Implemented the Auth middleware (#3)

This PR contains the work done to implement the `AuthMiddleware` middleware that, when plugged into a `Client` object, adds authentication parameters to any request the client would make.

Reviewed-on: #3
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/MarvelService/Internal/Extensions/TimeInterval+Computed.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the MarvelService open source project
+//
+// Copyright (c) -2025 Röck+Cöde VoF. and the MarvelService project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of MarvelService project authors
+//
+//===----------------------------------------------------------------------===
+
+import struct Foundation.TimeInterval
+
+extension TimeInterval {
+    
+    // MARK: Functions
+    
+    /// Converts a time interval to a string value.
+    /// - Returns: A time interval as a string.
+    var asString: String {
+        .init(format: "%f", self)
+    }
+    
+}

Sources/MarvelService/Internal/Use Cases/GenerateHashUseCase.swift

@@ -0,0 +1,60 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the MarvelService open source project
+//
+// Copyright (c) -2025 Röck+Cöde VoF. and the MarvelService project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of MarvelService project authors
+//
+//===----------------------------------------------------------------------===
+
+import CryptoKit
+
+import struct Foundation.Data
+import struct Foundation.TimeInterval
+
+/// A use case that generates a MD5 hash value, which is required to authenticate any request.
+struct GenerateHashUseCase {
+    
+    // MARK: Properties
+    
+    /// A private key.
+    private let privateKey: String
+    
+    /// A public key.
+    private let publicKey: String
+
+    // MARK: Initializers
+    
+    /// Initializes this use case.
+    /// - Parameters:
+    ///   - privateKey: A private key.
+    ///   - publicKey: A public key.
+    init(
+        privateKey: String,
+        publicKey: String,
+    ) {
+        self.privateKey = privateKey
+        self.publicKey = publicKey
+    }
+    
+    // MARK: Functions
+    
+    /// Generates a MD5 hash value out of a given public key, private key and a timestamp.
+    /// - Parameter timestamp: A timestamp that changes on a request-by-request basis.
+    /// - Returns: A MD5 hash generated out of a private key, an public key, and a timestamp.
+    func callAsFunction(
+        timestamp: TimeInterval
+    ) -> String {
+        let stringToHash = timestamp.asString + self.privateKey + self.publicKey
+        let dataToHash = Data(stringToHash.utf8)
+
+        return Insecure.MD5
+            .hash(data: dataToHash)
+            .map { String(format: "%02x", $0) }
+            .joined()
+    }
+    
+}

Sources/MarvelService/Public/Middlewares/AuthMiddleware.swift

@@ -0,0 +1,104 @@
+//===----------------------------------------------------------------------===
+//
+// This source file is part of the MarvelService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the MarvelService project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of MarvelService project authors
+//
+//===----------------------------------------------------------------------===
+
+import class OpenAPIRuntime.HTTPBody
+
+import protocol OpenAPIRuntime.ClientMiddleware
+
+import struct Foundation.Date
+import struct Foundation.TimeInterval
+import struct Foundation.URL
+import struct Foundation.URLComponents
+import struct HTTPTypes.HTTPRequest
+import struct HTTPTypes.HTTPResponse
+
+/// A middleware that attaches the necessary authentication parameters to the path of the request.
+public struct AuthMiddleware {
+    
+    // MARK: Properties
+    
+    /// A use case that generates a MD5 hash value to use as an authentication parameter.
+    private let hash: GenerateHashUseCase
+    
+    /// A Marvel API public key.
+    private let publicKey: String
+    
+    // MARK: Initializers
+
+    /// Initializes this middleware with private and public keys.
+    ///
+    /// The middleware attaches the required `apikey`, `ts`, and `hash` parameters to the URI path of the intercepted request.
+    /// This initializer should be used for server-side applications, as indicated in the [Marvel API documentation](https://developer.marvel.com/documentation/authorization)
+    ///
+    /// - Parameters:
+    ///   - privateKey: A Marvel API private key.
+    ///   - publicKey: A Marvel API public key.
+    public init(
+        privateKey: String,
+        publicKey: String
+    ) {
+        self.hash = .init(
+            privateKey: privateKey,
+            publicKey: publicKey
+        )
+        self.publicKey = publicKey
+    }
+    
+}
+
+// MARK: - ClientMiddleware
+
+extension AuthMiddleware: ClientMiddleware {
+    
+    // MARK: Functions
+    
+    public func intercept(
+        _ request: HTTPRequest,
+        body: HTTPBody?,
+        baseURL: URL,
+        operationID: String,
+        next: @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)
+    ) async throws -> (HTTPResponse, HTTPBody?) {
+        guard
+            let uriPath = request.path,
+            var urlComponents = URLComponents(string: uriPath)
+        else {
+            return try await next(request, body, baseURL)
+        }
+
+        let queryItems = urlComponents.queryItems ?? []
+        let timestamp = Date().timeIntervalSince1970
+
+        urlComponents.queryItems = queryItems + [
+            .init(name: "ts", value: timestamp.asString),
+            .init(name: "apikey", value: publicKey),
+            .init(name: "hash", value: hash(timestamp: timestamp))
+        ]
+        
+        let newPath = if let urlQuery = urlComponents.query {
+            urlComponents.path + "?" + urlQuery
+        } else {
+            urlComponents.path
+        }
+
+        let newRequest = HTTPRequest(
+            method: request.method,
+            scheme: request.scheme,
+            authority: request.authority,
+            path: newPath,
+            headerFields: request.headerFields
+        )
+
+        return try await next(newRequest, body, baseURL)
+    }
+    
+}