highlighter: autostop co-routine when not needed (#881)

* highlighter: autostop co-routine when not needed

* applied @Guldoman suggestions

data/core/doc/highlighter.lua

@@ -10,18 +10,17 @@ local Highlighter = Object:extend()
 
 function Highlighter:new(doc)
   self.doc = doc
+  self.running = false
   self:reset()
+end
 
 -- init incremental syntax highlighting
+function Highlighter:start()
+  if self.running then return end
+  self.running = true
   core.add_thread(function()
-    while true do
+    while self.first_invalid_line < self.max_wanted_line do
-      if self.first_invalid_line > self.max_wanted_line then
-        self.max_wanted_line = 0
-        coroutine.yield(1 / config.fps)
-
-      else
       local max = math.min(self.first_invalid_line + 40, self.max_wanted_line)
-
       local retokenized_from
       for i = self.first_invalid_line, max do
         local state = (i > 1) and self.lines[i - 1].state
@@ -42,10 +41,18 @@ function Highlighter:new(doc)
       core.redraw = true
       coroutine.yield()
     end
-    end
+    self.max_wanted_line = 0
+    self.running = false
   end, self)
 end
 
+local function set_max_wanted_lines(self, amount)
+  self.max_wanted_line = amount
+  if self.first_invalid_line < self.max_wanted_line then
+    self:start()
+  end
+end
+
 
 function Highlighter:reset()
   self.lines = {}
@@ -62,7 +69,7 @@ end
 
 function Highlighter:invalidate(idx)
   self.first_invalid_line = math.min(self.first_invalid_line, idx)
-  self.max_wanted_line = math.min(self.max_wanted_line, #self.doc.lines)
+  set_max_wanted_lines(self, math.min(self.max_wanted_line, #self.doc.lines))
 end
 
 function Highlighter:insert_notify(line, n)
@@ -101,7 +108,7 @@ function Highlighter:get_line(idx)
     self.lines[idx] = line
     self:update_notify(idx, 0)
   end
-  self.max_wanted_line = math.max(self.max_wanted_line, idx)
+  set_max_wanted_lines(self, math.max(self.max_wanted_line, idx))
   return line
 end