from decimal import Decimal

from django.core.exceptions import ObjectDoesNotExist

from django.db.models import Avg, Min, Max, Sum

from registrasion import models as rego

from cart import CartController

class InvoiceController(object):

    def __init__(self, invoice):

        self.invoice = invoice

    @classmethod

    def for_cart(cls, cart):

        ''' Returns an invoice object for a given cart at its current revision.

        If such an invoice does not exist, the cart is validated, and if valid,

        an invoice is generated.'''

        try:

            invoice = rego.Invoice.objects.get(

                cart=cart, cart_revision=cart.revision)

        except ObjectDoesNotExist:

            cart_controller = CartController(cart)

            cart_controller.validate_cart() # Raises ValidationError on fail.

            invoice = cls._generate(cart)

        return InvoiceController(invoice)

    @classmethod

    def resolve_discount_value(cls, item):

        try:

            condition = rego.DiscountForProduct.objects.get(

                discount=item.discount,

                product=item.product

            )

        except ObjectDoesNotExist:

            condition = rego.DiscountForCategory.objects.get(

                discount=item.discount,

                category=item.product.category

            )

        if condition.percentage is not None:

            value = item.product.price * (condition.percentage / 100)

        else:

            value = condition.price

        return value

    @classmethod

    def _generate(cls, cart):

        ''' Generates an invoice for the given cart. '''

        invoice = rego.Invoice.objects.create(

            user=cart.user,

            cart=cart,

            cart_revision=cart.revision,

            value=Decimal()

        )

        invoice.save()

        # TODO: calculate line items.

        product_items = rego.ProductItem.objects.filter(cart=cart)

        discount_items = rego.DiscountItem.objects.filter(cart=cart)

        invoice_value = Decimal()

        for item in product_items:

            line_item = rego.LineItem.objects.create(

                invoice=invoice,

                description=item.product.name,

                quantity=item.quantity,

                price=item.product.price,

            )

            line_item.save()

            invoice_value += line_item.quantity * line_item.price

        for item in discount_items:

            line_item = rego.LineItem.objects.create(

                invoice=invoice,

                description=item.discount.description,

                quantity=item.quantity,

                price=cls.resolve_discount_value(item) * -1,

            )

            line_item.save()

            invoice_value += line_item.quantity * line_item.price

        # TODO: calculate line items from discounts

        invoice.value = invoice_value

        invoice.save()

        return invoice

    def is_valid(self):

        ''' Returns true if the attached invoice is not void and it represents

        a valid cart. '''

        if self.invoice.void:

            return False

        if self.invoice.cart is not None:

            if self.invoice.cart.revision != self.invoice.cart_revision:

                return False

        return True

    def void(self):

        ''' Voids the invoice. '''

        self.invoice.void = True

    def pay(self, reference, amount):

        ''' Pays the invoice by the given amount. If the payment

        equals the total on the invoice, finalise the invoice.

        (NB should be transactional.)

        '''

        if self.invoice.cart is not None:

            cart = CartController(self.invoice.cart)

            cart.validate_cart() # Raises ValidationError if invalid

        ''' Adds a payment '''

        payment = rego.Payment.objects.create(

            invoice=self.invoice,

            reference=reference,

            amount=amount,

        )

        payment.save()

        payments = rego.Payment.objects .filter(invoice=self.invoice)

        agg = payments.aggregate(Sum("amount"))

        total = agg["amount__sum"]

        if total==self.invoice.value:

            self.invoice.paid = True

            cart = self.invoice.cart

            cart.active = False

            cart.save()

            self.invoice.save()