frontend: always calculate the snippet indexes from the beginning

We have been collecting incorrect indexes. They aren't calculated from
the end of the log but rather from the end of the previous node.

I am currently working on fixing our collected data.

frontend/src/app/contribute_events.cljs

@@ -4,7 +4,8 @@
    [lambdaisland.fetch :as fetch]
    [app.helpers :refer
     [current-path
-     remove-trailing-slash]]
+     remove-trailing-slash
+     previous-siblings]]
    [app.editor.core :refer [active-file]]
    [app.contribute-logic :refer
     [file-id
@@ -88,8 +89,26 @@
 
     (let [selection (.getSelection js/window)
           content (.toString selection)
+
+          ;; The position is calculated from the end of the last node
+          ;; This can be be either a previous snippet span or if the text
+          ;; longer than 65536 characters than it is implictily split into
+          ;; multiple sibling text nodes
           start (.-anchorOffset selection)
+
+          ;; Calculate the real starting index from the beginning of the log
+          offset (->> selection
+                      .-anchorNode
+                      previous-siblings
+                      (map #(.-textContent %))
+                      (map #(count %))
+                      (reduce +))
+          start (+ start offset)
+
+          ;; Index of the last snippet character. When parsing in python, don't
+          ;; forget to do text[start:end+1]
           end (+ start (count content) -1)
+
           snippet
           {:text content
            :start-index start

frontend/src/app/helpers.cljs

@@ -10,3 +10,9 @@
 
 (defn remove-trailing-slash [text]
   (clojure.string/replace text #"/$" ""))
+
+(defn previous-siblings [node]
+  (let [sibling (.-previousSibling node)]
+    (if-not sibling
+      []
+      (conj (previous-siblings sibling) sibling))))