first version

app/services/invoice/numbering.server.ts

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+import db from "../../db.server";
+import type { ShopSettings } from "@prisma/client";
+
+/**
+ * Allocates an invoice number for the given order, using the shop's
+ * configured numbering mode. For `prefix_sequential`, allocation is atomic
+ * across concurrent requests via Prisma's interactive transaction (with the
+ * counter row acting as the lock).
+ */
+export async function allocateInvoiceNumber(
+  settings: ShopSettings,
+  orderNumber: number,
+): Promise<string> {
+  const prefix = settings.invoicePrefix || "";
+
+  if (settings.numberingMode === "prefix_sequential") {
+    const next = await db.$transaction(async (tx) => {
+      const counter = await tx.invoiceCounter.upsert({
+        where: { shopDomain: settings.shopDomain },
+        create: {
+          shopDomain: settings.shopDomain,
+          lastValue: settings.invoiceSeed,
+        },
+        update: {},
+      });
+      const newValue = counter.lastValue + 1;
+      await tx.invoiceCounter.update({
+        where: { shopDomain: settings.shopDomain },
+        data: { lastValue: newValue },
+      });
+      return newValue;
+    });
+    return `${prefix}${next}`;
+  }
+
+  // Default: reuse the Shopify order number with the configured prefix.
+  return `${prefix}${orderNumber}`;
+}