diff --git a/src/cchardet/_cchardet.pyx b/src/cchardet/_cchardet.pyx
index db9be40..f03f1f9 100644
--- a/src/cchardet/_cchardet.pyx
+++ b/src/cchardet/_cchardet.pyx
@@ -9,21 +9,29 @@ cdef extern from "charsetdetect.h":
 
 def detect_with_confidence(char *msg):
     cdef csd_t csd = csd_open()
+
+    # すでにカウント済みの長さへアクセス
+    # strlenでは再度カウントすることになる
+    # https://github.com/python/cpython/blob/c30098c8c6014f3340a369a31df9c74bdbacc269/Include/bytesobject.h#L82
+    # https://github.com/python/cpython/blob/c30098c8c6014f3340a369a31df9c74bdbacc269/Objects/bytesobject.c#L2490
+    # https://github.com/python/cpython/blob/c30098c8c6014f3340a369a31df9c74bdbacc269/Include/object.h#L346
+    # https://github.com/python/cpython/blob/c30098c8c6014f3340a369a31df9c74bdbacc269/Objects/bytesobject.c#L2410
     cdef int length = len(msg)
+
     cdef int result = csd_consider(csd, msg, length)
     cdef float confidence = 0.0
     cdef const_char_ptr detected_charset
-    # ref: charsetdetect.cpp
+
     if result == 1: # Need more data
         detected_charset = csd_close2(csd, &confidence)
     elif result == 0: # Detected early
         detected_charset = csd_close2(csd, &confidence)
     else: # Error, signal with a negative number
         raise Exception("Error, signal with a negative number")
+
     if detected_charset:
         return detected_charset, confidence
-    else:
-        return None, None
+    return None, None
 
 cdef class Detector:
     cdef csd_t csd
@@ -70,5 +78,4 @@ cdef class Detector:
     def result(self):
         if len(self._detected_charset):
             return self._detected_charset, self._confidence
-        else:
-            return None, None
+        return None, None