//
//  ImageCacheService.swift
//  EmployeeDirectory
//
//  Created by Matt Bruce on 1/20/25.
//

import Foundation
import UIKit

/// A service that handles image caching using memory, disk, and network in priority order.
public class ImageCacheService {
    // MARK: - Properties
    public static let shared = ImageCacheService() // Default shared instance

    /// Memory cache for storing images in RAM.
    private let memoryCache = NSCache<NSString, UIImage>()
    
    /// File manager for handling disk operations.
    private let fileManager = FileManager.default
    
    /// Directory where cached images are stored on disk.
    private let cacheDirectory: URL = {
        FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)[0]
    }()
    
    // MARK: - Initializer
    
    public init() {}

    // MARK: - Public Methods
    
    /// Loads an image from memory, disk, or network.
    ///
    /// - Parameter url: The URL of the image to load.
    /// - Returns: The loaded `UIImage` or `nil` if the image could not be loaded.
    public func loadImage(from url: URL) async -> UIImage? {
        let uniqueKey = url.uniqueIdentifier
        let cacheKey = uniqueKey as NSString

        // Step 1: Check the memory cache
        if let cachedImage = memoryCache.object(forKey: cacheKey) {
            return cachedImage
        }
        
        // Step 2: Check the disk cache
        let diskImagePath = cacheDirectory.appendingPathComponent(uniqueKey)
        if let diskImage = UIImage(contentsOfFile: diskImagePath.path) {
            memoryCache.setObject(diskImage, forKey: cacheKey) // Cache in memory for faster access next time
            return diskImage
        }
        
        // Step 3: Fetch the image from the network
        if let networkImage = await fetchFromNetwork(url: url) {
            memoryCache.setObject(networkImage, forKey: cacheKey) // Cache in memory
            try? saveImageToDisk(image: networkImage, at: diskImagePath) // Cache on disk
            return networkImage
        }
        
        // Step 4: Return nil if all options fail
        return nil
    }
    
    // MARK: - Private Methods
    
    /// Fetches an image from the network.
    ///
    /// - Parameter url: The URL of the image to fetch.
    /// - Returns: The fetched `UIImage` or `nil` if the network request fails.
    private func fetchFromNetwork(url: URL) async -> UIImage? {
        do {
            let (data, _) = try await URLSession.shared.data(from: url)
            return UIImage(data: data)
        } catch {
            print("Failed to fetch image from network for URL: \(url). Error: \(error)")
            return nil
        }
    }
    
    /// Saves an image to disk at the specified path.
    ///
    /// - Parameters:
    ///   - image: The `UIImage` to save.
    ///   - path: The file path where the image should be saved.
    private func saveImageToDisk(image: UIImage, at path: URL) throws {
        guard let data = image.pngData() else { return }
        try data.write(to: path)
    }
}