(function(factory) {
   if (typeof define !== 'undefined' && define.amd) {
      define([], factory);
   } else if (typeof define !== 'undefined' && define.petal) {
      define(['promise-stream'], [], factory);
   } else if (typeof module !== 'undefined' && module.exports) {
      module.exports = factory();
   } else {
      window.PromiseStream = factory();
   }
})(function() {
   class PromiseStream {
      constructor() {
         this._current = new Promise((resolve) => {
            this._resolve = resolve;
         });
         this._next = this._current;
         this._value = undefined;
      }

      get promise() {
         /* waits for the _first_ update, or completes immediately */
         return this._current;
      }

      get next_promise() {
         /* waits for the _next_ update (which may be the first) */
         return this._next;
      }

      get value() {
         return this._value;
      }

      update(value) {
         if (value !== undefined && value !== this._value) {
            const resolve = this._resolve;
            const current = this._next;
            const next = new Promise((resolve) => {
               this._resolve = resolve;
            });

            this._current = current;
            this._next = next;
            this._value = value;

            resolve({ value, next });
         }
      }

      once(cb) {
         this.promise.then((v) => { cb(v.value); });
      }

      listen(cb) {
         let cancelled = false;

         function renew(v) {
            if (!cancelled) {
               cb(v.value);
               v.next.then(renew);
            }
         }

         this.promise.then(renew);

         return () => { cancelled = true; };
      }
   }

   return PromiseStream;
});