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import contextlib as cl
import functools
import itertools as it
import operator
import os.path as op
import sqlite3
import tempfile
import time

from .core import DEFAULT_SETTINGS, ENOVAL, Cache, Disk, Timeout
from .persistent import Deque, Index


class FanoutCache:
    """Cache that shards keys and values."""

    def __init__(
        self, directory=None, shards=8, timeout=0.010, disk=Disk, **settings
    ):
        """Initialize cache instance.

        :param str directory: cache directory
        :param int shards: number of shards to distribute writes
        :param float timeout: SQLite connection timeout
        :param disk: `Disk` instance for serialization
        :param settings: any of `DEFAULT_SETTINGS`

        """
        if directory is None:
            directory = tempfile.mkdtemp(prefix='diskcache-')
        directory = str(directory)
        directory = op.expanduser(directory)
        directory = op.expandvars(directory)

        default_size_limit = DEFAULT_SETTINGS['size_limit']
        size_limit = settings.pop('size_limit', default_size_limit) / shards

        self._count = shards
        self._directory = directory
        self._disk = disk
        self._shards = tuple(
            Cache(
                directory=op.join(directory, '%03d' % num),
                timeout=timeout,
                disk=disk,
                size_limit=size_limit,
                **settings,
            )
            for num in range(shards)
        )
        self._hash = self._shards[0].disk.hash
        self._caches = {}
        self._deques = {}
        self._indexes = {}

    @property
    def directory(self):
        """Cache directory."""
        return self._directory

    def __getattr__(self, name):
        safe_names = {'timeout', 'disk'}
        valid_name = name in DEFAULT_SETTINGS or name in safe_names
        assert valid_name, 'cannot access {} in cache shard'.format(name)
        return getattr(self._shards[0], name)

    @cl.contextmanager
    def transact(self, retry=True):
        """Context manager to perform a transaction by locking the cache.

        While the cache is locked, no other write operation is permitted.
        Transactions should therefore be as short as possible. Read and write
        operations performed in a transaction are atomic. Read operations may
        occur concurrent to a transaction.

        Transactions may be nested and may not be shared between threads.

        Blocks until transactions are held on all cache shards by retrying as
        necessary.

        >>> cache = FanoutCache()
        >>> with cache.transact():  # Atomically increment two keys.
        ...     _ = cache.incr('total', 123.4)
        ...     _ = cache.incr('count', 1)
        >>> with cache.transact():  # Atomically calculate average.
        ...     average = cache['total'] / cache['count']
        >>> average
        123.4

        :return: context manager for use in `with` statement

        """
        assert retry, 'retry must be True in FanoutCache'
        with cl.ExitStack() as stack:
            for shard in self._shards:
                shard_transaction = shard.transact(retry=True)
                stack.enter_context(shard_transaction)
            yield

    def set(self, key, value, expire=None, read=False, tag=None, retry=False):
        """Set `key` and `value` item in cache.

        When `read` is `True`, `value` should be a file-like object opened
        for reading in binary mode.

        If database timeout occurs then fails silently unless `retry` is set to
        `True` (default `False`).

        :param key: key for item
        :param value: value for item
        :param float expire: seconds until the key expires
            (default None, no expiry)
        :param bool read: read value as raw bytes from file (default False)
        :param str tag: text to associate with key (default None)
        :param bool retry: retry if database timeout occurs