Add category method for the hex number character set

and use a dispatch once to initialize the set only once

HTMLKit/HTMLInputStreamReader.m

@@ -8,6 +8,7 @@
 
 #import "HTMLInputStreamReader.h"
 #import "HTMLTokenizerCharacters.h"
+#import "NSCharacterSet+HTMLKit.h"
 
 #pragma mark - HTMLInputStreamReader
 
@@ -145,7 +146,7 @@
 
 - (BOOL)consumeHexNumber:(unsigned long long *)result
 {
-	NSCharacterSet *set = [NSCharacterSet characterSetWithCharactersInString:@"0123456789ABCDEFabcdef"];
+	NSCharacterSet *set = [NSCharacterSet HTMLHexNumberCharacterSet];
 
 	NSString *string = nil;
 	BOOL success = [_scanner scanCharactersFromSet:set intoString:&string];

HTMLKit/NSCharacterSet+HTMLKit.h

@@ -11,5 +11,6 @@
 @interface NSCharacterSet (HTMLKit)
 
 + (instancetype)HTMLWhitespaceCharacterSet;
++ (instancetype)HTMLHexNumberCharacterSet;
 
 @end

HTMLKit/NSCharacterSet+HTMLKit.m

@@ -12,7 +12,22 @@
 
 + (instancetype)HTMLWhitespaceCharacterSet
 {
-	return [NSCharacterSet characterSetWithCharactersInString:@" \t\n\r\f"];
+	static NSCharacterSet *set = nil;
+	static dispatch_once_t onceToken;
+	dispatch_once(&onceToken, ^{
+		set = [NSCharacterSet characterSetWithCharactersInString:@" \t\n\r\f"];
+	});
+	return set;
+}
+
++ (instancetype)HTMLHexNumberCharacterSet
+{
+	static NSCharacterSet *set = nil;
+	static dispatch_once_t onceToken;
+	dispatch_once(&onceToken, ^{
+		set = [NSCharacterSet characterSetWithCharactersInString:@"0123456789ABCDEFabcdef"];
+	});
+	return set;
 }
 
 @end