util: improve format() performance further

Replacing the regexp and replace function with a loop improves
performance by ~60-200%.

PR-URL: https://github.com/nodejs/node/pull/5360
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/util.js

@@ -17,7 +17,6 @@ function tryStringify(arg) {
   }
 }
 
-const formatRegExp = /%[sdj%]/g;
 exports.format = function(f) {
   if (typeof f !== 'string') {
     const objects = new Array(arguments.length);
@@ -27,30 +26,56 @@ exports.format = function(f) {
     return objects.join(' ');
   }
 
-  if (arguments.length === 1) return f;
-
-  const len = arguments.length;
-  const args = new Array(len);
-  var i;
-  for (i = 0; i < len; i++) {
-    args[i] = arguments[i];
-  }
-
-  i = 1;
-  var str = f.replace(formatRegExp, function(x) {
-    if (x === '%%') return '%';
-    if (i >= len) return x;
-    switch (x) {
-      case '%s': return String(args[i++]);
-      case '%d': return Number(args[i++]);
-      case '%j': return tryStringify(args[i++]);
-        // falls through
-      default:
-        return x;
+  var argLen = arguments.length;
+
+  if (argLen === 1) return f;
+
+  var str = '';
+  var a = 1;
+  var lastPos = 0;
+  for (var i = 0; i < f.length;) {
+    if (f.charCodeAt(i) === 37/*'%'*/ && i + 1 < f.length) {
+      switch (f.charCodeAt(i + 1)) {
+        case 100: // 'd'
+          if (a >= argLen)
+            break;
+          if (lastPos < i)
+            str += f.slice(lastPos, i);
+          str += Number(arguments[a++]);
+          lastPos = i = i + 2;
+          continue;
+        case 106: // 'j'
+          if (a >= argLen)
+            break;
+          if (lastPos < i)
+            str += f.slice(lastPos, i);
+          str += tryStringify(arguments[a++]);
+          lastPos = i = i + 2;
+          continue;
+        case 115: // 's'
+          if (a >= argLen)
+            break;
+          if (lastPos < i)
+            str += f.slice(lastPos, i);
+          str += String(arguments[a++]);
+          lastPos = i = i + 2;
+          continue;
+        case 37: // '%'
+          if (lastPos < i)
+            str += f.slice(lastPos, i);
+          str += '%';
+          lastPos = i = i + 2;
+          continue;
+      }
     }
-  });
-  while (i < len) {
-    const x = args[i++];
+    ++i;
+  }
+  if (lastPos === 0)
+    str = f;
+  else if (lastPos < f.length)
+    str += f.slice(lastPos);
+  while (a < argLen) {
+    const x = arguments[a++];
     if (x === null || (typeof x !== 'object' && typeof x !== 'symbol')) {
       str += ' ' + x;
     } else {