init

app/lib/pricing.ts

@@ -0,0 +1,41 @@
+import type { ItemId, StoreItem } from "@/app/types";
+import { CATALOG } from "./catalog";
+
+// ---------------------------------------------------------------------------
+// PriceProvider: the single seam between "where prices come from" and the
+// rest of the app. The cost math only ever calls `getItem`.
+//
+// To use live retailer prices later:
+//   1. Fetch prices on the SERVER (a Server Action or route handler) so the
+//      API key never reaches the browser.
+//   2. Build a snapshot `StoreItem[]` from the response.
+//   3. Pass `new SnapshotPriceProvider("Live grocery prices", snapshot)` into
+//      the grocery/planner functions instead of `averageGroceryProvider`.
+// The math stays synchronous and testable because it works on a snapshot.
+// ---------------------------------------------------------------------------
+
+export interface PriceProvider {
+  /** Shown in the UI so users know where prices came from. */
+  readonly label: string;
+  getItem(id: ItemId): StoreItem | undefined;
+}
+
+export class SnapshotPriceProvider implements PriceProvider {
+  private readonly byId: Map<ItemId, StoreItem>;
+
+  constructor(
+    readonly label: string,
+    items: StoreItem[],
+  ) {
+    this.byId = new Map(items.map((item) => [item.id, item]));
+  }
+
+  getItem(id: ItemId): StoreItem | undefined {
+    return this.byId.get(id);
+  }
+}
+
+export const averageGroceryProvider: PriceProvider = new SnapshotPriceProvider(
+  "Based on average grocery prices as of October 2026",
+  CATALOG,
+);