seek+tell and cached-seek optimisation

Author: arekbulski

kaitaistruct.py

@@ -95,18 +95,20 @@ def pos(self):
         return self._io.tell()
 
     def size(self):
+        if hasattr(self, 'size_cached'):
+            return self.size_cached
         # Python has no internal File object API function to get
         # current file / StringIO size, thus we use the following
         # trick.
         io = self._io
         # Remember our current position
         cur_pos = io.tell()
         # Seek to the end of the File object
-        io.seek(0, SEEK_END)
         # Remember position, which is equal to the full length
-        full_size = io.tell()
+        full_size = io.seek(0, SEEK_END)
         # Seek back to the current position
         io.seek(cur_pos)
+        self.size_cached = full_size
         return full_size
 
     # ========================================================================