import itertools

from django.db.models import Case
from django.db.models import F, Q
from django.db.models import Sum
from django.db.models import When
from django.db.models import Value

from registrasion.models import commerce
from registrasion.models import inventory

from .category import CategoryController
from .flag import FlagController


class ProductController(object):

    def __init__(self, product):
        self.product = product

    @classmethod
    def available_products(cls, user, category=None, products=None):
        ''' Returns a list of all of the products that are available per
        flag conditions from the given categories. '''
        if category is None and products is None:
            raise ValueError("You must provide products or a category")

        if category is not None:
            all_products = inventory.Product.objects.filter(category=category)
            all_products = all_products.select_related("category")
        else:
            all_products = []

        if products is not None:
            all_products = set(itertools.chain(all_products, products))

        categories = set(product.category for product in all_products)
        r = CategoryController.attach_user_remainders(user, categories)
        cat_quants = dict((c,c) for c in r)

        r = ProductController.attach_user_remainders(user, all_products)
        prod_quants = dict((p,p) for p in r)

        passed_limits = set(
            product
            for product in all_products
            if cat_quants[product.category].remainder > 0
            if prod_quants[product].remainder > 0
        )

        failed_and_messages = FlagController.test_flags(
            user, products=passed_limits
        )
        failed_conditions = set(i[0] for i in failed_and_messages)

        out = list(passed_limits - failed_conditions)
        out.sort(key=lambda product: product.order)

        return out


    @classmethod
    def attach_user_remainders(cls, user, products):
        '''

        Return:
            queryset(inventory.Product): A queryset containing items from
            ``product``, with an extra attribute -- remainder = the amount of
            this item that is remaining.
        '''

        ids = [product.id for product in products]
        products = inventory.Product.objects.filter(id__in=ids)

        cart_filter = (
            Q(productitem__cart__user=user) &
            Q(productitem__cart__status=commerce.Cart.STATUS_PAID)
        )

        quantity = When(
            cart_filter,
            then='productitem__quantity'
        )

        quantity_or_zero = Case(
            quantity,
            default=Value(0),
        )

        remainder = Case(
            When(limit_per_user=None, then=Value(99999999)),
            default=F('limit_per_user') - Sum(quantity_or_zero),
        )

        products = products.annotate(remainder=remainder)

        return products

    def user_quantity_remaining(self, user):
        ''' Returns the quantity of this product that the user add in the
        current cart. '''

        with_remainders = self.attach_user_remainders(user, [self.product])

        return with_remainders[0].remainder