Nearfield_Friends/Proximity/Services/Auth/AppleAuthProvider.swift

import Foundation
import AuthenticationServices
import CryptoKit

/// Apple "Sign in with Apple" — the primary, privacy-respecting auth path.
///
/// Sign in with Apple is ideal for Proximity because it is designed around
/// privacy: users can hide their real email (relay), and we get a stable
/// opaque user identifier without a social graph. This keeps *account*
/// identity cleanly separated from any *revealed* social identity.
final class AppleAuthProvider: NSObject, AuthProvider, ASAuthorizationControllerDelegate {

    var providerID: AuthProviderID { .apple }

    private var continuation: CheckedContinuation<AuthResult, Error>?
    private var currentNonce: String?

    // MARK: - AuthProvider

    func signIn() async throws -> AuthResult {
        let nonce = randomNonceString()
        currentNonce = nonce

        let request = ASAuthorizationAppleIDProvider().createRequest()
        request.requestedScopes = [.fullName, .email]
        request.nonce = sha256(nonce)

        let controller = ASAuthorizationController(authorizationRequests: [request])
        controller.delegate = self
        controller.presentationContextProvider = self
        controller.performRequests()

        return try await withCheckedThrowingContinuation { continuation in
            self.continuation = continuation
        }
    }

    func signOut() async throws {
        // Sign in with Apple has no server-side session to revoke here;
        // our backend session is revoked separately.
    }

    // MARK: - ASAuthorizationControllerDelegate

    func authorizationController(controller: ASAuthorizationController,
                                  didCompleteWithAuthorization authorization: ASAuthorization) {
        guard let credential = authorization.credential as? ASAuthorizationAppleIDCredential,
              let idTokenData = credential.identityToken,
              let idToken = String(data: idTokenData, encoding: .utf8) else {
            continuation?.resume(throwing: AuthError.invalidCredential)
            continuation = nil
            return
        }

        let profile = ProviderProfile(
            name: credential.fullName?.formatted(),
            email: credential.email,
            givenName: credential.fullName?.givenName,
            familyName: credential.fullName?.familyName
        )

        continuation?.resume(returning: AuthResult(
            providerID: .apple,
            idToken: idToken,
            nonce: currentNonce,
            profile: profile
        ))
        continuation = nil
    }

    func authorizationController(controller: ASAuthorizationController,
                                 didCompleteWithError error: Error) {
        continuation?.resume(throwing: error)
        continuation = nil
    }

    // MARK: - Nonce helpers

    private func randomNonceString(length: Int = 32) -> String {
        precondition(length > 0)
        let charset: [Character] =
            Array("0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._")
        var result = ""
        var remaining = length
        while remaining > 0 {
            let randoms: [UInt8] = (0..<16).map { _ in
                var r: UInt8 = 0
                SecRandomCopyBytes(kSecRandomDefault, 1, &r)
                return r
            }
            randoms.forEach { random in
                if remaining == 0 { return }
                if random < charset.count {
                    result.append(charset[Int(random)])
                    remaining -= 1
                }
            }
        }
        return result
    }

    private func sha256(_ input: String) -> String {
        let hashed = SHA256.hash(data: Data(input.utf8))
        return hashed.map { String(format: "%02x", $0) }.joined()
    }
}

/// Errors thrown by the auth layer.
enum AuthError: Error, LocalizedError {
    case invalidCredential
    case providerUnavailable

    var errorDescription: String? {
        switch self {
        case .invalidCredential: return "The sign-in credential was invalid."
        case .providerUnavailable: return "This sign-in option is unavailable."
        }
    }
}

// MARK: - Presentation context provider

extension AppleAuthProvider: ASAuthorizationControllerPresentationContextProviding {
    func presentationAnchor(for controller: ASAuthorizationController) -> ASPresentationAnchor {
        // In production, return the active window scene.
        ASPresentationAnchor()
    }
}