+ _chosen_interop_font = fonts[0];
+ } else {
+ set<string> used_ids;
+
+ /* Return the index of a _N at the end of a string, or string::npos */
+ auto underscore_number_position = [](string s) {
+ auto last_underscore = s.find_last_of("_");
+ if (last_underscore == string::npos) {
+ return string::npos;
+ }
+
+ for (auto i = last_underscore + 1; i < s.size(); ++i) {
+ if (!isdigit(s[i])) {
+ return string::npos;
+ }
+ }
+
+ return last_underscore;
+ };
+
+ /* Write fonts to _fonts, changing any duplicate IDs so that they are unique */
+ for (auto font: fonts) {
+ auto id = fix_id(font->id());
+ if (used_ids.find(id) == used_ids.end()) {
+ /* This ID is unique so we can just use it as-is */
+ _fonts.put(font, id);
+ used_ids.insert(id);
+ } else {
+ auto end = underscore_number_position(id);
+ if (end == string::npos) {
+ /* This string has no _N suffix, so add one */
+ id += "_0";
+ end = underscore_number_position(id);
+ }