import Foundation

struct UsageCacheEntry: Codable, Sendable {
    let fetchedAt: Date
    let payload: Data
}

actor UsageCache {
    static let shared = UsageCache()

    private let directory: URL
    private let fileManager = FileManager.default

    init() {
        let base = fileManager.urls(for: .cachesDirectory, in: .userDomainMask).first!
        directory = base.appendingPathComponent("ai-usage", isDirectory: true)
        try? fileManager.createDirectory(at: directory, withIntermediateDirectories: true)
    }

    func load(provider: ProviderID, maxAge: TimeInterval) -> UsageCacheEntry? {
        let url = cacheURL(for: provider)
        guard fileManager.fileExists(atPath: url.path),
              let data = try? Data(contentsOf: url),
              let entry = try? JSONDecoder().decode(UsageCacheEntry.self, from: data)
        else {
            return nil
        }
        if Date().timeIntervalSince(entry.fetchedAt) > maxAge {
            return nil
        }
        return entry
    }

    func loadStale(provider: ProviderID) -> UsageCacheEntry? {
        let url = cacheURL(for: provider)
        guard fileManager.fileExists(atPath: url.path),
              let data = try? Data(contentsOf: url),
              let entry = try? JSONDecoder().decode(UsageCacheEntry.self, from: data)
        else {
            return nil
        }
        return entry
    }

    func save(provider: ProviderID, payload: Data) {
        let entry = UsageCacheEntry(fetchedAt: Date(), payload: payload)
        guard let data = try? JSONEncoder().encode(entry) else { return }
        let url = cacheURL(for: provider)
        try? data.write(to: url, options: .atomic)
        try? fileManager.setAttributes([.posixPermissions: 0o600], ofItemAtPath: url.path)
    }

    private func cacheURL(for provider: ProviderID) -> URL {
        directory.appendingPathComponent("\(provider.rawValue)-usage.json")
    }
}