clean up decoding error reporting by counting lines instead of reading single...

clean up decoding error reporting by counting lines instead of reading single characters to find the error position

Cython/Compiler/Main.py

@@ -351,33 +351,30 @@ class Context(object):
         except UnicodeDecodeError, e:
             #import traceback
             #traceback.print_exc()
-            line = 1
-            column = 0
-            msg = e.args[-1]
-            position = e.args[2]
-            encoding = e.args[0]
-
-            with open(source_filename, "rb") as f:
-                byte_data = f.read()
-
-            # FIXME: make this at least a little less inefficient
-            for idx, c in enumerate(byte_data):
-                if c in (ord('\n'), '\n'):
-                    line += 1
-                    column = 0
-                if idx == position:
-                    break
-
-                column += 1
-
-            error((source_desc, line, column),
-                  "Decoding error, missing or incorrect coding=<encoding-name> "
-                  "at top of source (cannot decode with encoding %r: %s)" % (encoding, msg))
+            raise self._report_decode_error(source_desc, e)
 
         if Errors.num_errors > num_errors:
             raise CompileError()
         return tree
 
+    def _report_decode_error(self, source_desc, exc):
+        msg = exc.args[-1]
+        position = exc.args[2]
+        encoding = exc.args[0]
+
+        line = 1
+        column = idx = 0
+        with io.open(source_desc.filename, "r", encoding='iso8859-1', newline='') as f:
+            for line, data in enumerate(f, 1):
+                idx += len(data)
+                if idx >= position:
+                    column = position - (idx - len(data)) + 1
+                    break
+
+        return error((source_desc, line, column),
+                     "Decoding error, missing or incorrect coding=<encoding-name> "
+                     "at top of source (cannot decode with encoding %r: %s)" % (encoding, msg))
+
     def extract_module_name(self, path, options):
         # Find fully_qualified module name from the full pathname
         # of a source file.