added api logic, vitest, minimal testing ui

api/src/routes/income-preview.ts

@@ -0,0 +1,82 @@
+import { FastifyInstance } from "fastify";
+import { z } from "zod";
+
+const Body = z.object({ amountCents: z.number().int().nonnegative() });
+
+export default async function incomePreviewRoutes(app: FastifyInstance) {
+  app.post("/income/preview", async (req, reply) => {
+    const parsed = Body.safeParse(req.body);
+    if (!parsed.success) return reply.code(400).send({ message: "Invalid amount" });
+
+    const userId = req.userId;
+    let remaining = Math.max(0, parsed.data.amountCents | 0);
+
+    const [plans, cats] = await Promise.all([
+      app.prisma.fixedPlan.findMany({
+        where: { userId },
+        orderBy: [{ priority: "asc" }, { dueOn: "asc" }],
+        select: { id: true, name: true, totalCents: true, fundedCents: true, priority: true, dueOn: true },
+      }),
+      app.prisma.variableCategory.findMany({
+        where: { userId },
+        orderBy: [{ priority: "asc" }, { name: "asc" }],
+        select: { id: true, name: true, percent: true, isSavings: true, priority: true },
+      }),
+    ]);
+
+    // Fixed pass
+    const fixed: Array<{ id: string; name: string; amountCents: number }> = [];
+    for (const p of plans) {
+      if (remaining <= 0) break;
+      const needBig = (p.totalCents ?? 0n) - (p.fundedCents ?? 0n);
+      const need = Number(needBig > 0n ? needBig : 0n);
+      if (need <= 0) continue;
+      const give = Math.min(need, remaining);
+      fixed.push({ id: p.id, name: p.name, amountCents: give });
+      remaining -= give;
+    }
+
+    // Variable pass — largest remainder with savings-first tiebreak
+    const variable: Array<{ id: string; name: string; amountCents: number }> = [];
+    if (remaining > 0 && cats.length > 0) {
+      const totalPercent = cats.reduce((s, c) => s + (c.percent || 0), 0);
+      const normCats =
+        totalPercent === 100
+          ? cats
+          : cats.map((c) => ({
+              ...c,
+              percent: totalPercent > 0 ? (c.percent * 100) / totalPercent : 0,
+            }));
+
+      const base: number[] = new Array(normCats.length).fill(0);
+      const tie: { idx: number; remainder: number; isSavings: boolean; priority: number; name: string }[] = [];
+      let sumBase = 0;
+
+      normCats.forEach((c, idx) => {
+        const exact = (remaining * c.percent) / 100;
+        const floor = Math.floor(exact);
+        base[idx] = floor;
+        sumBase += floor;
+        tie.push({ idx, remainder: exact - floor, isSavings: !!c.isSavings, priority: c.priority, name: c.name });
+      });
+
+      let leftovers = remaining - sumBase;
+      tie.sort((a, b) => {
+        if (a.isSavings !== b.isSavings) return a.isSavings ? -1 : 1; // savings first
+        if (a.remainder !== b.remainder) return b.remainder - a.remainder;
+        if (a.priority !== b.priority) return a.priority - b.priority;
+        return a.name.localeCompare(b.name);
+      });
+      for (let i = 0; i < tie.length && leftovers > 0; i++, leftovers--) base[tie[i].idx] += 1;
+
+      normCats.forEach((c, idx) => {
+        const give = base[idx] || 0;
+        if (give > 0) variable.push({ id: c.id, name: c.name, amountCents: give });
+      });
+
+      remaining = leftovers;
+    }
+
+    return { fixed, variable, unallocatedCents: Math.max(0, remaining) };
+  });
+}