feat(api): add /api/convert endpoint with shared rates cache and query validation

app/api/convert/route.ts

@@ -0,0 +1,123 @@
+import { NextRequest, NextResponse } from "next/server";
+import { z } from "zod";
+
+import { convertAmount } from "@/lib/rates";
+import {
+  getLastCachedRates,
+  getRatesWithCache,
+  RATES_CACHE_CONTROL_VALUE,
+} from "@/lib/server/rates-cache";
+import { amountSchema } from "@/lib/validation";
+
+const querySchema = z.object({
+  from: z.string().trim().toUpperCase().min(1),
+  to: z.string().trim().toUpperCase().min(1),
+  amount: amountSchema,
+});
+
+export const revalidate = 300;
+
+export async function GET(request: NextRequest) {
+  const searchParams = request.nextUrl.searchParams;
+
+  const queryResult = querySchema.safeParse({
+    from: searchParams.get("from") ?? "",
+    to: searchParams.get("to") ?? "",
+    amount: searchParams.get("amount") ?? "",
+  });
+
+  if (!queryResult.success) {
+    return NextResponse.json(
+      {
+        message:
+          queryResult.error.issues[0]?.message ?? "Invalid query parameters",
+      },
+      { status: 400 },
+    );
+  }
+
+  const { amount, from, to } = queryResult.data;
+
+  try {
+    const rates = await getRatesWithCache();
+    const rateMap = new Map(rates.assets.map((asset) => [asset.code, asset]));
+
+    const fromAsset = rateMap.get(from);
+    const toAsset = rateMap.get(to);
+
+    if (!fromAsset || !toAsset) {
+      return NextResponse.json(
+        { message: "Unsupported currency or asset code" },
+        { status: 404 },
+      );
+    }
+
+    const converted = convertAmount(amount, fromAsset, toAsset);
+    const rate = fromAsset.usdPrice / toAsset.usdPrice;
+    const inverseRate = toAsset.usdPrice / fromAsset.usdPrice;
+
+    return NextResponse.json(
+      {
+        from,
+        to,
+        amount,
+        convertedAmount: converted,
+        rate,
+        inverseRate,
+        quoteCurrency: rates.quoteCurrency,
+        updatedAt: rates.updatedAt,
+        sources: rates.sources,
+      },
+      {
+        status: 200,
+        headers: {
+          "Cache-Control": RATES_CACHE_CONTROL_VALUE,
+        },
+      },
+    );
+  } catch (error) {
+    const cachedRates = getLastCachedRates();
+
+    if (cachedRates) {
+      const rateMap = new Map(
+        cachedRates.assets.map((asset) => [asset.code, asset]),
+      );
+      const fromAsset = rateMap.get(from);
+      const toAsset = rateMap.get(to);
+
+      if (fromAsset && toAsset) {
+        const converted = convertAmount(amount, fromAsset, toAsset);
+        const rate = fromAsset.usdPrice / toAsset.usdPrice;
+        const inverseRate = toAsset.usdPrice / fromAsset.usdPrice;
+
+        return NextResponse.json(
+          {
+            from,
+            to,
+            amount,
+            convertedAmount: converted,
+            rate,
+            inverseRate,
+            quoteCurrency: cachedRates.quoteCurrency,
+            updatedAt: cachedRates.updatedAt,
+            sources: cachedRates.sources,
+          },
+          {
+            status: 200,
+            headers: {
+              "Cache-Control": RATES_CACHE_CONTROL_VALUE,
+              "X-Cache-Fallback": "stale-on-error",
+            },
+          },
+        );
+      }
+    }
+
+    const message =
+      error instanceof Error
+        ? error.message
+        : "Unexpected error while converting amount";
+
+    return NextResponse.json({ message }, { status: 500 });
+  }
+}