From 2b33638a8dcf3f334225cd40f83533cca162f574 Mon Sep 17 00:00:00 2001
From: Martin Kavalar <martin@nextjournal.com>
Date: Mon, 22 May 2023 10:31:09 +0200
Subject: [PATCH] Improve render performance by only evaluating each render-fn
 once

---
 src/nextjournal/clerk/render.cljs |  5 ++++-
 src/nextjournal/clerk/view.clj    | 18 ++++++++++++++----
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/src/nextjournal/clerk/render.cljs b/src/nextjournal/clerk/render.cljs
index 86806eb0c..766e0be01 100644
--- a/src/nextjournal/clerk/render.cljs
+++ b/src/nextjournal/clerk/render.cljs
@@ -698,12 +698,15 @@
   (let [re-eval (fn [{:keys [form]}] (viewer/->viewer-fn form))]
     (w/postwalk (fn [x] (cond-> x (viewer/viewer-fn? x) re-eval)) doc)))
 
+(defn replace-viewer-fns [doc]
+  (w/postwalk-replace (:hash->viewer doc) (dissoc doc :hash->viewer)))
+
 (defn ^:export set-state! [{:as state :keys [doc]}]
   (when (contains? state :doc)
     (when (exists? js/window)
       ;; TODO: can we restore the scroll position when navigating back?
       (.scrollTo js/window #js {:top 0}))
-    (reset! !doc doc))
+    (reset! !doc (replace-viewer-fns doc)))
   ;; (when (and error (contains? @!doc :status))
   ;;   (swap! !doc dissoc :status))
   (when (remount? doc)
diff --git a/src/nextjournal/clerk/view.clj b/src/nextjournal/clerk/view.clj
index 6f4ea108e..9edf13c08 100644
--- a/src/nextjournal/clerk/view.clj
+++ b/src/nextjournal/clerk/view.clj
@@ -1,15 +1,25 @@
 (ns nextjournal.clerk.view
-  (:require [nextjournal.clerk.viewer :as v]
-            [hiccup.page :as hiccup]
+  (:require [clojure.java.io :as io]
+            [clojure.set :as set]
             [clojure.string :as str]
-            [clojure.java.io :as io])
+            [clojure.walk :as walk]
+            [hiccup.page :as hiccup]
+            [nextjournal.clerk.viewer :as v])
   (:import (java.net URI)))
 
+(defn ^:private extract-hash->viewer [presentation]
+  (into {}
+        (map (juxt :hash identity))
+        (keep :nextjournal/viewer (tree-seq (some-fn map? vector?) #(cond-> % (map? %) vals) presentation))))
+
 (defn doc->viewer
   ([doc] (doc->viewer {} doc))
   ([opts {:as doc :keys [ns file]}]
    (binding [*ns* ns]
-     (-> (merge doc opts) v/notebook v/present))))
+     (let [presentation (-> (merge doc opts) v/notebook v/present)
+           hash->viewer (extract-hash->viewer presentation)]
+       (assoc (walk/postwalk-replace (set/map-invert hash->viewer) presentation)
+              :hash->viewer hash->viewer)))))
 
 #_(doc->viewer (nextjournal.clerk/eval-file "notebooks/hello.clj"))
 #_(nextjournal.clerk/show! "notebooks/test.clj")