from pathlib import Path

from PIL import Image


def unique_output_path(path: Path) -> Path:
    if not path.exists():
        return path
    stem, suffix = path.stem, path.suffix
    n = 1
    while True:
        candidate = path.with_name(f"{stem}_{n}{suffix}")
        if not candidate.exists():
            return candidate
        n += 1


def convert_image(path: Path, max_width: int, quality: int = 85) -> Path:
    """Resize (only if wider than max_width) and convert an image to WebP,
    deleting the original. Returns the new file path. Raises on failure —
    caller decides how to handle/log."""
    with Image.open(path) as img:
        if img.width > max_width:
            ratio = max_width / img.width
            new_size = (max_width, round(img.height * ratio))
            img = img.resize(new_size, Image.LANCZOS)
        output = unique_output_path(path.with_suffix(".webp"))
        img.save(output, "WEBP", quality=quality, method=6)
    path.unlink()
    return output