Preallocate buffer size when reading data

lib/eventsource.js

@@ -15,6 +15,8 @@ var colon = 58
 var space = 32
 var lineFeed = 10
 var carriageReturn = 13
+// Beyond 256KB we could not observe any gain in performance
+var maxBufferAheadAllocation = 1024 * 256
 
 function hasBom (buf) {
   return bom.every(function (charCode, index) {
@@ -179,19 +181,36 @@ function EventSource (url, eventSourceInitDict) {
 
       // text/event-stream parser adapted from webkit's
       // Source/WebCore/page/EventSource.cpp
-      var isFirst = true
       var buf
+      var newBuffer
       var startingPos = 0
       var startingFieldLength = -1
+      var newBufferSize = 0
+      var bytesUsed = 0
+
       res.on('data', function (chunk) {
-        buf = buf ? Buffer.concat([buf, chunk]) : chunk
-        if (isFirst && hasBom(buf)) {
-          buf = buf.slice(bom.length)
+        if (!buf) {
+          buf = chunk
+          if (hasBom(buf)) {
+            buf = buf.slice(bom.length)
+          }
+          bytesUsed = buf.length
+        } else {
+          if (chunk.length > buf.length - bytesUsed) {
+            newBufferSize = (buf.length * 2) + chunk.length
+            if (newBufferSize > maxBufferAheadAllocation) {
+              newBufferSize = buf.length + chunk.length + maxBufferAheadAllocation
+            }
+            newBuffer = Buffer.alloc(newBufferSize)
+            buf.copy(newBuffer, 0, 0, bytesUsed)
+            buf = newBuffer
+          }
+          chunk.copy(buf, bytesUsed)
+          bytesUsed += chunk.length
         }
 
-        isFirst = false
         var pos = 0
-        var length = buf.length
+        var length = bytesUsed
 
         while (pos < length) {
           if (discardTrailingNewline) {
@@ -235,8 +254,10 @@ function EventSource (url, eventSourceInitDict) {
 
         if (pos === length) {
           buf = void 0
+          bytesUsed = 0
         } else if (pos > 0) {
-          buf = buf.slice(pos)
+          buf = buf.slice(pos, bytesUsed)
+          bytesUsed = buf.length
         }
       })
     })

test/eventsource_test.js

@@ -454,7 +454,7 @@ describe('Parser', function () {
   })
 
   it('parses relatively huge messages efficiently', function (done) {
-    this.timeout(1000)
+    this.timeout(200)
 
     createServer(function (err, server) {
       if (err) return done(err)