id);
+			return e.expiration_counter >= 10;
 		}), end(Extradata));
 	}
 }

src/ass_file.h

@@ -50,6 +50,7 @@ using EntryList = typename boost::intrusive::make_list<T, boost::intrusive::cons
 
 struct ExtradataEntry {
 	uint32_t id;
+	int expiration_counter;
 	std::string key;
 	std::string value;
 };

src/ass_parser.cpp

@@ -219,7 +219,7 @@ void AssParser::ParseExtradataLine(std::string const &data) {
 
 		// ensure next_extradata_id is always at least 1 more than the largest existing id
 		target->next_extradata_id = std::max(id+1, target->next_extradata_id);
-		target->Extradata.push_back(ExtradataEntry{id, std::move(key), std::move(value)});
+		target->Extradata.push_back(ExtradataEntry{id, 0, std::move(key), std::move(value)});
 	}
 }