import Foundation

/// Fiat prices for the two tickers the wallet supports, fetched from a public
/// price API. Cached briefly in memory and in UserDefaults so screens do not
/// refetch on every refresh, and so the currency conversion shows up instantly
/// on a cold start.
final class PriceService {
  static let shared = PriceService()

  struct Prices: Codable {
    var xrp: Double?
    var xmr: Double?
    var currency: String
    var fetchedAt: Date?
  }

  private static let cacheLifetime: TimeInterval = 60
  private static let defaultsKey = "koins.prices"

  private var memory: Prices?
  private var inFlight: Task<Prices, Never>?

  private init() {
    if let data = UserDefaults.standard.data(forKey: Self.defaultsKey),
       let saved = try? JSONDecoder().decode(Prices.self, from: data) {
      memory = saved
    }
  }

  func prices(currency: String) async -> Prices {
    if let memory,
       memory.currency == currency,
       let fetchedAt = memory.fetchedAt,
       Date().timeIntervalSince(fetchedAt) < Self.cacheLifetime {
      return memory
    }
    if let inFlight { return await inFlight.value }

    let task = Task<Prices, Never> { [weak self] in
      guard let self else { return Prices(currency: currency, fetchedAt: nil) }
      let fetched = await self.fetch(currency: currency)
      self.memory = fetched
      if let data = try? JSONEncoder().encode(fetched) {
        UserDefaults.standard.set(data, forKey: Self.defaultsKey)
      }
      return fetched
    }
    inFlight = task
    let result = await task.value
    inFlight = nil
    return result
  }

  private func fetch(currency: String) async -> Prices {
    if let prices = try? await fromCoinGecko(currency: currency) { return prices }
    return (try? await fromKraken(currency: currency)) ?? Prices(currency: currency, fetchedAt: nil)
  }

  private func fromCoinGecko(currency: String) async throws -> Prices {
    let code = currency.lowercased()
    guard let url = URL(string: "https://api.coingecko.com/api/v3/simple/price?ids=monero,ripple&vs_currencies=\(code)") else {
      throw XRPError.network("Invalid price endpoint")
    }
    let (data, response) = try await URLSession.shared.data(from: url)
    guard let http = response as? HTTPURLResponse, (200..<300).contains(http.statusCode) else {
      throw XRPError.network("CoinGecko request failed")
    }
    let json = try JSONSerialization.jsonObject(with: data) as? [String: Any] ?? [:]
    return Prices(
      xrp: (json["ripple"] as? [String: Any]).flatMap { ($0[code] as? NSNumber)?.doubleValue },
      xmr: (json["monero"] as? [String: Any]).flatMap { ($0[code] as? NSNumber)?.doubleValue },
      currency: currency,
      fetchedAt: Date())
  }

  /// Kraken only lists USD and EUR for both assets; anything else just yields
  /// no price rather than a wrong one.
  private func fromKraken(currency: String) async throws -> Prices {
    let supported: [String: (xrp: String, xmr: String)] = [
      "USD": ("XXRPZUSD", "XXMRZUSD"),
      "EUR": ("XXRPZEUR", "XXMRZEUR"),
    ]
    guard let pairs = supported[currency] else {
      throw XRPError.network("Kraken has no \(currency) pairs")
    }
    guard let url = URL(string: "https://api.kraken.com/0/public/Ticker?pair=\(pairs.xrp),\(pairs.xmr)") else {
      throw XRPError.network("Invalid price endpoint")
    }
    let (data, response) = try await URLSession.shared.data(from: url)
    guard let http = response as? HTTPURLResponse, (200..<300).contains(http.statusCode) else {
      throw XRPError.network("Kraken request failed")
    }
    let json = try JSONSerialization.jsonObject(with: data) as? [String: Any] ?? [:]
    let result = json["result"] as? [String: Any] ?? [:]

    func lastTrade(_ key: String) -> Double? {
      (result[key] as? [String: Any])
        .flatMap { entry in (entry["c"] as? [Any])?.first }
        .flatMap { ($0 as? String).flatMap(Double.init) }
    }
    return Prices(xrp: lastTrade(pairs.xrp),
                  xmr: lastTrade(pairs.xmr),
                  currency: currency,
                  fetchedAt: Date())
  }
}