feat(frontend): tail-store Zustand with FIFO cap 10k per slice

src/store/tail-store.test.ts

@@ -0,0 +1,131 @@
+// @vitest-environment node
+// `useTailStore` is a pure zustand store with no DOM / React dependencies,
+// so a node environment is fine. We import `getState()` and `setState`
+// from the store directly (no React render) and assert on the resulting
+// state shape.
+
+import { beforeEach, describe, expect, it } from "vitest";
+import type { Activity, Claim, Remittance } from "@/types";
+import { TAIL_CAP, useTailStore } from "./tail-store";
+
+function makeClaim(id: string, overrides: Partial<Claim> = {}): Claim {
+  return {
+    id,
+    patientName: `Patient ${id}`,
+    providerNpi: "1234567890",
+    payerName: "Medicaid",
+    cptCode: "99213",
+    billedAmount: 100,
+    receivedAmount: 0,
+    status: "submitted",
+    submissionDate: "2026-06-20T00:00:00Z",
+    ...overrides,
+  };
+}
+
+function makeRemittance(id: string, overrides: Partial<Remittance> = {}): Remittance {
+  return {
+    id,
+    claimId: `CLM-${id}`,
+    payerName: "Medicaid",
+    paidAmount: 100,
+    adjustmentAmount: 0,
+    receivedDate: "2026-06-20",
+    checkNumber: id,
+    status: "received",
+    ...overrides,
+  };
+}
+
+function makeActivity(id: string, overrides: Partial<Activity> = {}): Activity {
+  return {
+    id,
+    kind: "claim_submitted",
+    message: `Event ${id}`,
+    timestamp: "2026-06-20T00:00:00Z",
+    ...overrides,
+  };
+}
+
+describe("useTailStore", () => {
+  // Each test starts from a known-empty state. The store is module-level
+  // (singleton), so we use `reset()` to clear between cases — that's also
+  // what production code calls on stream open.
+  beforeEach(() => {
+    useTailStore.getState().reset("claims");
+    useTailStore.getState().reset("remittances");
+    useTailStore.getState().reset("activity");
+  });
+
+  it("test_add_claim_adds_new_claim_keyed_by_id", () => {
+    const { addClaim } = useTailStore.getState();
+    addClaim(makeClaim("CLM-1"));
+    addClaim(makeClaim("CLM-2"));
+    const { claims } = useTailStore.getState();
+    expect(Object.keys(claims)).toHaveLength(2);
+    expect(claims["CLM-1"]?.id).toBe("CLM-1");
+    expect(claims["CLM-2"]?.id).toBe("CLM-2");
+  });
+
+  it("test_add_claim_with_duplicate_id_is_noop", () => {
+    const { addClaim } = useTailStore.getState();
+    addClaim(makeClaim("CLM-1", { patientName: "Original" }));
+    addClaim(makeClaim("CLM-1", { patientName: "Updated" }));
+    const { claims } = useTailStore.getState();
+    expect(Object.keys(claims)).toHaveLength(1);
+    // First write wins; duplicates are silently dropped so the snapshot
+    // replay on reconnect doesn't trample the canonical row.
+    expect(claims["CLM-1"]?.patientName).toBe("Original");
+  });
+
+  it("test_reset_claims_clears_only_claims_slice", () => {
+    const { addClaim, addRemittance, addActivity, reset } = useTailStore.getState();
+    addClaim(makeClaim("CLM-1"));
+    addRemittance(makeRemittance("RMT-1"));
+    addActivity(makeActivity("ACT-1"));
+
+    reset("claims");
+    const s = useTailStore.getState();
+    expect(Object.keys(s.claims)).toHaveLength(0);
+    expect(Object.keys(s.remittances)).toHaveLength(1);
+    expect(s.activity).toHaveLength(1);
+  });
+
+  it("test_add_activity_appends", () => {
+    const { addActivity } = useTailStore.getState();
+    addActivity(makeActivity("ACT-1", { message: "first" }));
+    addActivity(makeActivity("ACT-2", { message: "second" }));
+    addActivity(makeActivity("ACT-3", { message: "third" }));
+    const { activity } = useTailStore.getState();
+    expect(activity).toHaveLength(3);
+    // Activity has no stable id; it lives in an array. Insertion order is
+    // the only ordering signal.
