// Copyright (c) 2021, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:async';

import 'package:analysis_server/protocol/protocol.dart';
import 'package:analysis_server/protocol/protocol_constants.dart';
import 'package:analysis_server/protocol/protocol_generated.dart';
import 'package:analysis_server/src/channel/channel.dart';

/// Return the stream of requests that is filtered to exclude requests for
/// which the client does not need actual responses.
///
/// If there is one completion request, and then another completion request,
/// then most probably the user continued typing, and there is no need to
/// compute results for the first request. But we will have to respond, an
/// empty response is enough.
///
/// Discarded requests are reported into [discardedRequests].
Stream<Request> debounceRequests(
  ServerCommunicationChannel channel,
  StreamController<Request> discardedRequests,
) {
  return _DebounceRequests(channel, discardedRequests).requests;
}

class _DebounceRequests {
  final ServerCommunicationChannel channel;
  final StreamController<Request> discardedRequests;
  late final Stream<Request> requests;

  _DebounceRequests(this.channel, this.discardedRequests) {
    var buffer = <Request>[];
    Timer? timer;

    requests = channel.requests.transform(
      StreamTransformer.fromHandlers(
        handleData: (request, sink) {
          buffer.add(request);
          // Accumulate requests for a short period of time.
          // When we were busy processing a request, the client could put
          // multiple requests into the event queue. So, when we look, we will
          // quickly get all of them. So, even 1 ms should be enough.
          timer ??= Timer(const Duration(milliseconds: 1), () {
            timer = null;
            var filtered = _filterCompletion(buffer);
            buffer = [];
            for (var request in filtered) {
              sink.add(request);
            }
          });
        },
      ),
    );
  }

  List<Request> _filterCompletion(List<Request> requests) {
    var reversed = <Request>[];
    var abortCompletionRequests = false;
    for (var request in requests.reversed) {
      if (request.method == ANALYSIS_REQUEST_UPDATE_CONTENT) {
        abortCompletionRequests = true;
      }
      if (request.method == COMPLETION_REQUEST_GET_SUGGESTIONS2) {
        if (abortCompletionRequests) {
          discardedRequests.add(request);
          var params = CompletionGetSuggestions2Params.fromRequest(request);
          var offset = params.offset;
          channel.sendResponse(
            CompletionGetSuggestions2Result(offset, 0, [], [], true)
                .toResponse(request.id),
          );
          continue;
        } else {
          abortCompletionRequests = true;
        }
      }
      reversed.add(request);
    }
    return reversed.reversed.toList();
  }
}