From bc430ea5a0b6839c30a642314b5f193c422ea175 Mon Sep 17 00:00:00 2001
From: Danijel Martinek <danijel@fraqtal.xyz>
Date: Tue, 5 May 2026 00:39:11 +0200
Subject: [PATCH] feat(auth): add signInUseCase (test red until DI lands)

---
 .../use-cases/sign-in.use-case.test.ts        | 52 +++++++++++++++++++
 .../application/use-cases/sign-in.use-case.ts | 34 ++++++++++++
 2 files changed, 86 insertions(+)
 create mode 100644 packages/auth/src/application/use-cases/sign-in.use-case.test.ts
 create mode 100644 packages/auth/src/application/use-cases/sign-in.use-case.ts

diff --git a/packages/auth/src/application/use-cases/sign-in.use-case.test.ts b/packages/auth/src/application/use-cases/sign-in.use-case.test.ts
new file mode 100644
index 0000000..df07299
--- /dev/null
+++ b/packages/auth/src/application/use-cases/sign-in.use-case.test.ts
@@ -0,0 +1,52 @@
+import { beforeEach, describe, expect, it } from "vitest";
+import { authContainer } from "@/di/container";
+import { AUTH_SYMBOLS } from "@/di/symbols";
+import { MockUsersRepository } from "@/infrastructure/repositories/mock-users.repository";
+import { MockAuthenticationService } from "@/infrastructure/services/mock-authentication.service";
+import type { IUsersRepository } from "@/application/repositories/users-repository.interface";
+import type { IAuthenticationService } from "@/application/services/authentication-service.interface";
+import { AuthenticationError } from "@/entities/errors";
+import { signInUseCase } from "./sign-in.use-case";
+
+describe("signInUseCase", () => {
+  let usersRepo: MockUsersRepository;
+  let authService: MockAuthenticationService;
+
+  beforeEach(() => {
+    if (authContainer.isBound(AUTH_SYMBOLS.IUsersRepository)) {
+      authContainer.unbind(AUTH_SYMBOLS.IUsersRepository);
+    }
+    if (authContainer.isBound(AUTH_SYMBOLS.IAuthenticationService)) {
+      authContainer.unbind(AUTH_SYMBOLS.IAuthenticationService);
+    }
+    usersRepo = new MockUsersRepository();
+    authService = new MockAuthenticationService(usersRepo);
+    authContainer
+      .bind<IUsersRepository>(AUTH_SYMBOLS.IUsersRepository)
+      .toConstantValue(usersRepo);
+    authContainer
+      .bind<IAuthenticationService>(AUTH_SYMBOLS.IAuthenticationService)
+      .toConstantValue(authService);
+  });
+
+  it("returns a session + cookie on valid credentials", async () => {
+    const result = await signInUseCase({
+      username: "alice",
+      password: "password_alice",
+    });
+    expect(result.session.userId).toBe("1");
+    expect(result.cookie.name).toBe("session");
+  });
+
+  it("throws AuthenticationError when user does not exist", async () => {
+    await expect(
+      signInUseCase({ username: "ghost", password: "anything" }),
+    ).rejects.toBeInstanceOf(AuthenticationError);
+  });
+
+  it("throws AuthenticationError on wrong password", async () => {
+    await expect(
+      signInUseCase({ username: "alice", password: "wrong" }),
+    ).rejects.toBeInstanceOf(AuthenticationError);
+  });
+});
diff --git a/packages/auth/src/application/use-cases/sign-in.use-case.ts b/packages/auth/src/application/use-cases/sign-in.use-case.ts
new file mode 100644
index 0000000..3a2c0b3
--- /dev/null
+++ b/packages/auth/src/application/use-cases/sign-in.use-case.ts
@@ -0,0 +1,34 @@
+import { AuthenticationError } from "../../entities/errors";
+import type { Cookie } from "../../entities/cookie";
+import type { Session } from "../../entities/session";
+import { authContainer } from "../../di/container";
+import { AUTH_SYMBOLS } from "../../di/symbols";
+import type { IUsersRepository } from "../repositories/users-repository.interface";
+import type { IAuthenticationService } from "../services/authentication-service.interface";
+
+export async function signInUseCase(input: {
+  username: string;
+  password: string;
+}): Promise<{ session: Session; cookie: Cookie }> {
+  const usersRepository = authContainer.get<IUsersRepository>(
+    AUTH_SYMBOLS.IUsersRepository,
+  );
+  const authService = authContainer.get<IAuthenticationService>(
+    AUTH_SYMBOLS.IAuthenticationService,
+  );
+
+  const existingUser = await usersRepository.getUserByUsername(input.username);
+  if (!existingUser) {
+    throw new AuthenticationError("User does not exist");
+  }
+
+  const validPassword = await authService.verifyPassword(
+    existingUser.passwordHash,
+    input.password,
+  );
+  if (!validPassword) {
+    throw new AuthenticationError("Incorrect username or password");
+  }
+
+  return await authService.createSession(existingUser);
+}