import SwiftUI

struct PostDetailsCarouselView: View {
  @EnvironmentObject var manager: BooruManager
  @EnvironmentObject var settings: SettingsManager
  let posts: [BooruPost]
  let focusedPost: BooruPost?
  @Binding var loadingStage: BooruPostLoadingState
  @State private var currentIndex: Int?
  private let cacheManager = ImageCacheManager.shared

  init(
    posts: [BooruPost],
    loadingStage: Binding<BooruPostLoadingState>,
    focusedPost: BooruPost? = nil
  ) {
    self.posts = posts
    self.focusedPost = focusedPost
    _loadingStage = loadingStage

    if let focused = focusedPost,
      let index = posts.firstIndex(where: { $0.id == focused.id })
    {
      self._currentIndex = State(initialValue: index)
    } else {
      self._currentIndex = State(initialValue: 0)
    }
  }

  func imageURL(post: BooruPost) -> URL? {
    switch settings.detailViewQuality {
    case .preview:
      post.previewURL

    case .sample:
      post.sampleURL

    case .original:
      post.fileURL
    }
  }

  var body: some View {
    ScrollView(.horizontal) {
      LazyHStack(spacing: 0) {
        ForEach(Array(posts.enumerated()), id: \.offset) { index, post in
          PostDetailsCarouselItemView(
            post: post,
            index: index,
            loadingStage: $loadingStage,
            imageURL: imageURL
          )
          #if os(iOS)
            .frame(width: UIScreen.main.bounds.width)
          #endif
        }
      }
      .scrollTargetLayout()
    }
    .scrollPosition(id: $currentIndex)
    .scrollIndicators(.hidden)
    .scrollTargetBehavior(.paging)
    .onChange(of: currentIndex) {
      guard let currentIndex else { return }

      Task(priority: .utility) {
        if currentIndex == posts.count - 1 { await manager.loadNextPage() }
      }

      preloadNearbyImages()
    }
    .onAppear(perform: preloadNearbyImages)
  }

  private func preloadNearbyImages() {
    let preloadRange = settings.preloadedCarouselImages

    guard preloadRange > 0 else { return }
    guard let currentIndex else { return }

    let startIndex = max(0, currentIndex - preloadRange)
    let endIndex = min(posts.count - 1, currentIndex + preloadRange)
    var urlsToPreload: [URL] = []

    for index in startIndex...endIndex {
      if let url = imageURL(post: posts[index]) {
        urlsToPreload.append(url)
      }

      urlsToPreload.append(posts[index].previewURL)
    }

    cacheManager.preloadImages(urlsToPreload)
  }
}