+    expect(activity.map((a) => a.message)).toEqual(["first", "second", "third"]);
+  });
+
+  it("test_fifo_cap_evicts_oldest_when_over_10000", () => {
+    // Insert 5 over the cap so the eviction policy must actually run.
+    // The plan calls for evicting the oldest 100 (or enough to be back at
+    // the cap) — we just assert: size == TAIL_CAP and the very-first
+    // items are gone.
+    //
+    // 10 005 individual `addClaim` calls is intentionally a stress test
+    // of the eviction path: each call rebuilds the dict + order array
+    // so the wall-clock cost is O(N^2) in the number of items ever
+    // inserted. Bump the per-test timeout from the default 5s to 60s
+    // so this case can complete in CI on slow runners.
+    const N = TAIL_CAP + 5;
+    const { addClaim } = useTailStore.getState();
+    for (let i = 0; i < N; i++) {
+      addClaim(makeClaim(`CLM-${i.toString().padStart(6, "0")}`));
+    }
+    const { claims } = useTailStore.getState();
+    expect(Object.keys(claims)).toHaveLength(TAIL_CAP);
+    // The five oldest (CLM-000000 .. CLM-000004) must be gone.
+    expect(claims["CLM-000000"]).toBeUndefined();
+    expect(claims["CLM-000004"]).toBeUndefined();
+    // The most recent (CLM-0010004) must be present.
+    const lastKey = `CLM-${(N - 1).toString().padStart(6, "0")}`;
+    expect(claims[lastKey]).toBeDefined();
+  }, 60_000);
+});

src/store/tail-store.ts

@@ -0,0 +1,161 @@
+// ---------------------------------------------------------------------------
+// Live-tail append-only store (sub-project 5, Phase 4 Task 17).
+//
+// Three independent slices (`claims` / `remittances` / `activity`) that
+// `useTailStream` (Phase 5) writes into as `item` events arrive. Reads
+// happen via `useMergedTail` (also Phase 5), which merges each slice with
+// the page's JSON fetch results and applies the page's filter.
+//
+// Design notes:
+// - `claims` and `remittances` are key-by-id maps (id is stable), so
+//   duplicate snapshots are dedup'd by `addClaim`/`addRemittance` (first
+//   write wins — the snapshot replay on reconnect must not trample the
+//   canonical row).
+// - `activity` is an append-only array (event ids aren't guaranteed to
+//   be unique across snapshots; the ActivityLog renders in arrival
+//   order).
+// - Each slice is FIFO-capped at `TAIL_CAP` (10 000) so a runaway tail
+//   can't grow the heap unbounded. Oldest entries are evicted in the
+//   add function itself — `reset` is what production calls when a new
+//   stream opens.
+// ---------------------------------------------------------------------------
+
+import { create } from "zustand";
+import type { Activity, Claim, Remittance } from "@/types";
+import type { TailResource } from "@/lib/tail-stream";
+
+/** Maximum number of items retained per slice before FIFO eviction kicks in. */
+export const TAIL_CAP = 10_000;
+
+/**
+ * Per-slice eviction batch. When the cap is exceeded, drop the oldest
+ * `EVICT_BATCH` entries in a single `set()` call. Picking a batch > 1
+ * amortizes the cost of eviction when a high-volume stream pushes many
+ * items per frame — and 100 is small enough that a 10 005-item insert
+ * still lands within one `set()` call.
+ *
+ * The store does NOT require this batch to be exactly the overflow
+ * amount — it always drains down to the cap, so a +5 overflow evicts 5
+ * even when EVICT_BATCH is 100. The batch is just an upper bound on how
+ * many we delete in a single pass.
+ */
+const EVICT_BATCH = 100;
+
+interface TailStore {
+  // --- Slices (keyed by id for the two that have stable ids) -----------
+  claims: Record<string, Claim>;
+  remittances: Record<string, Remittance>;
+  activity: Activity[];
+
+  // --- Insertion-order trackers (kept in sync with the dicts above) ----
+  // These are private to the store; consumers only read the dicts. We
+  // store them as part of the state so they reactively update with the
+  // same `set()` call (zustand shallow-merges, so the new array ref is
+  // what triggers a re-render in subscribers that select `claims`).
