Schedule XHR responses instead of delivering them on the second

Bandwidth calculation uses the length of the XHR so rounding delivery up to the nearest second can skew results. Instead, calculate the delivery time of XHRs and schedule a timeout to set the response body.

@@ -111,7 +111,7 @@
     fakeXhr = sinon.useFakeXMLHttpRequest();
     requests = [];
     fakeXhr.onCreate = function(xhr) {
-      xhr.startTime = t;
+      xhr.startTime = +new Date();
       xhr.delivered = 0;
       requests.push(xhr);
     };
@@ -159,10 +159,8 @@
           return left.time - right.time;
         });
 
-        // advance time and collect simulation results
-        for (t = i = 0; t < duration; clock.tick(1 * 1000), t++) {
-
-          // determine the bandwidth value at this moment
+        // pre-calculate the bandwidth at each second
+        for (t = i = 0; t < duration; t++) {
           while (bandwidths[i + 1] && bandwidths[i + 1].time <= t) {
             i++;
           }
@@ -170,52 +168,55 @@
             time: t,
             bandwidth: bandwidths[i].bandwidth
           });
+        }
 
-          // deliver responses if they're ready
-          requests.forEach(function(request, i) {
-            var arrival = request.startTime + propagationDelay,
-                segmentSize;
+        // advance time and collect simulation results
+        for (t = 0; t < duration; clock.tick(1000), t++) {
+          // schedule response deliveries
+          while (requests.length) {
+            (function(request) {
+              var segmentSize;
 
               // playlist responses
               if (/playlist-\d+$/.test(request.url)) {
                 // for simplicity, playlist responses have zero trasmission time
-              if (t === arrival) {
-                request.respond(200, null, playlistResponse(+requests[0].url.match(/\d+$/)));
-                // the request is completed
-                return requests.splice(requests.indexOf(request), 1);
-              }
-              return;
+                return setTimeout(function() {
+                  request.respond(200, null, playlistResponse(+request.url.match(/\d+$/)));
+                }, propagationDelay * 1000);
               }
 
               // segment responses
               segmentSize = +request.url.match(/(\d+)-\d+$/)[1] * segmentDuration;
               // segment response headers arrive after the propogation delay
-            if (t === arrival) {
+              setTimeout(function() {
+                var arrival = Math.ceil(+new Date() * 0.001);
                 results.playlists.push({
-                time: t,
+                  time: arrival,
                   bitrate: +request.url.match(/(\d+)-\d+$/)[1]
                 });
                 request.setResponseHeaders({
                   'Content-Type': 'video/mp2t'
                 });
-            }
-            // send the response body if all bytes have been delivered
-            if (request.delivered >= segmentSize) {
+
+                results.bandwidth.slice(arrival).every(function(value, i) {
+                  var remaining = segmentSize - request.delivered;
+                  if (remaining - value.bandwidth <= 0) {
+                    // send the response body once all bytes have been delivered
+                    setTimeout(function() {
                       buffered += segmentDuration;
                       request.status = 200;
                       request.response = new Uint8Array(segmentSize * 0.125);
                       request.setResponseBody('');
-              // the request is completed
-              return requests.splice(requests.indexOf(request), 1);
+                    }, ((remaining / value.bandwidth) + i) * 1000);
+                    return false;
                   }
-            // transmit the bits for this tick
-            if (t >= arrival) {
-              request.delivered += results.bandwidth[t].bandwidth;
-
+                  // record the bits for this tick
+                  request.delivered += value.bandwidth;
+                  return true;
+                });
+              }, propagationDelay * 1000);
+            })(requests.shift());
           }
-            // response has not arrived fully
-            return;
-          }, []);
 
           results.buffered.push({
             time: t,