/*
 * Copyright 2020, 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 Logging
import NIO

internal final class ServerInterceptorPipeline<Request, Response> {
  /// The `EventLoop` this RPC is being executed on.
  internal let eventLoop: EventLoop

  /// The path of the RPC in the format "/Service/Method", e.g. "/echo.Echo/Get".
  internal let path: String

  /// The type of the RPC, e.g. "unary".
  internal let type: GRPCCallType

  /// A logger.
  internal let logger: Logger

  /// The contexts associated with the interceptors stored in this pipeline. Contexts will be
  /// removed once the RPC has completed. Contexts are ordered from inbound to outbound, that is,
  /// the head is first and the tail is last.
  private var contexts: [ServerInterceptorContext<Request, Response>]?

  /// Returns the next context in the outbound direction for the context at the given index, if one
  /// exists.
  /// - Parameter index: The index of the `ServerInterceptorContext` which is requesting the next
  ///   outbound context.
  /// - Returns: The `ServerInterceptorContext` or `nil` if one does not exist.
  internal func nextOutboundContext(
    forIndex index: Int
  ) -> ServerInterceptorContext<Request, Response>? {
    return self.context(atIndex: index - 1)
  }

  /// Returns the next context in the inbound direction for the context at the given index, if one
  /// exists.
  /// - Parameter index: The index of the `ServerInterceptorContext` which is requesting the next
  ///   inbound context.
  /// - Returns: The `ServerInterceptorContext` or `nil` if one does not exist.
  internal func nextInboundContext(
    forIndex index: Int
  ) -> ServerInterceptorContext<Request, Response>? {
    return self.context(atIndex: index + 1)
  }

  /// Returns the context for the given index, if one exists.
  /// - Parameter index: The index of the `ServerInterceptorContext` to return.
  /// - Returns: The `ServerInterceptorContext` or `nil` if one does not exist for the given index.
  private func context(atIndex index: Int) -> ServerInterceptorContext<Request, Response>? {
    return self.contexts?[checked: index]
  }

  /// The context closest to the `NIO.Channel`, i.e. where inbound events originate. This will be
  /// `nil` once the RPC has completed.
  internal var head: ServerInterceptorContext<Request, Response>? {
    return self.contexts?.first
  }

  /// The context closest to the application, i.e. where outbound events originate. This will be
  /// `nil` once the RPC has completed.
  internal var tail: ServerInterceptorContext<Request, Response>? {
    return self.contexts?.last
  }

  internal init() {
    fatalError("Not implemented yet")
  }

  /// Emit a request part message into the interceptor pipeline.
  ///
  /// - Parameter part: The part to emit into the pipeline.
  /// - Important: This *must* to be called from the `eventLoop`.
  internal func receive(_ part: ServerRequestPart<Request>) {
    self.eventLoop.assertInEventLoop()
    self.head?.invokeReceive(part)
  }

  /// Write a response message into the interceptor pipeline.
  ///
  /// - Parameters:
  ///   - part: The response part to sent.
  ///   - promise: A promise to complete when the response part has been successfully written.
  /// - Important: This *must* to be called from the `eventLoop`.
  internal func send(_ part: ServerResponsePart<Response>, promise: EventLoopPromise<Void>?) {
    self.eventLoop.assertInEventLoop()

    if let tail = self.tail {
      tail.invokeSend(part, promise: promise)
    } else {
      promise?.fail(GRPCError.AlreadyComplete())
    }
  }
}