add a discounter function to the product model

uncloud_django_based/uncloud/uncloud_pay/models.py

@@ -28,9 +28,9 @@ class RecurringPeriod(models.TextChoices):
     ONE_TIME   = 'ONCE', _('Onetime')
     PER_YEAR   = 'YEAR', _('Per Year')
     PER_MONTH  = 'MONTH', _('Per Month')
-    PER_MINUTE = 'MINUTE', _('Per Minute')
     PER_DAY    = 'DAY', _('Per Day')
     PER_HOUR   = 'HOUR', _('Per Hour')
+    PER_MINUTE = 'MINUTE', _('Per Minute')
     PER_SECOND = 'SECOND', _('Per Second')
 
 
@@ -456,6 +456,10 @@ class Product(UncloudModel):
                               editable=False,
                               null=True)
 
+    # Default period for all products
+    default_recurring_period = RecurringPeriod.PER_MONTH
+
+
     @property
     def recurring_price(self, recurring_period=RecurringPeriod.PER_MONTH):
         pass # To be implemented in child.
@@ -474,3 +478,72 @@ class Product(UncloudModel):
 
     class Meta:
         abstract = True
+
+    def discounted_price_by_period(self, requested_period):
+        """
+        Each product has a standard recurring period for which
+        we define a pricing. I.e. VPN is usually year, VM is usually monthly.
+
+        The user can opt-in to use a different period, which influences the price:
+        The longer a user commits, the higher the discount.
+
+        Products can also be limited in the available periods. For instance
+        a VPN only makes sense to be bought for at least one day.
+
+        Rules are as follows:
+
+        given a standard recurring period of ..., changing to ... modifies price ...
+
+
+        # One month for free if buying / year, compared to a month: about 8.33% discount
+        per_year -> per_month -> /11
+        per_month -> per_year -> *11
+
+        # Month has 30.42 days on average. About 7.9% discount to go monthly
+        per_month -> per_day    -> /28
+        per_day   -> per_month  -> *28
+
+        # Day has 24h, give one for free
+        per_day   -> per_hour   -> /23
+        per_hour   -> per_day   -> /23
+
+
+        Examples
+
+        VPN @ 120CHF/y becomes
+        - 10.91 CHF/month (130.91 CHF/year)
+        - 0.39 CHF/day (142.21 CHF/year)
+
+        VM @ 15 CHF/month becomes
+        - 165 CHF/month (13.75 CHF/month)
+        - 0.54 CHF/day (16.30 CHF/month)
+
+        """
+
+
+        if self.default_recurring_period == RecurringPeriod.PER_YEAR:
+            if requested_period == RecurringPeriod.PER_YEAR:
+                return self.recurring_price
+            if requested_period == RecurringPeriod.PER_MONTH:
+                return self.recurring_price/11.
+            if requested_period == RecurringPeriod.PER_DAY:
+                return self.recurring_price/11./28.
+
+        elif self.default_recurring_period == RecurringPeriod.PER_MONTH:
+            if requested_period == RecurringPeriod.PER_YEAR:
+                return self.recurring_price*11
+            if requested_period == RecurringPeriod.PER_MONTH:
+                return self.recurring_price
+            if requested_period == RecurringPeriod.PER_DAY:
+                return self.recurring_price/28.
+
+        elif self.default_recurring_period == RecurringPeriod.PER_DAY:
+            if requested_period == RecurringPeriod.PER_YEAR:
+                return self.recurring_price*11*28
+            if requested_period == RecurringPeriod.PER_MONTH:
+                return self.recurring_price*28
+            if requested_period == RecurringPeriod.PER_DAY:
+                return self.recurring_price
+        else:
+            # FIXME: use the right type of exception here!
+            raise Exception("Did not implement the discounter for this case")