from decimal import Decimal
from django.db import models
from django.utils import timezone


class DiscountCalculator:
    """
    Калькулятор скидок для заказов.
    Рассчитывает применимые скидки и их суммы.
    """

    @staticmethod
    def get_available_discounts(scope=None, customer=None, auto_only=False):
        """
        Получить список доступных скидок.

        Args:
            scope: 'order', 'product', 'category' или None для всех
            customer: Customer для проверки условий
            auto_only: Только автоматические скидки

        Returns:
            QuerySet[Discount]: Активные скидки, отсортированные по приоритету
        """
        from discounts.models import Discount

        now = timezone.now()
        qs = Discount.objects.filter(is_active=True)

        # Фильтр по scope
        if scope:
            qs = qs.filter(scope=scope)

        # Фильтр по auto
        if auto_only:
            qs = qs.filter(is_auto=True)

        # Фильтр по дате
        qs = qs.filter(
            models.Q(start_date__isnull=True) | models.Q(start_date__lte=now),
            models.Q(end_date__isnull=True) | models.Q(end_date__gte=now)
        )

        # Фильтр по лимиту использований
        qs = qs.filter(
            models.Q(max_usage_count__isnull=True) |
            models.Q(current_usage_count__lt=models.F('max_usage_count'))
        )

        return qs.order_by('-priority', '-created_at')

    @staticmethod
    def calculate_order_discount(order, promo_code=None):
        """
        Рассчитать скидку на весь заказ.

        Args:
            order: Order объект
            promo_code: Строка промокода (опционально)

        Returns:
            dict: {
                'discount': Discount или None,
                'promo_code': PromoCode или None,
                'discount_amount': Decimal,
                'error': str или None
            }
        """
        from discounts.models import PromoCode
        from discounts.services.validator import DiscountValidator

        subtotal = Decimal(str(order.subtotal))
        result = {
            'discount': None,
            'promo_code': None,
            'discount_amount': Decimal('0'),
            'error': None
        }

        # 1. Проверяем промокод первым
        if promo_code:
            is_valid, promo, error = DiscountValidator.validate_promo_code(
                promo_code, order.customer, subtotal
            )

            if not is_valid:
                result['error'] = error
                return result

            discount = promo.discount

            # Проверяем scope скидки
            if discount.scope != 'order':
                result['error'] = "Этот промокод применяется только к товарам"
                return result

            result['discount'] = discount
            result['promo_code'] = promo
            result['discount_amount'] = discount.calculate_discount_amount(subtotal)
            return result

        # 2. Если нет промокода, проверяем автоматические скидки
        auto_discounts = DiscountCalculator.get_available_discounts(
            scope='order',
            auto_only=True
        )

        for discount in auto_discounts:
            # Проверяем мин. сумму заказа
            if discount.min_order_amount and subtotal < discount.min_order_amount:
                continue

            # Применяем первую подходящую автоматическую скидку
            result['discount'] = discount
            result['discount_amount'] = discount.calculate_discount_amount(subtotal)
            break

        return result

    @staticmethod
    def calculate_item_discount(order_item, available_discounts=None):
        """
        Рассчитать скидку на позицию заказа.

        Args:
            order_item: OrderItem объект
            available_discounts: Предварительно полученный список скидок

        Returns:
            dict: {
                'discount': Discount или None,
                'discount_amount': Decimal,
            }
        """
        result = {
            'discount': None,
            'discount_amount': Decimal('0')
        }

        # Определяем продукт
        product = None
        if order_item.product:
            product = order_item.product
        elif order_item.product_kit:
            product = order_item.product_kit

        if not product:
            return result

        base_amount = Decimal(str(order_item.price)) * Decimal(str(order_item.quantity))

        if not available_discounts:
            available_discounts = DiscountCalculator.get_available_discounts(
                scope='product',
                auto_only=True
            )

        for discount in available_discounts:
            # Проверяем, применяется ли скидка к этому товару
            if not discount.applies_to_product(product):
                continue

            result['discount'] = discount
            result['discount_amount'] = discount.calculate_discount_amount(base_amount)
            break

        # Проверяем скидки по категориям
        if not result['discount']:
            category_discounts = DiscountCalculator.get_available_discounts(
                scope='category',
                auto_only=True
            )

            for discount in category_discounts:
                if discount.applies_to_product(product):
                    result['discount'] = discount
                    result['discount_amount'] = discount.calculate_discount_amount(base_amount)
                    break

        return result

    @staticmethod
    def calculate_cart_discounts(cart_items, promo_code=None, customer=None):
        """
        Рассчитать скидки для корзины (применяется в POS до создания заказа).

        Args:
            cart_items: Список словарей {'type': 'product'|'kit', 'id': int, 'quantity': Decimal, 'price': Decimal}
            promo_code: Промокод (опционально)
            customer: Customer (опционально)

        Returns:
            dict: {
                'order_discount': {...},  # Как в calculate_order_discount
                'item_discounts': [
                    {'cart_index': int, 'discount': Discount, 'discount_amount': Decimal},
                    ...
                ],
                'total_discount': Decimal,
                'final_total': Decimal
            }
        """
        from products.models import Product, ProductKit

        cart_subtotal = Decimal('0')
        for item in cart_items:
            cart_subtotal += Decimal(str(item['price'])) * Decimal(str(item['quantity']))

        # Создаем фейковый объект для расчета скидки на заказ
        class FakeOrder:
            def __init__(self, subtotal, customer):
                self.subtotal = subtotal
                self.customer = customer

        fake_order = FakeOrder(cart_subtotal, customer)

        # Скидка на заказ
        order_discount = DiscountCalculator.calculate_order_discount(
            fake_order, promo_code
        )

        # Скидки на позиции
        item_discounts = []
        items_total_discount = Decimal('0')

        available_product_discounts = list(DiscountCalculator.get_available_discounts(
            scope='product',
            auto_only=True
        ))

        available_category_discounts = list(DiscountCalculator.get_available_discounts(
            scope='category',
            auto_only=True
        ))

        for idx, item in enumerate(cart_items):
            # Загружаем продукт
            product = None
            if item.get('type') == 'product':
                try:
                    product = Product.objects.get(id=item['id'])
                except Product.DoesNotExist:
                    pass
            elif item.get('type') == 'kit':
                try:
                    product = ProductKit.objects.get(id=item['id'])
                except ProductKit.DoesNotExist:
                    pass

            if not product:
                continue

            base_amount = Decimal(str(item['price'])) * Decimal(str(item['quantity']))

            # Проверяем скидки на товары
            applied_discount = None
            discount_amount = Decimal('0')

            for discount in available_product_discounts:
                if discount.applies_to_product(product):
                    applied_discount = discount
                    discount_amount = discount.calculate_discount_amount(base_amount)
                    break

            # Если не нашли скидку на товар, проверяем категории
            if not applied_discount:
                for discount in available_category_discounts:
                    if discount.applies_to_product(product):
                        applied_discount = discount
                        discount_amount = discount.calculate_discount_amount(base_amount)
                        break

            if applied_discount:
                item_discounts.append({
                    'cart_index': idx,
                    'discount': applied_discount,
                    'discount_amount': discount_amount
                })
                items_total_discount += discount_amount

        total_discount = order_discount['discount_amount'] + items_total_discount
        final_total = max(cart_subtotal - total_discount, Decimal('0'))

        return {
            'order_discount': order_discount,
            'item_discounts': item_discounts,
            'total_discount': total_discount,
            'final_total': final_total
        }