2 files changed, 121 insertions, 0 deletions
diff --git a/src/features/settings/logic/view-model.test.ts b/src/features/settings/logic/view-model.test.ts
new file mode 100644
index 0000000..93c4786
--- /dev/null
+++ b/src/features/settings/logic/view-model.test.ts
@@ -0,0 +1,61 @@
+import { describe, expect, it } from "vitest";
+import { MAX_CHAT_LIMIT, MIN_CHAT_LIMIT } from "../../../core/chunks";
+import { chatLimitChanged, parseChatLimit } from "./view-model";
+
+describe("parseChatLimit", () => {
+  it("parses a plain integer", () => {
+    expect(parseChatLimit("256")).toEqual({ ok: true, value: 256 });
+    expect(parseChatLimit("100")).toEqual({ ok: true, value: 100 });
+  });
+
+  it("trims surrounding whitespace", () => {
+    expect(parseChatLimit("  50  ")).toEqual({ ok: true, value: 50 });
+  });
+
+  it("floors a decimal", () => {
+    expect(parseChatLimit("100.9")).toEqual({ ok: true, value: 100 });
+  });
+
+  it("clamps below the floor to MIN_CHAT_LIMIT", () => {
+    expect(parseChatLimit("0")).toEqual({ ok: true, value: MIN_CHAT_LIMIT });
+    expect(parseChatLimit("-5")).toEqual({ ok: true, value: MIN_CHAT_LIMIT });
+    expect(parseChatLimit("5")).toEqual({ ok: true, value: MIN_CHAT_LIMIT });
+  });
+
+  it("clamps above the ceiling to MAX_CHAT_LIMIT", () => {
+    expect(parseChatLimit("99999999")).toEqual({ ok: true, value: MAX_CHAT_LIMIT });
+  });
+
+  it("rejects an empty string", () => {
+    expect(parseChatLimit("")).toEqual({ ok: false, error: expect.any(String) });
+    expect(parseChatLimit("   ")).toEqual({ ok: false, error: expect.any(String) });
+  });
+
+  it("rejects non-numeric input", () => {
+    expect(parseChatLimit("abc")).toEqual({ ok: false, error: expect.any(String) });
+    expect(parseChatLimit("twelve")).toEqual({ ok: false, error: expect.any(String) });
+  });
+});
+
+describe("chatLimitChanged", () => {
+  it("is false when the typed value normalizes to the current limit", () => {
+    expect(chatLimitChanged("256", 256)).toBe(false);
+  });
+
+  it("is true when the typed value differs", () => {
+    expect(chatLimitChanged("100", 256)).toBe(true);
+    expect(chatLimitChanged("256", 100)).toBe(true);
+  });
+
+  it("is false for empty / invalid input (nothing submittable)", () => {
+    expect(chatLimitChanged("", 256)).toBe(false);
+    expect(chatLimitChanged("abc", 256)).toBe(false);
+  });
+
+  it("is false when the typed value clamps to the current limit", () => {
+    // 5 clamps to MIN (10); current is already 10 → no change.
+    expect(chatLimitChanged("5", MIN_CHAT_LIMIT)).toBe(false);
+    // A huge value clamps to MAX; current is already MAX → no change.
+    expect(chatLimitChanged("99999999", MAX_CHAT_LIMIT)).toBe(false);
+  });
+});
diff --git a/src/features/settings/logic/view-model.ts b/src/features/settings/logic/view-model.ts
new file mode 100644
index 0000000..73f6d17
--- /dev/null
+++ b/src/features/settings/logic/view-model.ts
@@ -0,0 +1,60 @@
+import { normalizeChatLimit } from "../../../core/chunks";
+
+/**
+ * Pure core for the settings feature — zero DOM, zero effects, zero Svelte.
+ *
+ * The settings feature exposes FE-local, user-tunable settings. Currently the
+ * chat limit (max loaded chunks per conversation; the policy lives in
+ * `core/chunks/trim.ts`): this module is the view-model seam that parses +
+ * validates a typed value into a normalized limit, so the UI can disable submit
+ * and show an error on garbage input. The bound constants + normalization are
+ * OWNED by `core/chunks` (the single source of truth); this module never
+ * redefines them.
+ */
+
+// ── Injected port (consumer-defines-port; the composition root adapts the
+//    store's localStorage persistence to this shape). ──────────────────────────
+
+/** Outcome of persisting a chat-limit setting. */
+export type ChatLimitSaveResult =
+  | { readonly ok: true; readonly chatLimit: number }
+  | { readonly ok: false; readonly error: string };
+
+export type SaveChatLimit = (value: number) => Promise<ChatLimitSaveResult>;
+
+// ── chat-limit parse / validate ───────────────────────────────────────────────
+
+/** Result of parsing a typed chat-limit string. */
+export type ChatLimitParse =
+  | { readonly ok: true; readonly value: number }
+  | { readonly ok: false; readonly error: string };
+
+/**
+ * Parse a typed chat-limit string into a normalized limit (floored + clamped to
+ * [MIN_CHAT_LIMIT, MAX_CHAT_LIMIT] via `normalizeChatLimit`). Empty or
+ * non-numeric input is an ERROR (so the UI can disable submit + message), NOT a
+ * silent default — the default only applies at boot for an unset key, never
+ * from user typing.
+ */
+export function parseChatLimit(raw: string): ChatLimitParse {
+  const trimmed = raw.trim();
+  if (trimmed.length === 0) {
+    return { ok: false, error: "Enter a number." };
+  }
+  const n = Number(trimmed);
+  if (!Number.isFinite(n)) {
+    return { ok: false, error: "Must be a number." };
+  }
+  return { ok: true, value: normalizeChatLimit(n) };
+}
+
+/**
+ * Whether saving `typed` would change the `current` chat limit. A no-op save
+ * (empty/invalid, or equal after normalization) should be disabled. This is the
+ * dirty-check for the input — it must NOT mutate or clamp, only compare.
+ */
+export function chatLimitChanged(typed: string, current: number): boolean {
+  const parsed = parseChatLimit(typed);
+  if (!parsed.ok) return false;
+  return parsed.value !== current;
+}