import AppKit
import Foundation
import QuickLookThumbnailing

/// サムネイル生成。QuickLook 任せなので HEIC や RAW もそのまま扱える。
@MainActor
final class ThumbnailLoader {
    static let shared = ThumbnailLoader()

    private let cache = NSCache<NSString, NSImage>()

    private init() {
        cache.countLimit = 600
    }

    /// すでに読み込み済みなら即座に返す。スクロールで戻ってきたときに
    /// 一瞬プレースホルダが出るのを防ぐために使う。
    func cached(for url: URL, size: CGFloat) -> NSImage? {
        cache.object(forKey: Self.key(url, size))
    }

    func thumbnail(for url: URL, size: CGFloat) async -> NSImage? {
        let key = Self.key(url, size)
        if let cached = cache.object(forKey: key) { return cached }

        let scale = NSScreen.main?.backingScaleFactor ?? 2
        let request = QLThumbnailGenerator.Request(
            fileAt: url,
            size: CGSize(width: size, height: size),
            scale: scale,
            representationTypes: .thumbnail
        )

        guard let representation = try? await QLThumbnailGenerator.shared
            .generateBestRepresentation(for: request)
        else { return nil }

        let image = NSImage(cgImage: representation.cgImage, size: representation.contentRect.size)
        cache.setObject(image, forKey: key)
        return image
    }

    private static func key(_ url: URL, _ size: CGFloat) -> NSString {
        "\(url.path)|\(Int(size))" as NSString
    }
}