function showError(error) {
    const errorNode = document.querySelector('#error');
    if (errorNode.firstChild) {
      errorNode.removeChild(errorNode.firstChild);
    }
    errorNode.appendChild(document.createTextNode(error.message || error));
  }
  
  function loadScreens() {
    return fetch('/api/screens', {
      method: 'GET',
      headers: {
        'Accepts': 'application/json'
      }
    }).then(res => {
      return res.json();
    }).catch(showError);
  }
  
  function startSession(offer, screen) {
    return fetch('/api/session', {
      method: 'POST',
      body: JSON.stringify({
        offer,
        screen
      }),
      headers: {
        'Content-Type': 'application/json'
      }
    }).then(res => {
      return res.json();
    }).then(msg => {
      return msg.answer;
    });
  }
  
  function createOffer(pc, { audio, video }) {
    return new Promise((accept, reject) => {
      pc.onicecandidate = evt => {
        if (!evt.candidate) {
          
          // ICE Gathering finished 
          const { sdp: offer } = pc.localDescription;
          accept(offer);
        }
      };
      pc.createOffer({
        offerToReceiveAudio: audio,
        offerToReceiveVideo: video
      }).then(ld => {
        pc.setLocalDescription(ld)
      }).catch(reject)
    });
  }
  
  function startRemoteSession(screen, remoteVideoNode, stream) {
    let pc;
  
    return Promise.resolve().then(() => {
      pc = new RTCPeerConnection({
        iceServers: [{ urls: 'stun:stun.l.google.com:19302' }]
      });
      pc.ontrack = (evt) => {
        console.info('ontrack triggered');
        
        remoteVideoNode.srcObject = evt.streams[0];
        remoteVideoNode.play();
      };
  
      stream && stream.getTracks().forEach(track => {
        pc.addTrack(track, stream);
      })
      return createOffer(pc, { audio: false, video: true });
    }).then(offer => {
      console.info(offer);
      return startSession(offer, screen);
    }).then(answer => {
      console.info(answer);
      return pc.setRemoteDescription(new RTCSessionDescription({
        sdp: answer,
        type: 'answer'
      }));
    }).then(() => pc);
  }
  
  let peerConnection = null;
  document.addEventListener('DOMContentLoaded', () => {
    
    let selectedScreen = 0;
    const remoteVideo = document.querySelector('#remote-video');
    const screenSelect = document.querySelector('#screen-select');
    const startStop = document.querySelector('#start-stop');
    
    loadScreens().then(response => {
      while (screenSelect.firstChild) {
        screenSelect.removeChild(screenSelect.firstChild);
      }
      screenSelect.appendChild(document.createElement('option'));
      response.screens.forEach(screen => {
        const option = document.createElement('option');
        option.appendChild(document.createTextNode('Screen ' + (screen.index + 1)));
        option.setAttribute('value', screen.index);
        screenSelect.appendChild(option);
      });
    }).catch(showError);
  
    screenSelect.addEventListener('change', evt => {
      selectedScreen = parseInt(evt.currentTarget.value, 10);
    });
  
    const enableStartStop = (enabled) => {
      if (enabled) {
        startStop.removeAttribute('disabled');
      } else {
        startStop.setAttribute('disabled', '');
      }
    }
  
    const setStartStopTitle = (title) => {
      startStop.removeChild(startStop.firstChild);
      startStop.appendChild(document.createTextNode(title));
    }
  
    startStop.addEventListener('click', () => {
      enableStartStop(false);
  
      const userMediaPromise =  (adapter.browserDetails.browser === 'safari') ?
        navigator.mediaDevices.getUserMedia({ video: true }) : 
        Promise.resolve(null);
      if (!peerConnection) {
        userMediaPromise.then(stream => {
          return startRemoteSession(selectedScreen, remoteVideo, stream).then(pc => {
            remoteVideo.style.setProperty('visibility', 'visible');
            peerConnection = pc;
          }).catch(showError).then(() => {
            enableStartStop(true);
            setStartStopTitle('Stop');
          });
        })
      } else {
        peerConnection.close();
        peerConnection = null;
        enableStartStop(true);
        setStartStopTitle('Start');
        remoteVideo.style.setProperty('visibility', 'collapse');
      }
    });
  });
  
  window.addEventListener('beforeunload', () => {
    if (peerConnection) {
      peerConnection.close();
    }
  })