[plugin] Exporter: optimized JSON export (#8904)

I've implemented a better JSON export format that removes redundant lists and objects and introduces the `entries` key containing all the entries.

It also add `drawer` values from highlight so that user can use this piece of metadata to generate desirable output.

plugins/exporter.koplugin/clip.lua

@@ -256,6 +256,7 @@ function MyClipping:parseHighlight(highlights, bookmarks, book)
             clipping.time = self:getTime(item.datetime or "")
             clipping.text = self:getText(item.text)
             clipping.chapter = item.chapter
+            clipping.drawer = item.drawer
             for _, bookmark in pairs(bookmarks) do
                 if bookmark.datetime == item.datetime and bookmark.text then
                     local bookmark_quote = bookmark.text:match(pattern)

plugins/exporter.koplugin/main.lua

@@ -609,10 +609,47 @@ function Exporter:exportBooknotesToHTML(title, booknotes)
     end
 end
 
+function Exporter:prepareBooknotesForJSON(booknotes)
+    local exportable = {
+            title = booknotes.title,
+            author = booknotes.author,
+            entries = {},
+            exported = booknotes.exported,
+            file = booknotes.file
+    }
+    for _, entry in ipairs(booknotes) do
+        table.insert(exportable.entries, entry[1])
+    end
+    return exportable
+end
+
+-- This function should handle both multidocument export and single exports.
+-- For Single Exports, it will create a JSON file with a object ({}) as root node.
+-- For Multidocument export, it will create a JSON file with an array ([]) as root node.
+function Exporter:exportToJSON(clippings)
+    local file = io.open(self.json_clipping_file, "a")
+    if file then
+        local exportable = {}
+        if table.getn(clippings) == 1 then
+            -- We will handle single document export here.
+            exportable = self:prepareBooknotesForJSON(clippings[0])
+        else
+            for _, booknotes in ipairs(clippings) do
+                table.insert(exportable, self:prepareBooknotesForJSON(booknotes))
+            end
+        end
+        file:write(json.encode(exportable))
+        file:write("\n")
+        file:close()
+    end
+end
+
 function Exporter:exportBooknotesToJSON(title, booknotes)
+    logger.dbg("booknotes", booknotes)
     local file = io.open(self.json_clipping_file, "a")
     if file then
-        file:write(json.encode(booknotes))
+        local exportable = self:prepareBooknotesForJSON(booknotes)
+        file:write(json.encode(exportable))
         file:write("\n")
         file:close()
     end