from argparse import ArgumentParser
from itertools import filterfalse
from pathlib import Path
from logging import basicConfig, warning, WARNING
from shutil import copyfile

def main(music_folder: Path):
    # get the absolute path of the music folder from the argument
    music_folder = music_folder.resolve().absolute()

    # recursively traverse music_folder looking for .m3u files and, for each, ...
    for m3u in music_folder.rglob("*.m3u"):
        # create a copy in the same directory as a backup
        copyfile(m3u, m3u.with_suffix(".m3u.bak"))

        # read the contents of the file
        with m3u.open("r") as contents:
            lines = contents.readlines()
        
        # check that all non-comment lines refer to files in the music directory
        if not all(
            music_folder in Path(line).parents 
            if Path(line).is_absolute() 
            # join the parent path to the relative path in line, then resolve out the relative parts
            else music_folder in m3u.parent.joinpath(Path(line)).resolve().parents
            # ignore lines which start with # because they are comments, not paths to tracks
            for line in filterfalse(lambda l: l.strip().startswith("#"), lines)
        ):
            # if the file contains references to files outside of the music folder, skip the m3u file and continue to the next one
            warning(f"{m3u} contains references to tracks that are not in {music_folder} and will be skipped")
            continue
       
        with m3u.open("w") as updated_contents:
            for line in lines:
                # comment line.  just write it.
                if line.strip().startswith("#"):
                    updated_contents.write(line)
                # line contains an absolute path
                elif Path(line).is_absolute():
                    # get the path of the track file relative to the playlist file and write it in posix form (/ is path delimiter)
                    updated_contents.write(Path(line).relative_to(m3u, walk_up=True).as_posix())
                # line already contains a relative path
                else:
                    # normalize as a posix path
                    updated_contents.write(Path(line).as_posix())


if __name__ == "__main__":
    # setup logging to terminal
    basicConfig(
        level=WARNING,
    )
    
    # setup a parser for the CLI
    parser = ArgumentParser(
        description="Recursively traverse a music folder containing tracks and playlist files that reference them, replacing absolute paths in the playlists with equivalent relative paths."
    )

    parser.add_argument(
        "music_folder",
        required=True,
        type=Path
    )

    # parse the command line for the one and only argument
    args = parser.parse_args()

    main(args.music_folder)