# It would be nice if we could share the nix git cache, but as of the
# time of writing it is transitioning from gitv2 (deprecated) to gitv3
# (not ready yet), and trying to straddle them both is too far into nix
# implementation details for my comfort.  So we re-implement here half of
# nix's builtins.fetchGit.  :(

import argparse
import functools
import hashlib
import logging
import os
import subprocess
import sys
import time

from typing import Iterator, NamedTuple, Optional, TypeVar, Tuple, Union

import backoff

Path = str   # eg: "/home/user/.cache/git-cache/v1"
Repo = str   # eg: "https://github.com/NixOS/nixpkgs.git"
Ref = str    # eg: "master" or "v1.0.0"
Rev = str    # eg: "53a27350551844e1ed1a9257690294767389ef0d"
RefOrRev = Union[Ref, Rev]


class _LogEntry(NamedTuple):
    ref: Ref
    rev: Rev


T = TypeVar('T')


def _repo_hashname(repo: Repo) -> str:
    return hashlib.sha256(repo.encode()).hexdigest()


def git_cachedir(repo: Repo) -> Path:
    # Use xdg module when it's less painful to have as a dependency
    XDG_CACHE_HOME = Path(
        os.environ.get('XDG_CACHE_HOME', os.path.expanduser('~/.cache')))

    return Path(os.path.join(
        XDG_CACHE_HOME,
        'git-cache/v1',
        _repo_hashname(repo)))


def _log_filename(repo: Repo) -> Path:
    # Use xdg module when it's less painful to have as a dependency
    XDG_DATA_HOME = Path(
        os.environ.get('XDG_DATA_HOME', os.path.expanduser('~/.local/share')))

    return Path(os.path.join(
        XDG_DATA_HOME,
        'git-cache/v1',
        _repo_hashname(repo)))


def is_ancestor(repo: Repo, descendant: RefOrRev, ancestor: RefOrRev) -> bool:
    cachedir = git_cachedir(repo)
    logging.debug('Checking if %s is an ancestor of %s', ancestor, descendant)
    process = subprocess.run(['git',
                              '-C',
                              cachedir,
                              'merge-base',
                              '--is-ancestor',
                              ancestor,
                              descendant],
                             check=False)
    return process.returncode == 0


def verify_ancestry(
        repo: Repo,
        descendant: RefOrRev,
        ancestor: RefOrRev) -> None:
    if not is_ancestor(repo, descendant, ancestor):
        raise Exception('%s is not an ancestor of %s' % (ancestor, descendant))


def _read_fetch_log(repo: Repo) -> Iterator[_LogEntry]:
    filename = _log_filename(repo)
    if not os.path.exists(filename):
        return
    with open(filename, 'r') as f:
        for line in f:
            _, _, rev, ref = line.strip().split(maxsplit=3)
            yield _LogEntry(ref, rev)


def _last(it: Iterator[T]) -> Optional[T]:
    return functools.reduce(lambda a, b: b, it, None)


def _previous_fetched_rev(repo: Repo, ref: Ref) -> Optional[Rev]:
    return _last(entry.rev for entry in _read_fetch_log(
        repo) if entry.ref == ref)


def _log_fetch(repo: Repo, ref: Ref, rev: Rev) -> None:
    prev_rev = _previous_fetched_rev(repo, ref)
    if prev_rev is not None:
        verify_ancestry(repo, rev, prev_rev)
    filename = _log_filename(repo)
    os.makedirs(os.path.dirname(filename), exist_ok=True)
    with open(filename, 'a') as f:
        f.write('%s fetch %s %s\n' %
                (time.strftime('%Y-%m%d-%H:%M:%S%z'), rev, ref))


@backoff.on_exception(
    backoff.expo,
    subprocess.CalledProcessError,
    max_time=lambda: int(os.environ.get('BACKOFF_MAX_TIME', '30')))
def _git_fetch(cachedir: Path, repo: Repo, ref: Ref) -> None:
    # We don't use --force here because we want to abort and freak out if forced
    # updates are happening.
    subprocess.run(['git', '-C', cachedir, 'fetch', repo,
                    '%s:%s' % (ref, ref)], check=True)


def fetch(repo: Repo, ref: Ref) -> Tuple[Path, Rev]:
    cachedir = git_cachedir(repo)
    if not os.path.exists(cachedir):
        logging.debug("Initializing git repo")
        subprocess.run(['git', 'init', '--bare', cachedir],
                       check=True, stdout=sys.stderr)

    logging.debug('Fetching ref "%s" from %s', ref, repo)
    _git_fetch(cachedir, repo, ref)

    with open(os.path.join(cachedir, 'refs', 'heads', ref)) as rev_file:
        rev = Rev(rev_file.read(999).strip())
    verify_ancestry(repo, ref, rev)
    _log_fetch(repo, ref, rev)

    return cachedir, rev


def ensure_rev_available(repo: Repo, ref: Ref, rev: Rev) -> Path:
    cachedir = git_cachedir(repo)
    if os.path.exists(cachedir) and is_ancestor(repo, ref, rev):
        return cachedir

    logging.debug(
        'We do not have rev %s.  We will fetch ref "%s" and hope it appears.',
        rev, ref)
    fetch(repo, ref)
    logging.debug('Verifying that fetch retrieved rev %s', rev)
    subprocess.run(['git', '-C', cachedir, 'cat-file', '-e', rev], check=True)
    verify_ancestry(repo, ref, rev)

    return cachedir


def _main() -> None:
    parser = argparse.ArgumentParser(
        description='Cache remote git repositories locally.',
        epilog='example usage: git-cache https://github.com/NixOS/nixpkgs.git master')
    parser.add_argument(
        'repo',
        metavar='Repo',
        type=Repo,
        help='Git repository URL')
    parser.add_argument(
        'ref',
        metavar='Ref',
        type=Ref,
        help='Ref (branch or tag) in the git repo')
    parser.add_argument(
        'rev',
        metavar='Rev',
        type=Rev,
        nargs='?',
        help='Ensure that this revision is present.  ' +
        'If this revision is already present locally, no network operations are performed.')
    args = parser.parse_args()

    if args.rev is None:
        print('{1} {0}'.format(*fetch(args.repo, args.ref)))
    else:
        print(ensure_rev_available(args.repo, args.ref, args.rev))