import Foundation
import CryptoKit

/// Manages the user's **ephemeral rotating anonymous token**.
///
/// This is the heart of the privacy model. While present, the app broadcasts a
/// token that:
///  - contains **no identity** (no name, no persistent ID),
///  - **rotates** on a short interval (e.g. every 5 minutes) so it cannot be
///    used to track the user over time,
///  - is only meaningful to the Proximity backend, which maps two tokens that
///    were physically near each other into an anonymous encounter.
///
/// The token is derived from a secret + time window, so it changes
/// deterministically without needing to re-broadcast a new random value.
struct TokenManager {

    /// How long a single token remains valid before rotating.
    static let rotationInterval: TimeInterval = 5 * 60 // 5 minutes

    private let secret: SymmetricKey

    init(secret: SymmetricKey = SymmetricKey(size: .bits256)) {
        self.secret = secret
    }

    /// The current anonymous token for a given time window.
    /// Deterministic per window so both the broadcast and the backend agree.
    func currentToken(at date: Date = Date()) -> String {
        let window = Int(date.timeIntervalSince1970 / Self.rotationInterval)
        return token(forWindow: window)
    }

    /// The token for a specific rotation window.
    func token(forWindow window: Int) -> String {
        var data = Data()
        data.append(String(window).data(using: .utf8)!)
        let mac = HMAC<SHA256>.authenticationCode(for: data, using: secret)
        return Data(mac).base64EncodedString()
    }

    /// Whether a received remote token is still within a valid rotation window.
    func isValid(_ token: String, at date: Date = Date()) -> Bool {
        // Tokens are valid for the current window plus one prior window,
        // to tolerate clock skew between devices.
        let current = Int(date.timeIntervalSince1970 / Self.rotationInterval)
        for window in (current - 1)...current {
            if token(forWindow: window) == token { return true }
        }
        return false
    }
}