Nearfield_Friends/Proximity/Services/Auth/XProfileProvider.swift

import Foundation
import AuthenticationServices

/// Imports a user's X (Twitter) profile at reveal time.
///
/// X's API (v2) exposes a clean `GET /2/users/me` endpoint that returns the
/// authenticated user's handle, name, and bio — well-suited for enriching a
/// revealed connection. The OAuth 2.0 PKCE flow is used, and the token
/// exchange must happen server-side in production to protect the app secret.
final class XProfileProvider: NSObject, ProfileProvider {

    var providerID: ProfileProviderID { .x }

    private let clientID = "YOUR_X_CLIENT_ID"
    private let redirectURI = "com.proximity.app:/x"

    func fetchProfile() async throws -> ImportedProfile {
        // 1. OAuth 2.0 PKCE authorization.
        let verifier = generateCodeVerifier()
        let challenge = generateCodeChallenge(verifier)

        var components = URLComponents(string: "https://twitter.com/i/oauth2/authorize")!
        components.queryItems = [
            URLQueryItem(name: "response_type", value: "code"),
            URLQueryItem(name: "client_id", value: clientID),
            URLQueryItem(name: "redirect_uri", value: redirectURI),
            URLQueryItem(name: "scope", value: "users.read tweet.read"),
            URLQueryItem(name: "state", value: UUID().uuidString),
            URLQueryItem(name: "code_challenge", value: challenge),
            URLQueryItem(name: "code_challenge_method", value: "S256")
        ]
        guard let url = components.url else {
            throw ProfileImportError.authorizationFailed
        }

        let callback = try await presentAuthSession(url: url)
        guard let code = callback.queryItems?.first(where: { $0.name == "code" })?.value else {
            throw ProfileImportError.authorizationFailed
        }

        // 2. Exchange code for access token (production: server-side).
        let token = try await exchangeCode(code, verifier: verifier)

        // 3. Fetch the authenticated user's profile.
        var profileComponents = URLComponents(string: "https://api.twitter.com/2/users/me")!
        profileComponents.queryItems = [
            URLQueryItem(name: "user.fields", value: "name,username,description,profile_image_url")
        ]
        var request = URLRequest(url: profileComponents.url!)
        request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")

        let (data, _) = try await URLSession.shared.data(for: request)

        struct XResponse: Decodable {
            struct User: Decodable {
                let username: String
                let name: String?
                let description: String?
                let profile_image_url: String?
            }
            let data: User
        }

        let response = try JSONDecoder().decode(XResponse.self, from: data)
        let user = response.data

        return ImportedProfile(
            providerID: .x,
            handle: user.username,
            displayName: user.name,
            bio: user.description,
            avatarURL: user.profile_image_url.flatMap(URL.init(string:))
        )
    }

    // MARK: - Helpers

    private func presentAuthSession(url: URL) async throws -> URLComponents {
        let callback = try await withCheckedThrowingContinuation {
            (continuation: CheckedContinuation<URL, Error>) in
            let session = ASWebAuthenticationSession(
                url: url,
                callbackURLScheme: "com.proximity.app"
            ) { callback, error in
                if let error {
                    continuation.resume(throwing: error)
                } else if let callback {
                    continuation.resume(returning: callback)
                } else {
                    continuation.resume(throwing: ProfileImportError.authorizationFailed)
                }
            }
            session.presentationContextProvider = self
            session.start()
        }
        guard let components = URLComponents(url: callback, resolvingAgainstBaseURL: false) else {
            throw ProfileImportError.authorizationFailed
        }
        return components
    }

    private func exchangeCode(_ code: String, verifier: String) async throws -> String {
        // Production: POST to backend which exchanges code+verifier for a
        // bearer token using the client secret. Returns short-lived token.
        return "bearer-token"
    }

    // MARK: - PKCE

    private func generateCodeVerifier() -> String {
        var bytes = [UInt8](repeating: 0, count: 64)
        _ = SecRandomCopyBytes(kSecRandomDefault, bytes.count, &bytes)
        return Data(bytes).base64URLEncodedString()
    }

    private func generateCodeChallenge(_ verifier: String) -> String {
        let data = Data(verifier.utf8)
        let digest = SHA256.hash(data: data)
        return Data(digest).base64URLEncodedString()
    }
}

extension XProfileProvider: ASWebAuthenticationPresentationContextProviding {
    func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
        ASPresentationAnchor()
    }
}

// MARK: - SHA256 + base64url helpers

import CryptoKit

extension Data {
    /// Base64url (RFC 4648 §5) — no padding, URL-safe alphabet.
    func base64URLEncodedString() -> String {
        base64EncodedString()
            .replacingOccurrences(of: "+", with: "-")
            .replacingOccurrences(of: "/", with: "_")
            .replacingOccurrences(of: "=", with: "")
    }
}