Improve trip display name heuristics

src/loader/gtfs/trip.cc

@@ -190,44 +190,87 @@ void trip::interpolate() {
 
 std::string trip::display_name(timetable const& tt) const {
   auto const is_digit = [](char const x) { return x >= '0' && x <= '9'; };
-  if (route_->clasz_ == clasz::kBus) {
-    return route_->short_name_.empty() ? "Bus " + short_name_
-                                       : "Bus " + route_->short_name_;
-  } else if (route_->clasz_ == clasz::kTram) {
-    return route_->short_name_.empty() ? "Tram " + short_name_
-                                       : "Tram " + route_->short_name_;
-  }
 
-  auto const trip_name_is_number =
-      !short_name_.empty() && utl::all_of(short_name_, is_digit);
-  if (!route_->short_name_.starts_with("IC") &&
-      route_->agency_ != provider_idx_t::invalid() &&
-      tt.providers_[route_->agency_].long_name_ == "DB Fernverkehr AG") {
-    if (route_->clasz_ == clasz::kHighSpeed) {
-      return trip_name_is_number
-                 ? fmt::format("ICE {}", utl::parse<int>(short_name_))
-                 : fmt::format("ICE {}", route_->short_name_);
-    } else if (route_->clasz_ == clasz::kLongDistance) {
-      return trip_name_is_number
-                 ? fmt::format("IC {}", utl::parse<int>(short_name_))
-                 : fmt::format("IC {}", route_->short_name_);
+  // Depending on the class, there are different expectations from users
+  // if the special case doesn't work based on available data, fall back to
+  // generic code afterwards
+  switch (route_->clasz_) {
+    // For subway / metro, only the line matters since it's frequent enough.
+    case clasz::kSubway:
+    case clasz::kMetro:
+      if (!route_->short_name_.empty()) {
+        return route_->short_name_;
+      }
+      break;
+    // Deutsche Bahn heuristics to classify ICEs and ICs
+    case clasz::kHighSpeed:
+    case clasz::kLongDistance: {
+      auto const trip_name_is_number =
+          !short_name_.empty() && utl::all_of(short_name_, is_digit);
+      if (!route_->short_name_.starts_with("IC") &&
+          route_->agency_ != provider_idx_t::invalid() &&
+          tt.providers_[route_->agency_].long_name_ == "DB Fernverkehr AG") {
+        if (route_->clasz_ == clasz::kHighSpeed) {
+          return trip_name_is_number
+                     ? fmt::format("ICE {}", utl::parse<int>(short_name_))
+                     : fmt::format("ICE {}", route_->short_name_);
+        } else if (route_->clasz_ == clasz::kLongDistance) {
+          return trip_name_is_number
+                     ? fmt::format("IC {}", utl::parse<int>(short_name_))
+                     : fmt::format("IC {}", route_->short_name_);
+        }
+      }
+
+      break;
+    }
+    default: {
     }
   }
 
-  auto const starts_with_letter_and_ends_with_number =
-      [](std::string_view line_id) {
-        return !(line_id.front() >= '0' && line_id.front() <= '9') &&
-               (line_id.back() >= '0' && line_id.back() <= '9');
-      };
+  // route name looks like product name, trip name is number
+  auto const is_product_name = [=](std::string_view route) {
+    return utl::all_of(route, [=](char const c) { return !is_digit(c); });
+  };
+
+  auto const is_trip_number = [=](std::string_view trip_name) {
+    return utl::all_of(trip_name, is_digit);
+  };
 
-  if (starts_with_letter_and_ends_with_number(route_->short_name_)) {
-    return route_->short_name_;
-  } else if (trip_name_is_number) {
+  if (!route_->short_name_.empty() && is_product_name(route_->short_name_) &&
+      !short_name_.empty() && is_trip_number(short_name_)) {
     return fmt::format("{} {}", route_->short_name_,
                        utl::parse<int>(short_name_));
-  } else {
-    return fmt::format("{} {}", route_->short_name_, short_name_);
   }
+
+  // take the first thing that starts with a letter and ends with a number
+  auto const starts_with_letter_and_ends_with_number =
+      [=](std::string_view line_id) {
+        return !is_digit(line_id.front()) && is_digit(line_id.back());
+      };
+
+  auto precedence = std::array{std::string_view{short_name_},
+                               std::string_view{route_->short_name_},
+                               std::string_view{route_->long_name_}};
+
+  for (auto const candidate : precedence) {
+    if (candidate.empty()) {
+      continue;
+    }
+
+    if (starts_with_letter_and_ends_with_number(candidate)) {
+      return std::string(candidate);
+    }
+  }
+
+  // prefer trip name over route short name,
+  // prefer route short name over route long name
+  for (auto const candidate : precedence) {
+    if (!candidate.empty()) {
+      return std::string(candidate);
+    }
+  }
+
+  return {};
 }
 
 clasz trip::get_clasz(timetable const& tt) const {