try speeding up jump_to_page

src/render.cpp

@@ -178,11 +178,37 @@ Page_Info_Node *viewer_jump_to_page(App_State &app, u64 page_idx)
     return nullptr;
   page_idx = (page_idx + app.rndata.n_pages) % app.rndata.n_pages;
   
-  // @Speed
+  // // @Speed
-  Page_Info_Node *page = app.rndata.pages;
+  // Page_Info_Node *page = app.rndata.pages;
-  for (u64 i = 0; i < page_idx; ++i) {
+  // for (u64 i = 0; i < page_idx; ++i) {
-    page = page->next;    
+  //   page = page->next;    
-    assert(page);
+  //   assert(page);
+  // }
+
+  // Find the page_idx-th page. We could just iterate n times over the pages list,
+  // but skimming through the clusters first should be faster in most cases.
+  Page_Info_Node *page = nullptr;
+  for (u64 i = 0; i < app.rndata.n_clusters - 1; ++i) {
+    const Cluster_Info &cluster = app.rndata.clusters[i];
+    const Cluster_Info &next_cluster = app.rndata.clusters[i];
+    if (cluster.first_page_idx <= page_idx && page_idx < next_cluster.first_page_idx) {
+      u64 idx_in_cluster = page_idx - cluster.first_page_idx;
+      page = cluster.first_page; 
+      for (u64 j = 0; j < idx_in_cluster; ++j) {
+        page = page->next;
+      }
+      break;
+    }
+  }
+  if (!page) {
+    // page was not in the first N - 1 clusters, so it must be in the last one.
+    const Cluster_Info &cluster = app.rndata.clusters[app.rndata.n_clusters - 1];
+    assert(page_idx >= cluster.first_page_idx);
+    u64 idx_in_cluster = page_idx - cluster.first_page_idx;
+    page = cluster.first_page; 
+    for (u64 j = 0; j < idx_in_cluster; ++j) {
+      page = page->next;
+    }
   }
 
   app.viewer.latest_page_gone_to = page_idx;