added base examples and drills

kata/billing/original/README.md

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+# billing original
+
+Messy invoice finalization logic mixing pricing rules, time, logging, mutation, and file I/O.

kata/billing/original/invoiceService.js

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+const fs = require("fs");
+
+function finalizeInvoice(filePath, invoiceId, customerType) {
+  const raw = fs.readFileSync(filePath, "utf8");
+  const state = JSON.parse(raw);
+  const invoice = state.invoices.find((item) => item.id === invoiceId);
+
+  if (!invoice) {
+    throw new Error("Invoice not found");
+  }
+
+  let subtotal = 0;
+
+  for (const line of invoice.lines) {
+    subtotal = subtotal + line.quantity * line.unitPrice;
+  }
+
+  let discount = 0;
+
+  if (customerType === "vip") {
+    discount = subtotal * 0.15;
+  }
+
+  if (subtotal > 200) {
+    discount = discount + 10;
+  }
+
+  const taxed = (subtotal - discount) * 1.1;
+  invoice.status = "finalized";
+  invoice.total = Math.round(taxed * 100) / 100;
+  invoice.finalizedAt = new Date().toISOString();
+
+  fs.writeFileSync(filePath, JSON.stringify(state, null, 2));
+  console.log("Invoice finalized", invoiceId);
+
+  return invoice;
+}
+
+module.exports = { finalizeInvoice };

kata/billing/original/state.json

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+{
+  "invoices": [
+    {
+      "id": "inv-1",
+      "status": "draft",
+      "total": 0,
+      "finalizedAt": null,
+      "lines": [
+        { "description": "Hosting", "quantity": 2, "unitPrice": 60 },
+        { "description": "Support", "quantity": 1, "unitPrice": 95 }
+      ]
+    }
+  ]
+}