diff --git a/cc-pic.py b/cc-pic.py
index ba9dcf2..1cfe32b 100644
--- a/cc-pic.py
+++ b/cc-pic.py
@@ -55,7 +55,7 @@ class Converter:
         brightest_i, brightest_l = 0, 0
         darkest_i, darkest_l = 0, 768
         for oy, line in enumerate(self.PIX_BITS):
-            for ox, bit in enumerate(line):
+            for ox in range(len(line)):
                 pix = self._imgdata[x + ox, y + oy]
                 brightness = self._brightness(pix)
                 if brightness > brightest_l:
@@ -64,6 +64,7 @@ class Converter:
                     darkest_l, darkest_i = brightness, pix
         return darkest_i, brightest_i
 
+    @lru_cache()
     def _is_darker(self, bg: int, fg: int, c: int) -> bool:
         return self._distance(bg, c) < self._distance(fg, c)