Implemented the MakeAuthorizationHeaderUseCase use case.

Libraries/Package.swift

@@ -4,6 +4,7 @@ import PackageDescription
 
 let package = Package(
     name: "Libraries",
+    platforms: [.iOS(.v15)],
     products: [
         .library(
             name: "Libraries",

Libraries/Sources/APIService/Use Cases/MakeAuthorizationHeaderUseCase.swift

@@ -0,0 +1,46 @@
+//
+//  MakeAuthorizationHeaderUseCase.swift
+//  APIService
+//
+//  Created by Javier Cicchelli on 04/12/2022.
+//  Copyright © 2022 Röck+Cöde. All rights reserved.
+//
+
+import Foundation
+
+struct MakeAuthorizationHeaderUseCase {
+    func callAsFunction(
+        username: String,
+        password: String
+    ) throws -> [String: String] {
+        guard !username.isEmpty else { throw MakeAuthorizationHeaderError.usernameIsEmpty }
+        guard !password.isEmpty else { throw MakeAuthorizationHeaderError.passwordIsEmpty }
+        
+        let loginString = String(format: .Formats.usernameAndPassword, username, password)
+        
+        guard let loginData = loginString.data(using: .utf8) else {
+            throw MakeAuthorizationHeaderError.loginDataNotCreated
+        }
+        
+        let loginBase64 = loginData.base64EncodedString()
+        
+        return [.Header.Keys.authorization: String(format: .Formats.authorizationValue, loginBase64)]
+    }
+}
+
+// MARK: - Errors
+
+enum MakeAuthorizationHeaderError: Error {
+    case usernameIsEmpty
+    case passwordIsEmpty
+    case loginDataNotCreated
+}
+
+// MARK: - String+Formats
+
+private extension String {
+    enum Formats {
+        static let usernameAndPassword = "%@:%@"
+        static let authorizationValue = "Basic %@"
+    }
+}

Libraries/Tests/APIServiceTests/Cases/Use Cases/MakeAuthorizationHeaderUseCaseTests.swift

@@ -0,0 +1,73 @@
+//
+//  MakeAuthorizationHeaderUseCaseTests.swift
+//  APIServiceTests
+//
+//  Created by Javier Cicchelli on 04/12/2022.
+//  Copyright © 2022 Röck+Cöde. All rights reserved.
+//
+
+import XCTest
+
+@testable import APIService
+
+final class MakeAuthorizationHeaderUseCaseTests: XCTestCase {
+    
+    // MARK: Properties
+    
+    let makeAuthHeader = MakeAuthorizationHeaderUseCase()
+    
+    var username: String!
+    var password: String!
+    
+    var result: [String: String]!
+    
+    // MARK: Test cases
+    
+    func test_withCorrectUsernameAndPassword() throws {
+        // GIVEN
+        username = "username"
+        password = "password"
+        
+        // WHEN
+        
+        result = try makeAuthHeader(username: username, password: password)
+        
+        // THEN
+        XCTAssertEqual(result.count, 1)
+        XCTAssertEqual(result[.Header.Keys.authorization], "Basic dXNlcm5hbWU6cGFzc3dvcmQ=")
+    }
+    
+    func test_withEmptyUsername() throws {
+        // GIVEN
+        username = ""
+        password = "password"
+        
+        // WHEN & THEN
+        XCTAssertThrowsError(try makeAuthHeader(username: username, password: password)) { error in
+            XCTAssertEqual(error as? MakeAuthorizationHeaderError, .usernameIsEmpty)
+        }
+    }
+    
+    func test_withEmptyPassword() throws {
+        // GIVEN
+        username = "username"
+        password = ""
+        
+        // WHEN & THEN
+        XCTAssertThrowsError(try makeAuthHeader(username: username, password: password)) { error in
+            XCTAssertEqual(error as? MakeAuthorizationHeaderError, .passwordIsEmpty)
+        }
+    }
+    
+    func test_withEmptyUsernameAndPassword() throws {
+        // GIVEN
+        username = ""
+        password = ""
+        
+        // WHEN & THEN
+        XCTAssertThrowsError(try makeAuthHeader(username: username, password: password)) { error in
+            XCTAssertEqual(error as? MakeAuthorizationHeaderError, .usernameIsEmpty)
+        }
+    }
+    
+}