import itertools
import operator

from pip._vendor.six.moves import collections_abc  # type: ignore

from pip._internal.utils.compat import lru_cache
from pip._internal.utils.typing import MYPY_CHECK_RUNNING

if MYPY_CHECK_RUNNING:
    from typing import Callable, Iterator, Optional, Set

    from pip._vendor.packaging.version import _BaseVersion

    from .base import Candidate


def _deduplicated_by_version(candidates):
    # type: (Iterator[Candidate]) -> Iterator[Candidate]
    returned = set()  # type: Set[_BaseVersion]
    for candidate in candidates:
        if candidate.version in returned:
            continue
        returned.add(candidate.version)
        yield candidate


def _insert_installed(installed, others):
    # type: (Candidate, Iterator[Candidate]) -> Iterator[Candidate]
    """Iterator for ``FoundCandidates``.

    This iterator is used when the resolver prefers to upgrade an
    already-installed package. Candidates from index are returned in their
    normal ordering, except replaced when the version is already installed.

    Since candidates from index are already sorted by reverse version order,
    `sorted()` here would keep the ordering mostly intact, only shuffling the
    already-installed candidate into the correct position. We put the already-
    installed candidate in front of those from the index, so it's put in front
    after sorting due to Python sorting's stableness guarentee.
    """
    candidates = sorted(
        itertools.chain([installed], others),
        key=operator.attrgetter("version"),
        reverse=True,
    )
    return iter(candidates)


class FoundCandidates(collections_abc.Sequence):
    """A lazy sequence to provide candidates to the resolver.

    The intended usage is to return this from `find_matches()` so the resolver
    can iterate through the sequence multiple times, but only access the index
    page when remote packages are actually needed. This improve performances
    when suitable candidates are already installed on disk.
    """
    def __init__(
        self,
        get_others,  # type: Callable[[], Iterator[Candidate]]
        installed,  # type: Optional[Candidate]
        prefers_installed,  # type: bool
    ):
        self._get_others = get_others
        self._installed = installed
        self._prefers_installed = prefers_installed

    def __getitem__(self, index):
        # type: (int) -> Candidate
        # Implemented to satisfy the ABC check. This is not needed by the
        # resolver, and should not be used by the provider either (for
        # performance reasons).
        raise NotImplementedError("don't do this")

    def __iter__(self):
        # type: () -> Iterator[Candidate]
        if not self._installed:
            candidates = self._get_others()
        elif self._prefers_installed:
            candidates = itertools.chain([self._installed], self._get_others())
        else:
            candidates = _insert_installed(self._installed, self._get_others())
        return _deduplicated_by_version(candidates)

    def __len__(self):
        # type: () -> int
        # Implemented to satisfy the ABC check. This is not needed by the
        # resolver, and should not be used by the provider either (for
        # performance reasons).
        raise NotImplementedError("don't do this")

    @lru_cache(maxsize=1)
    def __bool__(self):
        # type: () -> bool
        if self._prefers_installed and self._installed:
            return True
        return any(self)

    __nonzero__ = __bool__  # XXX: Python 2.