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 .batch import BatchController
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))

        category_remainders = CategoryController.user_remainders(user)
        product_remainders = ProductController.user_remainders(user)

        passed_limits = set(
            product
            for product in all_products
            if category_remainders[product.category.id] > 0
            if product_remainders[product.id] > 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
    @BatchController.memoise
    def user_remainders(cls, user):
        '''

        Return:
            Mapping[int->int]: A dictionary that maps the product ID to the
            user's remainder for that product.
        '''

        products = inventory.Product.objects.all()

        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 dict((product.id, product.remainder) for product in products)