fix bug in chunks building

we were discarding like 90% of the pages due to buggy insertion of pages
in the linked list

src/render.cpp

@@ -179,15 +179,8 @@ void viewer_jump_to_cluster(App_State &app, u64 cluster_idx)
   assert(app.rndata.n_clusters > 0);
   cluster_idx = (cluster_idx + app.rndata.n_clusters) % app.rndata.n_clusters;
 
-  // @Speed: this is slow! Consider an acceleration structure, or maybe we can reuse
-  // Page_Info_Groups + binary search? (depends on whether cluster_idx are sorted)
-  Page_Info_Node *page = app.rndata.pages;
-  for (u64 i = 0; i < app.rndata.n_pages; ++i) {
-    if (page->cluster_id == cluster_idx)
-      break;
-    page = page->next;    
-    assert(page);
-  }
+  Page_Info_Node *page = app.rndata.clusters[cluster_idx].first_page;
+  assert(page);
 
   app.viewer.highlighted_cluster = cluster_idx;
   viewer_jump_to(app, page->range.start);

src/rntuple.cpp

@@ -96,6 +96,8 @@ void gather_ntuple_metadata(Arena *arena, RMicroFileReader &reader, const RNTupl
       
   chr::time_point start_t = chr::high_resolution_clock::now();
 
+  Cluster_Info_Node *clusters = arena_push_array<Cluster_Info_Node>(arena, descriptor.GetNActiveClusters());
+
   // gather clusters and pages metadata
   for (const RClusterDescriptor &cluster_desc : descriptor.GetClusterIterable()) {
     ++n_clusters;
@@ -111,7 +113,12 @@ void gather_ntuple_metadata(Arena *arena, RMicroFileReader &reader, const RNTupl
         pinfo->n_elems = page_info.fHasChecksum ? -page_info.fNElements : page_info.fNElements;
         pinfo->cluster_id = cluster_desc.GetId();
 
-        if (!pinfo_head) {
+        Cluster_Info_Node &cluster = clusters[pinfo->cluster_id];
+        if (!cluster.first_page || pinfo->range.start < cluster.first_page->range.start) {
+          cluster.first_page = pinfo;
+        }
+
+        if (UNLIKELY(!pinfo_head)) {
           // first node inserted
           assert(!pinfo_tail);
           pinfo_head = pinfo_tail = pinfo;
@@ -141,6 +148,7 @@ void gather_ntuple_metadata(Arena *arena, RMicroFileReader &reader, const RNTupl
                     prev->next = pinfo;
                     pinfo->prev = prev;
                   }
+                  node->prev = pinfo;
                   pinfo->next = node;
                   inserted = true;
                   break;
@@ -157,6 +165,7 @@ void gather_ntuple_metadata(Arena *arena, RMicroFileReader &reader, const RNTupl
                     prev->next = pinfo;
                     pinfo->prev = prev;
                   }
+                  node->prev = pinfo;
                   pinfo->next = node;
                   inserted = true;
                   break;
@@ -231,6 +240,9 @@ void gather_ntuple_metadata(Arena *arena, RMicroFileReader &reader, const RNTupl
     ++n_groups;
   }
 
+  // verify that we added all pages to chunks
+  assert(idx == n_pages);
+
   Page_Info_Group &last_group = groups[n_groups - 1];
   last_group.range.len = pinfo_tail->range.end() - last_group.range.start;
 
@@ -250,6 +262,7 @@ void gather_ntuple_metadata(Arena *arena, RMicroFileReader &reader, const RNTupl
   rndata.cluster_groups = cluster_groups;
   rndata.n_cluster_groups = cg_idx;
   rndata.tot_page_list_size = tot_page_list_size;
+  rndata.clusters = clusters;
   rndata.n_clusters = n_clusters;
 }
 

src/rntuple.h

@@ -35,6 +35,10 @@ struct Page_Info_Chunk {
   u32 first_group;
 };
 
+struct Cluster_Info_Node {
+  Page_Info_Node *first_page;
+};
+
 struct Cluster_Group_Info {
   Byte_Range rng_page_list;
 };
@@ -78,6 +82,7 @@ struct RNTuple_Data {
   u64 n_cluster_groups;
   u64 tot_page_list_size;
 
+  Cluster_Info_Node *clusters;
   u64 n_clusters;
 
   Page_Info_Group *page_groups;