import subprocess
import json
import functools

nix_tarball_drv = "/nix/store/h0m0794x8d02hyrwkby3asajnz2s2g6n-nix-1.11.15.tar.xz.drv"

@functools.lru_cache()
def load_derivation(drv):
    data = subprocess.check_output(['nix', 'show-derivation', drv]).decode()
    return json.loads(data)[drv]

def get_drv_deps(drv):
    data = load_derivation(drv)
    return data['inputDrvs'].keys()

def is_fixed_output(drv):
    data = load_derivation(drv)
    try:
        data['outputs']['out']['hash']
        return True
    except KeyError:
        return False

def is_substitutable_or_built(drv):
    return subprocess.call(["nix-store", "-Q", "-j0", "--realise", drv]) == 0

def needed_fixed_output_drvs(drv, already_seen=None):
    "Outputs the unbuilt fixed-output-derivations needed to build this derivation, in topologically sorted order."
    print(drv)
    if already_seen is None:
        already_seen = set()
    if drv in already_seen:
        return []
    already_seen.add(drv)
    if is_substitutable_or_built(drv):
        return []
    ret = []
    for dep in get_drv_deps(drv):
        ret += needed_fixed_output_drvs(dep, already_seen)
    if is_fixed_output(drv):
        ret.append(drv)
    return ret