perf(utilities): optimize case-sensitive tag replace with hash map

The lookup cost of a hash map is O(1) instead of `indexOf` which
is O(N).

Generate a hash map of the case-sensitive tag names, rename
`fixTagCase` to `formatTagName`, and use the hash map.

lib/utilities.js

@@ -1,9 +1,23 @@
 'use strict';
 
-var CASE_SENSITIVE_TAG_NAMES = require('./constants').CASE_SENSITIVE_TAG_NAMES
-var CASE_SENSITIVE_TAG_NAMES_LOWERCASE = CASE_SENSITIVE_TAG_NAMES.map(function(tag) {
-    return tag.toLowerCase();
-});
+var CASE_SENSITIVE_TAG_NAMES = require('./constants').CASE_SENSITIVE_TAG_NAMES;
+
+var caseSensitiveTagNamesMap = {};
+var tagName;
+for (var i = 0, len = CASE_SENSITIVE_TAG_NAMES.length; i < len; i++) {
+    tagName = CASE_SENSITIVE_TAG_NAMES[i];
+    caseSensitiveTagNamesMap[tagName.toLowerCase()] = tagName;
+}
+
+/**
+ * Gets case-sensitive tag name.
+ *
+ * @param  {String}           tagName - The lowercase tag name.
+ * @return {String|undefined}
+ */
+function getCaseSensitiveTagName(tagName) {
+    return caseSensitiveTagNamesMap[tagName];
+}
 
 /**
  * Format DOM attributes to an associative array.
@@ -25,20 +39,19 @@ function formatAttributes(attributes) {
 }
 
 /**
- * Correct the case of tag names. For case-sensitive SVG tags, return the proper
- * tag name; for all other tags, return the lowercased name.
+ * Corrects the tag name if it is case-sensitive (SVG).
+ * Otherwise, returns the lowercase tag name (HTML).
  *
- * @param  {String} tagName - The tag name to correct.
- * @return {String}         - The corrected tag name.
+ * @param  {String} tagName - The lowercase tag name.
+ * @return {String}         - The formatted tag name.
  */
-function fixTagCase(tagName) {
-    var correctedName = tagName.toLowerCase();
-    var caseSensitiveTagIndex = CASE_SENSITIVE_TAG_NAMES_LOWERCASE.indexOf(correctedName);
-    // fix case if element is case-sensitive SVG
-    if (caseSensitiveTagIndex !== -1) {
-        correctedName = CASE_SENSITIVE_TAG_NAMES[caseSensitiveTagIndex];
+function formatTagName(tagName) {
+    tagName = tagName.toLowerCase();
+    var caseSensitiveTagName = getCaseSensitiveTagName(tagName);
+    if (caseSensitiveTagName) {
+        return caseSensitiveTagName;
     }
-    return correctedName;
+    return tagName;
 }
 
 /**
@@ -75,8 +88,8 @@ function formatDOM(nodes, parentObj, directive) {
 
         // set the node name if it's not "#text" or "#comment"
         // e.g., "div"
-        if (node.nodeName.indexOf('#') !== 0) {
-            nodeObj.name = fixTagCase(node.nodeName);
+        if (node.nodeName[0] !== '#') {
+            nodeObj.name = formatTagName(node.nodeName);
             // also, nodes of type "tag" have "attribs"
             nodeObj.attribs = {}; // default
             if (node.attributes && node.attributes.length) {