import 'dart:async';
import 'dart:math';
import 'package:veilid_support/veilid_support.dart';

import '../../../proto/proto.dart' as proto;

import '../../../tools/tools.dart';
import 'author_input_source.dart';
import 'message_integrity.dart';

class AuthorInputQueue {
  AuthorInputQueue._({
    required TypedKey author,
    required AuthorInputSource inputSource,
    required int inputPosition,
    required proto.Message? previousMessage,
    required void Function(Object, StackTrace?) onError,
    required MessageIntegrity messageIntegrity,
  })  : _author = author,
        _onError = onError,
        _inputSource = inputSource,
        _previousMessage = previousMessage,
        _messageIntegrity = messageIntegrity,
        _inputPosition = inputPosition;

  static Future<AuthorInputQueue?> create({
    required TypedKey author,
    required AuthorInputSource inputSource,
    required proto.Message? previousMessage,
    required void Function(Object, StackTrace?) onError,
  }) async {
    // Get ending input position
    final inputPosition = await inputSource.getTailPosition() - 1;

    // Create an input queue for the input source
    final queue = AuthorInputQueue._(
        author: author,
        inputSource: inputSource,
        inputPosition: inputPosition,
        previousMessage: previousMessage,
        onError: onError,
        messageIntegrity: await MessageIntegrity.create(author: author));

    // Rewind the queue's 'inputPosition' to the first unreconciled message
    if (!await queue._rewindInputToAfterLastMessage()) {
      return null;
    }

    return queue;
  }

  ////////////////////////////////////////////////////////////////////////////
  // Public interface

  /// Get the input source for this queue
  AuthorInputSource get inputSource => _inputSource;

  /// Get the author of this queue
  TypedKey get author => _author;

  /// Get the current message that needs reconciliation
  Future<proto.Message?> getCurrentMessage() async {
    try {
      // if we have a current message already, return it
      if (_currentMessage != null) {
        return _currentMessage;
      }

      // Get the window
      final currentWindow = await _updateWindow(clampInputPosition: false);
      if (currentWindow == null) {
        return null;
      }
      final currentElement =
          currentWindow.elements[_inputPosition - currentWindow.firstPosition];
      return _currentMessage = currentElement.value;
      // Catch everything so we can avoid ParallelWaitError
      // ignore: avoid_catches_without_on_clauses
    } catch (e, st) {
      log.error('Exception getting current message: $e:\n$st\n');
      _currentMessage = null;
      return null;
    }
  }

  /// Move the reconciliation cursor (_inputPosition) forward on the input
  /// queue and tees up the next message for processing
  /// Returns true if there is more work to do
  /// Returns false if there are no more messages to reconcile in this queue
  Future<bool> advance() async {
    // Move current message to previous
    _previousMessage = await getCurrentMessage();
    _currentMessage = null;

    while (true) {
      // Advance to next position
      _inputPosition++;

      // Get more window if we need to
      final currentMessage = await getCurrentMessage();
      if (currentMessage == null) {
        return false;
      }

      if (_previousMessage != null) {
        // Ensure the timestamp is not moving backward
        if (currentMessage.timestamp < _previousMessage!.timestamp) {
          log.warning('timestamp backward: ${currentMessage.timestamp}'
              ' < ${_previousMessage!.timestamp}');
          continue;
        }
      }

      // Verify the id chain for the message
      final matchId =
          await _messageIntegrity.generateMessageId(_previousMessage);
      if (matchId.compare(currentMessage.idBytes) != 0) {
        log.warning('id chain invalid: $matchId != ${currentMessage.idBytes}');
        continue;
      }

      // Verify the signature for the message
      if (!await _messageIntegrity.verifyMessage(currentMessage)) {
        log.warning('invalid message signature: $currentMessage');
        continue;
      }

      break;
    }
    return true;
  }

  ////////////////////////////////////////////////////////////////////////////
  // Internal implementation

  /// Walk backward from the tail of the input queue to find the first
  /// message newer than our last reconciled message from this author
  /// Returns false if no work is needed
  Future<bool> _rewindInputToAfterLastMessage() async {
    // Iterate windows over the inputSource
    InputWindow? currentWindow;
    outer:
    while (true) {
      // Get more window if we need to
      currentWindow = await _updateWindow(clampInputPosition: true);
      if (currentWindow == null) {
        // Window is not available or things are empty so this
        // queue can't work right now
        return false;
      }

