# swap_link.py
# Replace a link file with another one pointing to a new target
#

from pathlib import Path
from os.path import relpath # won't be necessary with Python 3.12 thanks to walk_up arg in relative_to()

def swap_link(lnk, tgt):
    #lnk: symbolic link to swap, str
    #tgt: target for new link, str
    lnpath = Path(lnk)
    if not lnpath.is_symlink():
        raise Exception("First argument is not a symbolic link")
    elif lnpath.exists():
        raise Exception("Link is not broken")

    tpath = Path(tgt)
    if tpath.is_symlink():
        if not tpath.exists():
            raise Exception("Target is also a broken link!")
        elif tpath.readlink().resolve() == lnpath.resolve() :
            raise Exception("Target is a link to link to be fixed...")
    
    tmp_suffix = 0
    tmp_prefix = 'temp'
    tmp_name = tmp_prefix + str(tmp_suffix)
    tmp_path = Path(lnpath.parent / tmp_name)
    while tmp_path.exists() or tmp_path.is_symlink():
        tmp_suffix += 1
        tmp_name = tmp_prefix + str(tmp_suffix)
        tmp_path = Path(lnk.parent / tmp_name)

    sym_path = relpath(tgt, lnk)[3:]

    try:
        tmp_path.symlink_to(sym_path)
    except Exception:
        print(f"Attempted to create tmp link: {tmp_name}")
        raise Exception("Failed to create new symlink. Check permissions!")

    try:
        lnpath.unlink()
    except Exception:
        tmp_path.unlink()
        raise Exception("Failed to remove broken link. Check permissions!")
    
    tmp_path.rename(lnk)