Do not rewrite file unless necessary

pre_commit_hooks/end_of_file_fixer.py

@@ -1,51 +1,43 @@
 import argparse
-import os
-from typing import IO
 from typing import Optional
 from typing import Sequence
 
 
-def fix_file(file_obj: IO[bytes]) -> int:
+def _process_file(file_obj: bytes) -> bytes:
     # Test for newline at end of file
     # Empty files will throw IOError here
-    try:
-        file_obj.seek(-1, os.SEEK_END)
-    except OSError:
-        return 0
-    last_character = file_obj.read(1)
-    # last_character will be '' for an empty file
-    if last_character not in {b'\n', b'\r'} and last_character != b'':
-        # Needs this seek for windows, otherwise IOError
-        file_obj.seek(0, os.SEEK_END)
-        file_obj.write(b'\n')
-        return 1
 
-    while last_character in {b'\n', b'\r'}:
-        # Deal with the beginning of the file
-        if file_obj.tell() == 1:
-            # If we've reached the beginning of the file and it is all
-            # linebreaks then we can make this file empty
-            file_obj.seek(0)
-            file_obj.truncate()
-            return 1
+    while len(file_obj):
+        if file_obj[-2:] == b'\r\n':
+            if len(file_obj) == 2:
+                return b''
+            elif file_obj[-3:-2] not in {b'\n', b'\r'}:
+                return file_obj
+            else:
+                file_obj = file_obj[:-2]
+        elif file_obj[-1:] in {b'\n', b'\r'}:
+            if len(file_obj) == 1:
+                return b''
+            elif file_obj[-2:-1] not in {b'\n', b'\r'}:
+                return file_obj
+            else:
+                file_obj = file_obj[:-1]
+        else:
+            return file_obj + b'\n'
 
-        # Go back two bytes and read a character
-        file_obj.seek(-2, os.SEEK_CUR)
-        last_character = file_obj.read(1)
+    return file_obj
 
-    # Our current position is at the end of the file just before any amount of
-    # newlines.  If we find extraneous newlines, then backtrack and trim them.
-    position = file_obj.tell()
-    remaining = file_obj.read()
-    for sequence in (b'\n', b'\r\n', b'\r'):
-        if remaining == sequence:
-            return 0
-        elif remaining.startswith(sequence):
-            file_obj.seek(position + len(sequence))
-            file_obj.truncate()
-            return 1
 
-    return 0
+def _fix_file(filename: str) -> bool:
+    with open(filename, mode='rb') as file_processed:
+        file_content = file_processed.read()
+    newcontent = _process_file(file_content)
+    if newcontent != file_content:
+        with open(filename, mode='wb') as file_processed:
+            file_processed.write(newcontent)
+        return True
+    else:
+        return False
 
 
 def main(argv: Optional[Sequence[str]] = None) -> int:
@@ -57,11 +49,9 @@ def main(argv: Optional[Sequence[str]] = None) -> int:
 
     for filename in args.filenames:
         # Read as binary so we can read byte-by-byte
-        with open(filename, 'rb+') as file_obj:
-            ret_for_file = fix_file(file_obj)
-            if ret_for_file:
-                print(f'Fixing {filename}')
-            retv |= ret_for_file
+        if _fix_file(filename):
+            print(f'Fixing {filename}')
+            retv = 1
 
     return retv