#!/usr/bin/env python3
"""
rename_to_first4.py

Usage:
    python3 rename_to_first4.py <directory>

Renames each file in <directory> so its name becomes the first
four characters of the original filename (before the extension).
Keeps the file extension and avoids overwriting existing files.
"""
import os
import sys
from pathlib import Path

def main():
    if len(sys.argv) != 2:
        print("Usage: rename_to_first4.py <directory>")
        sys.exit(1)

    directory = Path(sys.argv[1])
    if not directory.is_dir():
        print(f"Error: '{directory}' is not a directory.")
        sys.exit(1)

    for f in directory.iterdir():
        if not f.is_file():
            continue

        stem = f.stem[:4]  # first 4 letters of filename (no extension)
        new_name = f"{stem}{f.suffix}"
        new_path = directory / new_name

        # avoid overwriting existing files
        counter = 1
        while new_path.exists():
            new_name = f"{stem}_{counter}{f.suffix}"
            new_path = directory / new_name
            counter += 1

        print(f"Renaming: {f.name} -> {new_name}")
        f.rename(new_path)

    print("✅ Done renaming all files.")

if __name__ == "__main__":
    main()