import Foundation
import UIKit

/// Album art: a memory cache over a big disk URLCache, so downloaded albums keep their covers
/// offline. Stands in for Coil on Android.
final class ArtLoader: @unchecked Sendable {
    static let shared = ArtLoader()

    private let memory = NSCache<NSURL, UIImage>()
    private let session: URLSession
    private let lock = NSLock()
    private var inflight: [URL: Task<UIImage?, Never>] = [:]

    init() {
        let dir = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)[0].appendingPathComponent("art", isDirectory: true)
        let cfg = URLSessionConfiguration.default
        cfg.urlCache = URLCache(memoryCapacity: 16 << 20, diskCapacity: 512 << 20, directory: dir)
        cfg.requestCachePolicy = .returnCacheDataElseLoad
        cfg.timeoutIntervalForRequest = 20
        session = URLSession(configuration: cfg)
        memory.totalCostLimit = 64 << 20
    }

    func cached(_ url: URL) -> UIImage? { memory.object(forKey: url as NSURL) }

    func image(_ url: URL) async -> UIImage? {
        if let img = cached(url) { return img }
        if let t = lock.withLock({ inflight[url] }) { return await t.value }
        let t = Task<UIImage?, Never> { [session] in
            guard let (data, resp) = try? await session.data(from: url),
                  (resp as? HTTPURLResponse).map({ (200..<300).contains($0.statusCode) }) ?? true,
                  let img = UIImage(data: data) else { return nil }
            let decoded = await img.byPreparingForDisplay() ?? img
            return decoded
        }
        lock.withLock { inflight[url] = t }
        let img = await t.value
        lock.withLock { inflight[url] = nil }
        if let img {
            memory.setObject(img, forKey: url as NSURL, cost: Int(img.size.width * img.size.height * img.scale * img.scale * 4))
        }
        return img
    }

    func prefetch(_ url: URL) {
        Task.detached(priority: .utility) { _ = await self.image(url) }
    }
}