(function() {
  var manifestPromise = null;
  var loadedScripts = Object.create(null);

  function overlayBasePath() {
    return (window.APP_BASE || '/') + 'overlay/';
  }

  function fetchManifest() {
    return fetch(overlayBasePath() + 'manifest.json', { cache: 'no-store' })
      .then(function(response) {
        if (response.status === 404) {
          return null;
        }
        if (!response.ok) {
          throw new Error('overlay manifest request failed');
        }
        return response.json();
      })
      .catch(function() {
        return null;
      });
  }

  function loadScript(path) {
    if (loadedScripts[path]) {
      return loadedScripts[path];
    }
    loadedScripts[path] = new Promise(function(resolve, reject) {
      var script = document.createElement('script');
      script.src = overlayBasePath() + path.replace(/^\/+/, '');
      script.async = false;
      script.onload = function() { resolve(); };
      script.onerror = function() { reject(new Error('overlay script load failed')); };
      document.head.appendChild(script);
    }).catch(function() {
      return null;
    });
    return loadedScripts[path];
  }

  function load() {
    if (manifestPromise) {
      return manifestPromise;
    }
    manifestPromise = fetchManifest().then(function(manifest) {
      if (!(manifest && manifest.slots && window.CrankUiSlots)) {
        return null;
      }
      var tasks = Object.entries(manifest.slots)
        .filter(function(entry) {
          return window.CrankUiSlots.isAllowed(entry[0]) && typeof entry[1] === 'string';
        })
        .map(function(entry) {
          return loadScript(entry[1]);
        });
      return Promise.all(tasks).then(function() {
        return manifest;
      });
    });
    return manifestPromise;
  }

  async function render(root, context) {
    await load();
    if (window.CrankUiSlots) {
      window.CrankUiSlots.renderAll(root, context);
    }
  }

  window.CrankOverlay = {
    load: load,
    render: render,
  };
}());