add average mpg to fuel log list

Gas Man/FuelLogs/FuelLogListView.swift

@@ -20,7 +20,7 @@ struct FuelLogListView: View {
     @State private var selectedVehicleID: UUID? = nil
     @State private var showAddVehicleSheet = false
 
-    // For tracking the previous odometer reading
+    // For tracking the previous odometer reading (used for validation)
     @State private var previousOdometer: Double? = nil
     @State private var showOdometerAlert = false
     @State private var isUpdatingCalculation = false
@@ -44,9 +44,32 @@ struct FuelLogListView: View {
         }
     }
     
+    // Computed property to calculate average MPG for full-tank logs.
+    // MPG for an individual log is computed as:
+    //   (current.odometer - next.odometer) / current.fuelVolume
+    private var averageMPG: Double? {
+        let logs = filteredFuelLogs
+        guard logs.count > 1 else { return nil }
+        var mpgValues: [Double] = []
+        // Iterate over logs except the oldest.
+        for i in 0..<logs.count - 1 {
+            let current = logs[i]
+            let next = logs[i + 1]
+            // Only include if fullTank is true, fuelVolume is positive, and odometer difference is positive.
+            if current.fullTank, current.fuelVolume > 0, current.odometer > next.odometer {
+                let mpg = (current.odometer - next.odometer) / current.fuelVolume
+                mpgValues.append(mpg)
+            }
+        }
+        guard !mpgValues.isEmpty else { return nil }
+        let total = mpgValues.reduce(0, +)
+        return total / Double(mpgValues.count)
+    }
+    
     var body: some View {
         NavigationView {
             if vehicles.isEmpty {
+                // Empty state: no vehicles exist.
                 VStack(spacing: 20) {
                     Text("No vehicles found.")
                         .font(.headline)
@@ -67,6 +90,7 @@ struct FuelLogListView: View {
                 }
             } else {
                 List {
+                    // Vehicle Picker Section
                     Section {
                         Picker("Vehicle", selection: $selectedVehicleID) {
                             ForEach(vehicles, id: \.id) { vehicle in
@@ -77,6 +101,22 @@ struct FuelLogListView: View {
                         .pickerStyle(MenuPickerStyle())
                     }
                     
+                    // Average MPG Section
+                    Section {
+                        HStack {
+                            Text("Average MPG:")
+                            Spacer()
+                            if let avg = averageMPG {
+                                Text("\(avg, specifier: "%.1f")")
+                                    .bold()
+                            } else {
+                                Text("N/A")
+                                    .foregroundColor(.secondary)
+                            }
+                        }
+                    }
+                    
+                    // Fuel Logs Section (filtered by selected vehicle)
                     ForEach(filteredFuelLogs.indices, id: \.self) { index in
                         let log = filteredFuelLogs[index]
                         let distance: Double? = {
@@ -131,6 +171,7 @@ struct FuelLogListView: View {
         }
     }
     
+    // Set default vehicle selection on appear.
     private func setDefaultVehicle() {
         if selectedVehicleID == nil {
             if let lastFuelLog = fuelLogs.first, let vehicle = lastFuelLog.vehicle {