jsPsych.extensions['webgazer'] = (function () {

  var extension = {};

  // private state for the extension
  // extension authors can define public functions to interact
  // with the state. recommend not exposing state directly
  // so that state manipulations are checked.
  var state = {};

  // required, will be called at jsPsych.init
  // should return a Promise
  extension.initialize = function (params) {
    return new Promise(function(resolve, reject){
      if (typeof params.webgazer === 'undefined') {
        if (window.webgazer) {
          state.webgazer = window.webgazer;
        } else {
          reject(new Error('webgazer extension failed to initialize. webgazer.js not loaded. Load webgazer.js before calling jsPsych.init()'));
        }
      } else {
        state.webgazer = params.webgazer;
      }
  
      // sets up event handler for webgazer data
      state.webgazer.setGazeListener(handleGazeDataUpdate);
  
      // starts webgazer, and once it initializes we stop mouseCalibration and
      // pause webgazer data.
      state.webgazer.begin().then(function () {
        extension.stopMouseCalibration();
        extension.pause();
        resolve();
      })
  
      // hide video by default
      extension.hideVideo();
  
      // hide predictions by default
      extension.hidePredictions();
    })
  }

  // required, will be called when the trial starts (before trial loads)
  extension.on_start = function () {
    state.currentTrialData = [];
  }

  // required will be called when the trial loads
  extension.on_load = function () {

    // set current trial start time
    state.currentTrialStart = performance.now();

    // resume data collection
    state.webgazer.resume();

    // set internal flag
    state.activeTrial = true;
  }

  // required, will be called when jsPsych.finishTrial() is called
  // must return data object to be merged into data.
  extension.on_finish = function () {
    // pause the eye tracker
    state.webgazer.pause();

    // set internal flag
    state.activeTrial = false;

    // send back the gazeData
    return {
      gazeData: JSON.stringify(state.currentTrialData)
    }
  }

  extension.faceDetected = function () {
    return state.webgazer.getTracker().predictionReady;
  }

  extension.showPredictions = function () {
    state.webgazer.showPredictionPoints(true);
  }

  extension.hidePredictions = function () {
    state.webgazer.showPredictionPoints(false);
  }

  extension.showVideo = function () {
    state.webgazer.showVideo(true);
    state.webgazer.showFaceOverlay(true);
    state.webgazer.showFaceFeedbackBox(true);
  }

  extension.hideVideo = function () {
    state.webgazer.showVideo(false);
    state.webgazer.showFaceOverlay(false);
    state.webgazer.showFaceFeedbackBox(false);
  }

  extension.resume = function () {
    state.webgazer.resume();
  }

  extension.pause = function () {
    state.webgazer.pause();
  }

  extension.stopMouseCalibration = function () {
    state.webgazer.removeMouseEventListeners()
  }

  extension.startMouseCalibration = function () {
    state.webgazer.addMouseEventListeners()
  }

  extension.calibratePoint = function (x, y) {
    state.webgazer.recordScreenPosition(x, y, 'click');
  }

  extension.setRegressionType = function (regression_type) {
    var valid_regression_models = ['ridge', 'weigthedRidge', 'threadedRidge'];
    if (valid_regression_models.includes(regression_type)) {
      state.webgazer.setRegression(regression_type)
    } else {
      console.warn('Invalid regression_type parameter for webgazer.setRegressionType. Valid options are ridge, weightedRidge, and threadedRidge.')
    }
  }

  extension.getCurrentPrediction = function () {
    return state.webgazer.getCurrentPrediction();
  }

  // extension.addGazeDataUpdateListener(listener){
  //   state.webgazer.setGazeListener(listener);
  // }

  function handleGazeDataUpdate(gazeData, elapsedTime) {
    if (gazeData !== null && state.activeTrial) {
      var d = {
        x: gazeData.x,
        y: gazeData.y,
        t: performance.now() - state.currentTrialStart
      }
      state.currentTrialData.push(d); // add data to current trial's data
    }

  }

  return extension;

})();