Discounts: Fix edge case in computation (#4126)

* Add new test case for discounts:

Two discounts:
- "For every 1 item1, you get three item2 for 10 % off."
- "For every 1 item1, you get five item3 for 10 % off."
Cart: 2x item1, 2x item2, 6x item3
Expected result: 2x item1 full price, 2x item2 discounted, 5x item3 discounted, 1x item3 full price

* Fix discount calculation bug

* Update src/pretix/base/models/discount.py

Co-authored-by: Raphael Michel <michel@rami.io>

* Update src/pretix/base/models/discount.py

Co-authored-by: Richard Schreiber <schreiber@rami.io>

---------

Co-authored-by: Raphael Michel <michel@rami.io>
Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/base/models/discount.py

@@ -23,6 +23,7 @@
 from collections import defaultdict
 from decimal import Decimal
 from itertools import groupby
+from math import ceil
 from typing import Dict, Optional, Tuple
 
 from django.core.exceptions import ValidationError
@@ -272,7 +273,7 @@ class Discount(LoggedModel):
 
             # Prevent over-consuming of items, i.e. if our discount is "buy 2, get 1 free", we only
             # want to match multiples of 3
-            n_groups = min(len(condition_idx_group) // self.condition_min_count, len(benefit_idx_group))
+            n_groups = min(len(condition_idx_group) // self.condition_min_count, ceil(len(benefit_idx_group) / self.benefit_only_apply_to_cheapest_n_matches))
             consume_idx = condition_idx_group[:n_groups * self.condition_min_count]
             benefit_idx = benefit_idx_group[:n_groups * self.benefit_only_apply_to_cheapest_n_matches]
         else:

src/tests/base/test_pricing_discount.py

@@ -51,6 +51,11 @@ def item2(event):
     return event.items.create(name='Ticket II', default_price=Decimal('50.00'))
 
 
+@pytest.fixture
+def item3(event):
+    return event.items.create(name='Ticket III', default_price=Decimal('42.00'))
+
+
 @pytest.fixture
 def voucher(event):
     return event.vouchers.create()
@@ -1338,3 +1343,66 @@ def test_multiple_discounts_with_benefit_condition_overlap(event, item, item2):
 
     new_prices = [p for p, d in apply_discounts(event, 'web', positions)]
     assert sorted(new_prices) == sorted(expected)
+
+
+@pytest.mark.django_db
+@scopes_disabled()
+def test_multiple_discounts_with_same_condition(event, item, item2, item3):
+    # "For every 1 item1, you get three item2 for 10 % off." + "For every 1 item1, you get five item3 for 10 % off."
+    d1 = Discount(
+        event=event,
+        condition_min_count=1,
+        condition_all_products=False,
+        benefit_only_apply_to_cheapest_n_matches=3,
+        benefit_discount_matching_percent=10,
+        benefit_same_products=False,
+        position=1,
+    )
+    d1.save()
+    d1.condition_limit_products.add(item)
+    d1.benefit_limit_products.add(item2)
+
+    d2 = Discount(
+        event=event,
+        condition_min_count=1,
+        condition_all_products=False,
+        benefit_only_apply_to_cheapest_n_matches=5,
+        benefit_discount_matching_percent=10,
+        benefit_same_products=False,
+        position=2,
+    )
+    d2.save()
+    d2.condition_limit_products.add(item)
+    d2.benefit_limit_products.add(item3)
+
+    positions = (
+        (item3.pk, None, Decimal('42.00'), False, False, Decimal('0.00')),
+        (item3.pk, None, Decimal('42.00'), False, False, Decimal('0.00')),
+        (item3.pk, None, Decimal('42.00'), False, False, Decimal('0.00')),
+        (item3.pk, None, Decimal('42.00'), False, False, Decimal('0.00')),
+        (item3.pk, None, Decimal('42.00'), False, False, Decimal('0.00')),
+        (item3.pk, None, Decimal('42.00'), False, False, Decimal('0.00')),
+        (item2.pk, None, Decimal('50.00'), False, False, Decimal('0.00')),
+        (item2.pk, None, Decimal('50.00'), False, False, Decimal('0.00')),
+        (item.pk, None, Decimal('23.00'), False, False, Decimal('0.00')),
+        (item.pk, None, Decimal('23.00'), False, False, Decimal('0.00')),
+    )
+    expected = (
+        # both item1 remain full price
+        Decimal('23.00'),
+        Decimal('23.00'),
+        # 5 item3 discounted
+        Decimal('37.80'),
+        Decimal('37.80'),
+        Decimal('37.80'),
+        Decimal('37.80'),
+        Decimal('37.80'),
+        # 2 item2 discounted
+        Decimal('45.00'),
+        Decimal('45.00'),
+        # 1 item3 remains untouched
+        Decimal('42.00'),
+    )
+
+    new_prices = [p for p, d in apply_discounts(event, 'web', positions)]
+    assert sorted(new_prices) == sorted(expected)