doorcontrol: Add annotations that decode the hex card number

doorcontrol/admin.py

@@ -30,9 +30,10 @@ class HIDEventAdmin(admin.ModelAdmin):
         "event_type",
         IsRedFilter,
     ]
+    readonly_fields = ["decoded_card_number"]
 
     def get_queryset(self, request):
-        return super().get_queryset(request).with_is_red()
+        return super().get_queryset(request).with_is_red().with_decoded_card_number()
 
     @admin.display(boolean=True)
     def is_red(self, obj):

doorcontrol/models.py

@@ -1,5 +1,6 @@
 from django.db import models
-from django.db.models import ExpressionWrapper, Q
+from django.db.models import ExpressionWrapper, F, Func, Q
+from django.db.models.functions import Mod
 
 
 class HIDEventQuerySet(models.QuerySet):
@@ -28,6 +29,42 @@ class HIDEventQuerySet(models.QuerySet):
             )
         )
 
+    def with_decoded_card_number(self):
+        # TODO: CONV and BIT_COUNT are MySQL/MariaDB specific
+        class Conv(Func):
+            function = "CONV"
+            arity = 3
+            # This is technically not true, but fine for my purposes
+            output_field = models.IntegerField()
+
+        class BitCount(Func):
+            function = "BIT_COUNT"
+            arity = 1
+
+        return (
+            self.alias(card_number=Conv(F("raw_card_number"), 16, 10))
+            .alias(more_than_26_bits=F("card_number").bitrightshift(26))
+            .annotate(card_is_26_bit=Q(more_than_26_bits=0))
+            .alias(
+                parity_a=Mod(
+                    BitCount(F("card_number").bitrightshift(1).bitand(0xFFF)), 2
+                ),
+                parity_b=Mod(
+                    BitCount(F("card_number").bitrightshift(13).bitand(0xFFF)), 2
+                ),
+            )
+            .annotate(
+                card_is_valid_26_bit=~Q(parity_a=F("card_number").bitand(1))
+                & Q(parity_b=F("card_number").bitrightshift(25).bitand(1))
+            )
+            .annotate(
+                card_number_26_bit=F("card_number").bitrightshift(1).bitand(0xFFFF),
+                card_facility_code_26_bit=F("card_number")
+                .bitrightshift(17)
+                .bitand(0xFF),
+            )
+        )
+
 
 class HIDEvent(models.Model):
     objects = HIDEventQuerySet.as_manager()
@@ -121,6 +158,18 @@ class HIDEvent(models.Model):
     def __str__(self):
         return f"{self.door_name} {self.timestamp} - {self.description}"
 
+    def decoded_card_number(self) -> str:
+        """Requires annotations from `with_decoded_card_number`"""
+        if self.raw_card_number is None:
+            return None
+        elif self.card_is_26_bit:
+            if self.card_is_valid_26_bit:
+                return f"{self.card_facility_code_26_bit} - {self.card_number_26_bit}"
+            else:
+                return "Invalid"
+        else:
+            return "Not 26 bit card"
+
     class Meta:
         constraints = [
             models.UniqueConstraint(