fix w kalkulacji last price

app/domain/costs.py

@@ -34,3 +34,25 @@ def calculate_net_price_costs(net_price, vat_rate=23, vat_deduction_percent=50):
         "deductible_vat": deductible_vat,
         "effective_cost": effective_cost,
     }
+
+
+def calculate_net_price_comparison(retail_gross_price, settlement_net_price, liters=1, vat_rate=23):
+    """Compare retail and settlement prices on a net-to-net basis.
+
+    Retail price is supplied gross (pump price), while settlement/Last Price is net.
+    VAT is excluded from the saving because recoverable tax is not a commercial saving.
+    """
+    retail_gross = Decimal(str(retail_gross_price))
+    settlement_net = Decimal(str(settlement_net_price))
+    quantity = Decimal(str(liters))
+    rate = Decimal(str(vat_rate)) / Decimal("100")
+    retail_net = retail_gross / (Decimal("1") + rate)
+    saving_per_liter = max(retail_net - settlement_net, Decimal("0"))
+    return {
+        "retail_net_price": retail_net,
+        "settlement_net_price": settlement_net,
+        "saving_net_per_liter": saving_per_liter,
+        "retail_net_total": retail_net * quantity,
+        "settlement_net_total": settlement_net * quantity,
+        "saving_net_total": saving_per_liter * quantity,
+    }