path: improve POSIX path.join() performance

Performance gains are ~4x (~1.5us), but still much slower than a naive
approach. There is some duplicate work done between join(), normalize()
and normalizeArray() so additional optimizations are possible.

Note that this only improves the POSIX implementation.

Thanks to @isaacs and @othiym23 for helping with this optimization.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>

lib/path.js

@@ -348,12 +348,17 @@ if (isWindows) {
   // posix version
   exports.normalize = function(path) {
     var isAbsolute = exports.isAbsolute(path),
-        trailingSlash = path.substr(-1) === '/';
+        trailingSlash = path[path.length - 1] === '/',
+        segments = path.split('/'),
+        nonEmptySegments = [];
 
     // Normalize the path
-    path = normalizeArray(path.split('/').filter(function(p) {
-      return !!p;
-    }), !isAbsolute).join('/');
+    for (var i = 0; i < segments.length; i++) {
+      if (segments[i]) {
+        nonEmptySegments.push(segments[i]);
+      }
+    }
+    path = normalizeArray(nonEmptySegments, !isAbsolute).join('/');
 
     if (!path && !isAbsolute) {
       path = '.';
@@ -372,13 +377,21 @@ if (isWindows) {
 
   // posix version
   exports.join = function() {
-    var paths = Array.prototype.slice.call(arguments, 0);
-    return exports.normalize(paths.filter(function(p, index) {
-      if (!util.isString(p)) {
+    var path = '';
+    for (var i = 0; i < arguments.length; i++) {
+      var segment = arguments[i];
+      if (!util.isString(segment)) {
         throw new TypeError('Arguments to path.join must be strings');
       }
-      return p;
-    }).join('/'));
+      if (segment) {
+        if (!path) {
+          path += segment;
+        } else {
+          path += '/' + segment;
+        }
+      }
+    }
+    return exports.normalize(path);
   };