Avoid duplicate Best Value rank crashes

FuelBoardTests/Tests/FuelBoardSharedTests/FuelSavingsEstimateTests.swift

@@ -213,6 +213,27 @@ final class FuelSavingsEstimateTests: XCTestCase {
                        "baseline must be recalculated from the latest journey values when routes arrive")
     }
 
+    func testRankingToleratesDuplicateStationIDs() {
+        let baseline = station("baseline", price: 145.9)
+        let duplicateA = station("duplicate", price: 138.9, lat: 51.55)
+        let duplicateB = station("duplicate", price: 139.9, lat: 51.56)
+        let pool = [baseline, duplicateA, duplicateB]
+
+        let ranked = FuelSavingsEstimator.ranked(
+            pool,
+            fuel: .e10,
+            baseline: baseline,
+            settings: settings,
+            amount: amount,
+            journeyMiles: { s in
+                s.id == "baseline" ? self.straightLine(2.0) : self.straightLine(7.0)
+            }
+        )
+
+        XCTAssertEqual(ranked.count, pool.count)
+        XCTAssertEqual(ranked.filter { $0.id == "duplicate" }.count, 2)
+    }
+
     func testStationWithoutAPriceIsUnavailable() {
         let baseline = station("baseline", price: 145.9)
         let noPrice = station("no-price", price: nil)

Shared/FuelSavings.swift

@@ -499,11 +499,12 @@ enum FuelSavingsEstimator {
                 pricePencePerLitre: station.prices[fuel] ?? .infinity
             )
         }
-        let order = Dictionary(
-            uniqueKeysWithValues: FuelSavingsCalculator.sortedForWorthTheDrive(ranked)
-                .enumerated()
-                .map { ($0.element.stationID, $0.offset) }
-        )
+        var order: [String: Int] = [:]
+        for (offset, rankedStation) in FuelSavingsCalculator.sortedForWorthTheDrive(ranked).enumerated() {
+            if order[rankedStation.stationID] == nil {
+                order[rankedStation.stationID] = offset
+            }
+        }
         return stations.sorted { lhs, rhs in
             let left = order[lhs.id] ?? Int.max
             let right = order[rhs.id] ?? Int.max