import SwiftUI

struct WallpaperItem: View {
  let wallpaper: Wallpaper
  let action: () -> Void
  @ObservedObject var favoritesManager = FavoritesManager.shared
  @State private var retryID = UUID()
  @State private var attempt = 0
  @State private var isHovering = false

  // Transient thumbnail failures (flaky network) are common; retry a few
  // times with growing backoff before giving up quietly.
  let maxAttempts: Int
  let retryDelayNanoseconds: UInt64

  init(
    wallpaper: Wallpaper, action: @escaping () -> Void, attempt: Int = 0,
    maxAttempts: Int = 4, retryDelayNanoseconds: UInt64 = 800_000_000
  ) {
    self.wallpaper = wallpaper
    self.action = action
    self.maxAttempts = maxAttempts
    self.retryDelayNanoseconds = retryDelayNanoseconds
    _attempt = State(initialValue: attempt)
  }

  var body: some View {
    Button(action: action) {
      AsyncImage(url: URL(string: wallpaper.thumbs.small)) { thumbnail(for: $0) }
        .id(retryID)
        .frame(height: 100)
        .clipped()
        .cornerRadius(8)
        .overlay(
          RoundedRectangle(cornerRadius: 8)
            .stroke(isHovering ? Color.accentColor : Color.clear, lineWidth: 2)
        )
        .shadow(radius: isHovering ? 4 : 0)
        .overlay(
          // Favorite Button
          Group {
            if isHovering || favoritesManager.isFavorite(wallpaper) {
              Button(action: toggleFavorite) {
                Image(systemName: favoritesManager.isFavorite(wallpaper) ? "heart.fill" : "heart")
                  .foregroundColor(favoritesManager.isFavorite(wallpaper) ? .red : .white)
                  .shadow(radius: 2)
                  .font(.system(size: 16, weight: .bold))
                  .padding(6)
                  .background(Color.black.opacity(0.3))
                  .clipShape(Circle())
              }
              .buttonStyle(.plain)
              .padding(6)
              .transition(.opacity)
            }
          },
          alignment: .topTrailing
        )
        .scaleEffect(isHovering ? 1.02 : 1.0)
        .animation(.spring(response: 0.3), value: isHovering)
    }
    .buttonStyle(.plain)
    .onHover { isHovering = $0 }
    .help("\(wallpaper.resolution)\n\(wallpaper.purity)")
  }

  func thumbnail(for phase: AsyncImagePhase) -> AnyView {
    if case .success(let image) = phase {
      return AnyView(image.resizable().aspectRatio(contentMode: .fill))
    }
    if case .failure = phase {
      return AnyView(
        Rectangle().fill(Color.gray.opacity(0.2)).overlay(
          attempt < maxAttempts
            ? AnyView(ProgressView())
            : AnyView(Image(systemName: "photo").foregroundColor(.gray.opacity(0.6)))
        ).task { await retryAfterFailure() })
    }
    return AnyView(Rectangle().fill(Color.gray.opacity(0.2)).overlay(ProgressView()))
  }

  /// Exponential-backoff retry. Each successive failure doubles the wait
  /// (`base * 2^attempt`) so a flaky host gets a chance to recover without
  /// hammering it, while still bounding the total wait under
  /// `maxAttempts * base` for the default parameters.
  ///
  /// Cancellation-safe: bails out immediately when the hosting task is
  /// already cancelled, and exits early if `Task.sleep` itself is
  /// interrupted mid-wait so a disappearing row never advances the
  /// attempt counter or rebinds `retryID`.
  /// Exposes the doubling backoff schedule for tests so callers can
  /// assert the wait times without having to actually sleep.
  func backoffDelay(forAttempt attempt: Int) -> UInt64 {
    let safe = max(0, attempt)
    let multiplier = UInt64(1) << safe
    return multiplier &* retryDelayNanoseconds
  }

  func retryAfterFailure() async {
    guard attempt < maxAttempts else { return }
    if Task.isCancelled { return }

    // Doubling schedule: 1x, 2x, 4x, 8x, ... `attempt` stays small
    // (≤ maxAttempts), so the shift never overflows UInt64.
    let multiplier = UInt64(1) << attempt
    let delay = multiplier &* retryDelayNanoseconds
    do {
      try await Task.sleep(nanoseconds: delay)
    } catch {
      return
    }
    guard !Task.isCancelled else { return }
    attempt += 1
    retryID = UUID()
  }

  /// Test seam: read-only access to the internal retry counter.
  var exposedAttempt: Int { attempt }

  func toggleFavorite() { favoritesManager.toggleFavorite(wallpaper) }
}