      // Iterate through current window backward
      for (var i = currentWindow.elements.length - 1;
          i >= 0 && _inputPosition >= 0;
          i--, _inputPosition--) {
        final elem = currentWindow.elements[i];

        // If we've found an input element that is older or same time as our
        // last reconciled message for this author, or we find the message
        // itself then we stop
        if (_previousMessage != null) {
          if (elem.value.authorUniqueIdBytes
                      .compare(_previousMessage!.authorUniqueIdBytes) ==
                  0 ||
              elem.value.timestamp <= _previousMessage!.timestamp) {
            break outer;
          }
        }
      }
      // If we're at the beginning of the inputSource then we stop
      if (_inputPosition < 0) {
        break;
      }
    }

    // _inputPosition points to either before the input source starts
    // or the position of the previous element. We still need to set the
    // _currentMessage to the previous element so advance() can compare
    // against it if we can.
    if (_inputPosition >= 0) {
      _currentMessage = currentWindow
          .elements[_inputPosition - currentWindow.firstPosition].value;
    }

    // After this advance(), the _inputPosition and _currentMessage should
    // be equal to the first message to process and the current window to
    // process should not be empty if there is work to do
    return advance();
  }

  /// Slide the window toward the current position and load the batch around it
  Future<InputWindow?> _updateWindow({required bool clampInputPosition}) async {
    final inputTailPosition = await _inputSource.getTailPosition();
    if (inputTailPosition == 0) {
      return null;
    }

    // Handle out-of-range input position
    if (clampInputPosition) {
      _inputPosition = min(max(_inputPosition, 0), inputTailPosition - 1);
    } else if (_inputPosition < 0 || _inputPosition >= inputTailPosition) {
      return null;
    }

    // Check if we are still in the window
    final currentWindow = _currentWindow;

    int firstPosition;
    int lastPosition;
    if (currentWindow != null) {
      firstPosition = currentWindow.firstPosition;
      lastPosition = currentWindow.lastPosition;

      // Slide the window if we need to
      if (_inputPosition >= firstPosition && _inputPosition <= lastPosition) {
        return currentWindow;
      } else if (_inputPosition < firstPosition) {
        // Slide it backward, current position is now last
        firstPosition = max((_inputPosition - _maxWindowLength) + 1, 0);
        lastPosition = _inputPosition;
      } else if (_inputPosition > lastPosition) {
        // Slide it forward, current position is now first
        firstPosition = _inputPosition;
        lastPosition =
            min((_inputPosition + _maxWindowLength) - 1, inputTailPosition - 1);
      }
    } else {
      // need a new window, start with the input position at the end
      lastPosition = _inputPosition;
      firstPosition = max((_inputPosition - _maxWindowLength) + 1, 0);
    }

    // Get another input batch futher back
    final avCurrentWindow = await _inputSource.getWindow(
        firstPosition, lastPosition - firstPosition + 1);

    final asErr = avCurrentWindow.asError;
    if (asErr != null) {
      _onError(asErr.error, asErr.stackTrace);
      _currentWindow = null;
      return null;
    }
    final asLoading = avCurrentWindow.asLoading;
    if (asLoading != null) {
      _currentWindow = null;
      return null;
    }

    final nextWindow = avCurrentWindow.asData!.value;
    if (nextWindow == null || nextWindow.length == 0) {
      _currentWindow = null;
      return null;
    }

    // Handle out-of-range input position
    // Doing this again because getWindow is allowed to return a smaller
    // window than the one requested, possibly due to DHT consistency
    // fluctuations and race conditions
    if (clampInputPosition) {
      _inputPosition = min(max(_inputPosition, nextWindow.firstPosition),
          nextWindow.lastPosition);
    } else if (_inputPosition < nextWindow.firstPosition ||
        _inputPosition > nextWindow.lastPosition) {
      return null;
    }

    return _currentWindow = nextWindow;
  }

  ////////////////////////////////////////////////////////////////////////////

  /// The author of this messages in the input source
  final TypedKey _author;

  /// The input source we're pulling messages from
  final AuthorInputSource _inputSource;

  /// What to call if an error happens
  final void Function(Object, StackTrace?) _onError;

  /// The message integrity validator
  final MessageIntegrity _messageIntegrity;

  /// The last message we reconciled/output
  proto.Message? _previousMessage;

  /// The current message we're looking at
  proto.Message? _currentMessage;

  /// The current position in the input source that we are looking at
  int _inputPosition;

  /// The current input window from the InputSource;
  InputWindow? _currentWindow;

  /// Desired maximum window length
  static const int _maxWindowLength = 256;
}