Improve performance due to slow indexing (#273)

Newer versions of tantivy require more memory during the indexing phase.
Otherwise the indexing phase will be a lot slower than in previous
versions.

See quickwit-oss/tantivy#2156 for details.

src/course_library.rs

@@ -550,8 +550,8 @@ impl LocalCourseLibrary {
             reader: None,
         };
 
-        // Initialize the search index writer with an initial arena size of 50 MB.
-        let mut index_writer = library.index.writer(50_000_000)?;
+        // Initialize the search index writer with an initial arena size of 150 MB.
+        let mut index_writer = library.index.writer(150_000_000)?;
 
         // Convert the list of paths to ignore into absolute paths.
         let absolute_root = library_root.canonicalize()?;

src/scheduler.rs

@@ -746,11 +746,15 @@ impl DepthFirstScheduler {
             if let Some(superseded_by) = superseded_by {
                 all_superseded_by.extend(superseded_by);
             }
+
             let superseded_by = self.data.get_superseded_by(&candidate.course_id);
             if let Some(superseded_by) = superseded_by {
                 all_superseded_by.extend(superseded_by);
             }
         }
+        if all_superseded_by.is_empty() {
+            return candidates;
+        }
 
         // Filter out the superseding lessons and courses with scores lower than the superseding
         // score.