// Copyright 2026 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import Foundation
import XCTest

@available(macOS 12.0, watchOS 8.0, *)
class MockURLProtocol: URLProtocol, @unchecked Sendable {
  typealias MockURLRequestHandler = (URLRequest) throws -> (
    URLResponse,
    AsyncLineSequence<URL.AsyncBytes>?
  )

  private nonisolated(unsafe) static var _requestHandlers = [MockURLRequestHandler]()

  nonisolated(unsafe) static var errorToThrowMidStream: Error?
  nonisolated(unsafe) static var stopLoadingExpectation: XCTestExpectation?
  nonisolated(unsafe) static var neverFinishes: Bool = false

  nonisolated(unsafe) static var requestHandler: MockURLRequestHandler? {
    get {
      assert(
        _requestHandlers.count <= 1,
        "More than one request handler is configured; use `requestHandlers` instead."
      )
      return _requestHandlers.first
    }
    set {
      _requestHandlers = newValue.map { [$0] } ?? []
    }
  }

  nonisolated(unsafe) static var requestHandlersQueue: [MockURLRequestHandler] {
    get { _requestHandlers }
    set { _requestHandlers = newValue }
  }

  private var _loadingTask: Task<Void, Never>?
  private let lock = NSLock()

  private var loadingTask: Task<Void, Never>? {
    get {
      lock.lock()
      defer { lock.unlock() }
      return _loadingTask
    }
    set {
      lock.lock()
      _loadingTask = newValue
      lock.unlock()
    }
  }

  override class func canInit(with request: URLRequest) -> Bool {
    #if os(watchOS)
      print("MockURLProtocol cannot be used on watchOS.")
      return false
    #else
      return true
    #endif // os(watchOS)
  }

  override class func canonicalRequest(for request: URLRequest) -> URLRequest { return request }

  override func startLoading() {
    guard let client = client else {
      fatalError("`client` is nil.")
    }

    guard !MockURLProtocol.requestHandlersQueue.isEmpty else {
      fatalError("No request handlers left in the queue. Unexpected network call.")
    }
    let requestHandler = MockURLProtocol.requestHandlersQueue.removeFirst()

    loadingTask = Task {
      do {
        let (response, stream) = try requestHandler(self.request)
        client.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
        if let stream = stream {
          for try await line in stream {
            if Task.isCancelled { return }
            guard let data = line.data(using: .utf8) else {
              fatalError("Failed to convert \"\(line)\" to UTF8 data.")
            }
            client.urlProtocol(self, didLoad: data)
            // Add a newline character since AsyncLineSequence strips them when reading line by
            // line;
            // without the following, the whole file is delivered as a single line.
            client.urlProtocol(self, didLoad: "\n".data(using: .utf8)!)
          }
        }
        if let errorToThrow = MockURLProtocol.errorToThrowMidStream {
          // Sleep guarantees the error is thrown mid-stream (after URLSession yields the stream to
          // the consumer) rather than pre-stream, preventing test coupling to undocumented
          // URLSession
          // internal buffer sizes.
          try? await Task.sleep(nanoseconds: 2_000_000_000)
          if Task.isCancelled { return }
          client.urlProtocol(self, didFailWithError: errorToThrow)
        } else if !MockURLProtocol.neverFinishes {
          if Task.isCancelled { return }
          client.urlProtocolDidFinishLoading(self)
        }
      } catch {
        if Task.isCancelled { return }

        client.urlProtocol(self, didFailWithError: error)

        let nsError = error as NSError
        let isMissingFile = nsError.domain == NSCocoaErrorDomain && nsError
          .code == NSFileReadNoSuchFileError

        if !isMissingFile {
          XCTFail("Unexpected failure in MockURLProtocol: \(error.localizedDescription)")
        }
      }
    }
  }

  override func stopLoading() {
    loadingTask?.cancel()
    MockURLProtocol.stopLoadingExpectation?.fulfill()
  }
}