"Database cache backend."

from django.core.cache.backends.base import BaseCache

from django.db import connection, transaction, DatabaseError

import base64, time

from datetime import datetime

try:

    import cPickle as pickle

except ImportError:

    import pickle

class CacheClass(BaseCache):

    def __init__(self, table, params):

        BaseCache.__init__(self, params)

        self._table = table

        max_entries = params.get('max_entries', 300)

        try:

            self._max_entries = int(max_entries)

        except (ValueError, TypeError):

            self._max_entries = 300

        cull_frequency = params.get('cull_frequency', 3)

        try:

            self._cull_frequency = int(cull_frequency)

        except (ValueError, TypeError):

            self._cull_frequency = 3

    def get(self, key, default=None):

        cursor = connection.cursor()

        cursor.execute("SELECT cache_key, value, expires FROM %s WHERE cache_key = %%s" % self._table, [key])

        row = cursor.fetchone()

        if row is None:

            return default

        now = datetime.now()

        if row[2] < now:

            cursor.execute("DELETE FROM %s WHERE cache_key = %%s" % self._table, [key])

            transaction.commit_unless_managed()

            return default

        return pickle.loads(base64.decodestring(row[1]))

    def set(self, key, value, timeout=None):

        self._base_set('set', key, value, timeout)

    def add(self, key, value, timeout=None):

        return self._base_set('add', key, value, timeout)

    def _base_set(self, mode, key, value, timeout=None):

        if timeout is None:

            timeout = self.default_timeout

        cursor = connection.cursor()

        cursor.execute("SELECT COUNT(*) FROM %s" % self._table)

        num = cursor.fetchone()[0]

        now = datetime.now().replace(microsecond=0)

        exp = datetime.fromtimestamp(time.time() + timeout).replace(microsecond=0)

        if num > self._max_entries:

            self._cull(cursor, now)

        encoded = base64.encodestring(pickle.dumps(value, 2)).strip()

        cursor.execute("SELECT cache_key FROM %s WHERE cache_key = %%s" % self._table, [key])

        try:

            if mode == 'set' and cursor.fetchone():

                cursor.execute("UPDATE %s SET value = %%s, expires = %%s WHERE cache_key = %%s" % self._table, [encoded, str(exp), key])

            else:

                cursor.execute("INSERT INTO %s (cache_key, value, expires) VALUES (%%s, %%s, %%s)" % self._table, [key, encoded, str(exp)])

        except DatabaseError:

            # To be threadsafe, updates/inserts are allowed to fail silently

            return False

        else:

            transaction.commit_unless_managed()

            return True

    def delete(self, key):

        cursor = connection.cursor()

        cursor.execute("DELETE FROM %s WHERE cache_key = %%s" % self._table, [key])

        transaction.commit_unless_managed()

    def has_key(self, key):

        cursor = connection.cursor()

        cursor.execute("SELECT cache_key FROM %s WHERE cache_key = %%s" % self._table, [key])

        return cursor.fetchone() is not None

    def _cull(self, cursor, now):

        if self._cull_frequency == 0:

            cursor.execute("DELETE FROM %s" % self._table)

        else:

            cursor.execute("DELETE FROM %s WHERE expires < %%s" % self._table, [str(now)])

            cursor.execute("SELECT COUNT(*) FROM %s" % self._table)

            num = cursor.fetchone()[0]

            if num > self._max_entries:

                cursor.execute("SELECT cache_key FROM %s ORDER BY cache_key LIMIT 1 OFFSET %%s" % self._table, [num / self._cull_frequency])

                cursor.execute("DELETE FROM %s WHERE cache_key < %%s" % self._table, [cursor.fetchone()[0]])