+  claimOrder: string[];
+  remitOrder: string[];
+
+  // --- Setters ---------------------------------------------------------
+  addClaim: (c: Claim) => void;
+  addRemittance: (r: RemitListItem) => void;
+  addActivity: (a: Activity) => void;
+  reset: (resource: TailResource) => void;
+}
+
+/**
+ * The spec's sketch uses the placeholder name `RemitListItem` for the
+ * remittance shape; locally we just use the existing `Remittance` type
+ * from `@/types` (same value, no need to add a duplicate).
+ */
+type RemitListItem = Remittance;
+
+function evictOldest(
+  order: string[],
+  dict: Record<string, Claim | Remittance>,
+  batch: number,
+): { order: string[]; dict: Record<string, Claim | Remittance> } {
+  if (order.length <= TAIL_CAP) return { order, dict };
+  // Drain down to the cap; never evict more than `batch` per call so a
+  // 5-item overflow evicts 5, but a 1 000-item overflow evicts 100 in
+  // this pass and the remaining 900 in subsequent add() calls.
+  const toDrop = Math.min(batch, order.length - TAIL_CAP);
+  const dropped = order.slice(0, toDrop);
+  const nextOrder = order.slice(toDrop);
+  // Object.assign is consistently faster than `{ ...dict }` in V8 for
+  // large dicts; we follow up with `delete` for each evicted id.
+  const nextDict: Record<string, Claim | Remittance> = Object.assign({}, dict);
+  for (const id of dropped) delete nextDict[id];
+  return { order: nextOrder, dict: nextDict };
+}
+
+export const useTailStore = create<TailStore>((set) => ({
+  claims: {},
+  remittances: {},
+  activity: [],
+  claimOrder: [],
+  remitOrder: [],
+
+  addClaim: (c) =>
+    set((s) => {
+      // Dedup: first write wins. The snapshot replay on reconnect
+      // produces the same id repeatedly; we want the first occurrence
+      // to stick so the canonical row isn't overwritten by an older
+      // version that happened to be in the snapshot.
+      if (s.claims[c.id]) return s;
+      // Object.assign is faster than `{ ...s.claims, [c.id]: c }` for
+      // large dicts; this hot path is called once per `item` event.
+      const nextClaims: Record<string, Claim> = Object.assign({}, s.claims, {
+        [c.id]: c,
+      });
+      const nextOrder = s.claimOrder.concat(c.id);
+      if (nextOrder.length > TAIL_CAP) {
+        const { order, dict } = evictOldest(nextOrder, nextClaims, EVICT_BATCH);
+        return { claims: dict as Record<string, Claim>, claimOrder: order };
+      }
+      return { claims: nextClaims, claimOrder: nextOrder };
+    }),
+
+  addRemittance: (r) =>
+    set((s) => {
+      if (s.remittances[r.id]) return s;
+      const nextRemits: Record<string, Remittance> = Object.assign(
+        {},
+        s.remittances,
+        { [r.id]: r },
+      );
+      const nextOrder = s.remitOrder.concat(r.id);
+      if (nextOrder.length > TAIL_CAP) {
+        const { order, dict } = evictOldest(nextOrder, nextRemits, EVICT_BATCH);
+        return {
+          remittances: dict as Record<string, Remittance>,
+          remitOrder: order,
+        };
+      }
+      return { remittances: nextRemits, remitOrder: nextOrder };
+    }),
+
+  addActivity: (a) =>
+    set((s) => {
+      // Activity has no stable id, so it's a plain append. FIFO cap
+      // evicts the oldest with a single `slice` (the typical case is
+      // +1, +1, +1; an extreme burst falls back to multiple slice
+      // passes).
+      let next = [...s.activity, a];
+      if (next.length > TAIL_CAP) {
+        next = next.slice(next.length - TAIL_CAP);
+      }
+      return { activity: next };
+    }),
+
+  reset: (resource) =>
+    set(() => {
+      switch (resource) {
+        case "claims":
+          return { claims: {}, claimOrder: [] };
+        case "remittances":
+          return { remittances: {}, remitOrder: [] };
+        case "activity":
+          return { activity: [] };
+      }
+    }),
+}));