pre-commit hook for linting only modified lines of cpp files with clang-format

pre-commit

#!/usr/bin/python3

# Author: julianknodt
# -- Description
# A git pre-commit hook that will run clang-format on only modified lines.

import subprocess
import os


def main():
  DRY = False
  extensions = [".cpp", ".h", ".hpp", ".cxx"]

  out = subprocess.check_output(["git", "diff", "--cached", "-U0"])
  out = out.decode('utf-8')
  curr_file = None

  result = 0
  for l in out.splitlines():
    if l.startswith("diff --git"):
      file_path = l.split()[-1]
      assert(file_path.startswith("b/"))
      curr_file = file_path[2:]
    if l.startswith("@@"):
      assert(curr_file is not None)
      if not any(curr_file.endswith(e) for e in extensions): continue

      new = l.split()[2]
      assert(new.startswith("+"))
      start, count = new[1:].split(",")
      cmds = [
        f"clang-format -i {curr_file} --lines={start}:{start+count}",
        f"git add {curr_file}",
      ]
      run = print if DRY else os.system
      for c in cmds: result |= (run(c) or 0)
  exit(result)

if __name__ == "__main__": main()