/*
 * Script tag fetch
 *
 * When load.metadata.scriptLoad is true, we load via script tag injection.
 */
(function() {

  if (typeof document != 'undefined')
    var head = document.getElementsByTagName('head')[0];

  var curSystem;

  // if doing worker executing, this is set to the load record being executed
  var workerLoad = null;
  
  // interactive mode handling method courtesy RequireJS
  var ieEvents = head && (function() {
    var s = document.createElement('script');
    var isOpera = typeof opera !== 'undefined' && opera.toString() === '[object Opera]';
    return s.attachEvent && !(s.attachEvent.toString && s.attachEvent.toString().indexOf('[native code') < 0) && !isOpera;
  })();

  // IE interactive-only part
  // we store loading scripts array as { script: <script>, load: {...} }
  var interactiveLoadingScripts = [];
  var interactiveScript;
  function getInteractiveScriptLoad() {
    if (interactiveScript && interactiveScript.script.readyState === 'interactive')
      return interactiveScript.load;

    for (var i = 0; i < interactiveLoadingScripts.length; i++)
      if (interactiveLoadingScripts[i].script.readyState == 'interactive') {
        interactiveScript = interactiveLoadingScripts[i];
        return interactiveScript.load;
      }
  }
  
  // System.register, System.registerDynamic, AMD define pipeline
  // this is called by the above methods when they execute
  // we then run the reduceRegister_ collection function either immediately
  // if we are in IE and know the currently executing script (interactive)
  // or later if we need to wait for the synchronous load callback to know the script
  var loadingCnt = 0;
  var registerQueue = [];
  hook('pushRegister_', function(pushRegister) {
    return function(register) {
      // if using eval-execution then skip
      if (pushRegister.call(this, register))
        return false;

      // if using worker execution, then we're done
      if (workerLoad)
        this.reduceRegister_(workerLoad, register);

      // detect if we know the currently executing load (IE)
      // if so, immediately call reduceRegister
      else if (ieEvents)
        this.reduceRegister_(getInteractiveScriptLoad(), register);

      // otherwise, add to our execution queue
      // to call reduceRegister on sync script load event
      else if (loadingCnt)
        registerQueue.push(register);

      // if we're not currently loading anything though
      // then do the reduction against a null load
      // (out of band named define or named register)
      // note even in non-script environments, this catch is used
      else
        this.reduceRegister_(null, register);

      return true;
    };
  });

  function webWorkerImport(loader, load) {
    return new Promise(function(resolve, reject) {
      if (load.metadata.integrity)
        reject(new Error('Subresource integrity checking is not supported in web workers.'));

      workerLoad = load;
      try {
        importScripts(load.address);
      }
      catch(e) {
        workerLoad = null;
        reject(e);
      }
      workerLoad = null;

      // if nothing registered, then something went wrong
      if (!load.metadata.entry)
        reject(new Error(load.address + ' did not call System.register or AMD define'));

      resolve('');
    });
  }

  // override fetch to use script injection
  hook('fetch', function(fetch) {
    return function(load) {
      var loader = this;

      if (!load.metadata.scriptLoad || (!isBrowser && !isWorker))
        return fetch.call(this, load);

      if (isWorker)
        return webWorkerImport(loader, load);

      return new Promise(function(resolve, reject) {
        var s = document.createElement('script');
        
        s.async = true;
        
        if (load.metadata.integrity)
          s.setAttribute('integrity', load.metadata.integrity);

        if (ieEvents) {
          s.attachEvent('onreadystatechange', complete);
          interactiveLoadingScripts.push({
            script: s,
            load: load
          });
        }
        else {
          s.addEventListener('load', complete, false);
          s.addEventListener('error', error, false);
        }

        loadingCnt++;

        curSystem = __global.System;

        s.src = load.address;
        head.appendChild(s);

        function complete(evt) {
          if (s.readyState && s.readyState != 'loaded' && s.readyState != 'complete')
            return;

          loadingCnt--;

          // complete call is sync on execution finish
          // (in ie already done reductions)
          if (!load.metadata.entry && !registerQueue.length) {
            loader.reduceRegister_(load);
          }
          else if (!ieEvents) {
            for (var i = 0; i < registerQueue.length; i++)
              loader.reduceRegister_(load, registerQueue[i]);
            registerQueue = [];
          }

          cleanup();

          // if nothing registered, then something went wrong
          if (!load.metadata.entry && !load.metadata.bundle)
            reject(new Error(load.name + ' did not call System.register or AMD define'));

          resolve('');
        }

        function error(evt) {
          cleanup();
          reject(new Error('Unable to load script ' + load.address));
        }

        function cleanup() {
          __global.System = curSystem;

          if (s.detachEvent) {
            s.detachEvent('onreadystatechange', complete);
            for (var i = 0; i < interactiveLoadingScripts.length; i++)
              if (interactiveLoadingScripts[i].script == s) {
                if (interactiveScript && interactiveScript.script == s)
                  interactiveScript = null;
                interactiveLoadingScripts.splice(i, 1);
              }
          }
          else {
            s.removeEventListener('load', complete, false);
            s.removeEventListener('error', error, false);
          }

          head.removeChild(s);
        }
      });
    };
  });
})();
