// LocalASRModelDownloadClient.swift
// OSGKeyboard · Shared
//
// URLSession download with byte-level progress and pause/resume (macOS local model installs).

import Foundation

#if os(macOS)

public struct LocalASRDownloadProgressUpdate: Sendable {
    public let bytesReceived: Int64
    public let bytesTotal: Int64

    public var fraction: Double {
        guard bytesTotal > 0 else { return 0 }
        return min(1, max(0, Double(bytesReceived) / Double(bytesTotal)))
    }
}

/// Controls an in-flight URLSession download; supports pause via resume data.
public final class LocalASRModelDownloadController: NSObject, URLSessionDownloadDelegate, @unchecked Sendable {
    private let destinationURL: URL
    private let onProgress: @Sendable (LocalASRDownloadProgressUpdate) -> Void
    private lazy var delegateSession: URLSession = {
        URLSession(configuration: .default, delegate: self, delegateQueue: nil)
    }()

    private var remoteURL: URL?
    private var task: URLSessionDownloadTask?
    private var completionContinuation: CheckedContinuation<Void, Error>?
    private var isPausing = false
    private var finished = false

    init(
        destinationURL: URL,
        onProgress: @escaping @Sendable (LocalASRDownloadProgressUpdate) -> Void
    ) {
        self.destinationURL = destinationURL
        self.onProgress = onProgress
        super.init()
    }

    /// Runs until the archive is fully written to `destinationURL` (survives pause/resume).
    public func download(from remoteURL: URL) async throws {
        self.remoteURL = remoteURL
        try await withCheckedThrowingContinuation { (continuation: CheckedContinuation<Void, Error>) in
            completionContinuation = continuation
            startTask(resumeData: nil)
        }
    }

    public func pause() async throws -> Data {
        guard task != nil, !finished else {
            throw LocalASRModelManagerError.downloadFailed("No active download to pause.")
        }
        return try await withCheckedThrowingContinuation { (continuation: CheckedContinuation<Data, Error>) in
            isPausing = true
            task?.cancel(byProducingResumeData: { [weak self] data in
                guard let self else { return }
                self.isPausing = false
                if let data {
                    continuation.resume(returning: data)
                } else {
                    continuation.resume(throwing: LocalASRModelManagerError.downloadFailed("Pause failed."))
                }
            })
        }
    }

    /// Continues a paused download; `download(from:)` must still be awaiting.
    public func resumeFromPause(_ resumeData: Data) {
        finished = false
        startTask(resumeData: resumeData)
    }

    public func cancel() {
        finished = true
        task?.cancel()
        completionContinuation?.resume(throwing: CancellationError())
        completionContinuation = nil
        delegateSession.invalidateAndCancel()
    }

    private func startTask(resumeData: Data?) {
        if let resumeData {
            task = delegateSession.downloadTask(withResumeData: resumeData)
        } else if let remoteURL {
            task = delegateSession.downloadTask(with: remoteURL)
        }
        task?.resume()
    }

    // MARK: - URLSessionDownloadDelegate

    public func urlSession(
        _ session: URLSession,
        downloadTask: URLSessionDownloadTask,
        didWriteData bytesWritten: Int64,
        totalBytesWritten: Int64,
        totalBytesExpectedToWrite: Int64
    ) {
        onProgress(
            LocalASRDownloadProgressUpdate(
                bytesReceived: totalBytesWritten,
                bytesTotal: max(totalBytesExpectedToWrite, 1)
            )
        )
    }

    public func urlSession(
        _ session: URLSession,
        downloadTask: URLSessionDownloadTask,
        didFinishDownloadingTo location: URL
    ) {
        guard !finished else { return }
        finished = true
        do {
            let fm = FileManager.default
            if fm.fileExists(atPath: destinationURL.path) {
                try fm.removeItem(at: destinationURL)
            }
            try fm.moveItem(at: location, to: destinationURL)
            completionContinuation?.resume()
        } catch {
            completionContinuation?.resume(
                throwing: LocalASRModelManagerError.downloadFailed(error.localizedDescription)
            )
        }
        completionContinuation = nil
        session.finishTasksAndInvalidate()
    }

    public func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        guard !finished else { return }
        if isPausing { return }
        if let error {
            finished = true
            completionContinuation?.resume(
                throwing: LocalASRModelManagerError.downloadFailed(error.localizedDescription)
            )
            completionContinuation = nil
            session.finishTasksAndInvalidate()
        }
    }
}

public enum LocalASRModelDownloadClient {

    public static func makeController(
        destinationURL: URL,
        onProgress: @escaping @Sendable (LocalASRDownloadProgressUpdate) -> Void
    ) -> LocalASRModelDownloadController {
        LocalASRModelDownloadController(destinationURL: destinationURL, onProgress: onProgress)
    }
}

#endif