// we define a __exec for globally-scoped execution
// used by module format implementations
var __exec;

(function() {

  var hasBtoa = typeof btoa != 'undefined';

  function getSource(load) {
    var lastLineIndex = load.source.lastIndexOf('\n');

    // wrap ES formats with a System closure for System global encapsulation
    var wrap = load.metadata.format != 'global';

    var sourceMap = load.metadata.sourceMap;
    if (sourceMap) {
      if (typeof sourceMap != 'object')
        throw new TypeError('load.metadata.sourceMap must be set to an object.');

      sourceMap = JSON.stringify(sourceMap);
    }

    return (wrap ? '(function(System, SystemJS, require) {' : '') + load.source + (wrap ? '\n})(System, System);' : '')
        // adds the sourceURL comment if not already present
        + (load.source.substr(lastLineIndex, 15) != '\n//# sourceURL=' 
          ? '\n//# sourceURL=' + load.address + (sourceMap ? '!transpiled' : '') : '')
        // add sourceMappingURL if load.metadata.sourceMap is set
        + (sourceMap && hasBtoa && '\n//# sourceMappingURL=data:application/json;base64,' + btoa(unescape(encodeURIComponent(sourceMap))) || '');
  }

  var curLoad;

  // System.register, System.registerDynamic, AMD define pipeline
  // if currently evalling code here, immediately reduce the registered entry against the load record
  hook('pushRegister_', function() {
    return function(register) {
      if (!curLoad)
        return false;

      this.reduceRegister_(curLoad, register);
      return true;
    };
  });

  // System clobbering protection (mostly for Traceur)
  var curSystem;
  var callCounter = 0;
  function preExec(loader, load) {
    curLoad = load;
    if (callCounter++ == 0)
      curSystem = __global.System;
    __global.System = __global.SystemJS = loader; 
  }
  function postExec() {
    if (--callCounter == 0)
      __global.System = __global.SystemJS = curSystem;
    curLoad = undefined;
  }

  __exec = function(load) {
    if ((load.metadata.integrity || load.metadata.nonce) && supportsScriptExec)
      return scriptExec.call(this, load);
    try {
      preExec(this, load);
      curLoad = load;
      (0, eval)(getSource(load));
      postExec();
    }
    catch(e) {
      postExec(); 
      throw addToError(e, 'Evaluating ' + load.address);
    }
  };

  var supportsScriptExec = false;
  if (isBrowser && typeof document != 'undefined' && document.getElementsByTagName) {
    var scripts = document.getElementsByTagName('script');
    $__curScript = scripts[scripts.length - 1];

    if (!(window.chrome && window.chrome.extension || navigator.userAgent.match(/^Node\.js/)))
      supportsScriptExec = true;
  }

  // script execution via injecting a script tag into the page
  // this allows CSP integrity and nonce to be set for CSP environments
  var head;
  function scriptExec(load) {
    if (!head)
      head = document.head || document.body || document.documentElement;

    var script = document.createElement('script');
    script.text = getSource(load, false);
    var onerror = window.onerror;
    var e;
    window.onerror = function(_e) {
      e = addToError(_e, 'Evaluating ' + load.address);
    }
    preExec(this, load);

    if (load.metadata.integrity)
      script.setAttribute('integrity', load.metadata.integrity);
    if (load.metadata.nonce)
      script.setAttribute('nonce', load.metadata.nonce);

    head.appendChild(script);
    head.removeChild(script);
    postExec();
    window.onerror = onerror;
    if (e)
      throw e;
  }

})();