/*
 * Copyright 2019, gRPC Authors All rights reserved.
 *
 * 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 SwiftProtobuf
import NIO
import NIOHTTP1
import Logging

/// Handles unary calls. Calls the observer block with the request message.
///
/// - The observer block is implemented by the framework user and returns a future containing the call result.
/// - To return a response to the client, the framework user should complete that future
///   (similar to e.g. serving regular HTTP requests in frameworks such as Vapor).
public class UnaryCallHandler<RequestMessage: Message, ResponseMessage: Message>: BaseCallHandler<RequestMessage, ResponseMessage> {
  public typealias EventObserver = (RequestMessage) -> EventLoopFuture<ResponseMessage>
  private var eventObserver: EventObserver?

  private var callContext: UnaryResponseCallContext<ResponseMessage>?

  public init(callHandlerContext: CallHandlerContext, eventObserverFactory: (UnaryResponseCallContext<ResponseMessage>) -> EventObserver) {
    super.init(callHandlerContext: callHandlerContext)
    let callContext = UnaryResponseCallContextImpl<ResponseMessage>(
      channel: self.callHandlerContext.channel,
      request: self.callHandlerContext.request,
      errorDelegate: self.callHandlerContext.errorDelegate,
      logger: self.callHandlerContext.logger
    )

    self.callContext = callContext
    self.eventObserver = eventObserverFactory(callContext)
    callContext.responsePromise.futureResult.whenComplete { _ in
      // When done, reset references to avoid retain cycles.
      self.eventObserver = nil
      self.callContext = nil
    }
  }

  public override func processMessage(_ message: RequestMessage) throws {
    guard let eventObserver = self.eventObserver,
      let context = self.callContext else {
      self.logger.error("processMessage(_:) called before the call started or after the call completed")
      throw GRPCError.server(.tooManyRequests)
    }

    let resultFuture = eventObserver(message)
    resultFuture
      // Fulfill the response promise with whatever response (or error) the framework user has provided.
      .cascade(to: context.responsePromise)
    self.eventObserver = nil
  }

  public override func endOfStreamReceived() throws {
    if self.eventObserver != nil {
      throw GRPCError.server(.noRequestsButOneExpected)
    }
  }

  override func sendErrorStatus(_ status: GRPCStatus) {
    callContext?.responsePromise.fail(status)
  }
}