fix(fallback): use fractional part of setpoint as extra shift chance

shiftregister/fallback/models.py

@@ -2,7 +2,9 @@ import math
 import secrets
 from base64 import urlsafe_b64encode
 from datetime import datetime, time
+from random import random
 
+import sentry_sdk
 from django.db.models import Count, Exists, ExpressionWrapper, Max, OuterRef, Sum
 from django.db.models.fields import DateTimeField
 from django.db.models.lookups import LessThan
@@ -74,17 +76,23 @@ class TeamMember(models.Model):
         free_slot_count = free_bucket.annotate(
             needed_helpers=F("required_helpers") - F("fallback_count")
         ).aggregate(sum=Sum("needed_helpers"))["sum"]
+
+        quota = global_preferences["helper__fallback_quota"]
+        number_of_team_members = TeamMember.objects.count()
+        max_shifts_per_member = total_slot_count / max(
+            number_of_team_members * quota, 1
+        )
+
         active_team_members = (
             TeamMember.objects.filter(~Q(fallback_shifts=None)).count() + 1
         )
+        shifts_per_member = total_slot_count / active_team_members
 
-        quota = global_preferences["helper__fallback_quota"]
-        number_of_team_members = TeamMember.objects.count()
-        max_shifts_per_member = math.ceil(
-            total_slot_count / max((number_of_team_members * quota), 1)
+        extra_chance, shift_count = math.modf(
+            min(max_shifts_per_member, shifts_per_member)
         )
-        shifts_per_member = math.ceil(total_slot_count / (active_team_members))
-        shift_count = min(max_shifts_per_member, shifts_per_member)
+        if extra_chance > random():
+            shift_count += 1
 
         blocked_times = []
         for shift in self.fallback_shifts.all():
@@ -174,6 +182,9 @@ class TeamMember(models.Model):
                     break
             if not assignment:
                 print("could not find any matching assignments to take away")
+                sentry_sdk.capture_message(
+                    "could not find any matching assignments to take away"
+                )
                 return
             shifts_needed -= 1
             blocked_times.append(