[SyntaxModel] Improve the performance of searching URLs in comments (#5214)

[SyntaxModel] When searching URLs in doc comments, reduce the number of protocol name comparisons by looking ahead more characters, NFC. rdar://28298506

Searching URL in doc comments can be expensive. We used to look for
every colon as an indicator of potential URLs. However, this is not
efficient enough. Suggested by Ben, we further divide protocols into
categories so that most protocols can use "://" as an indicator of its
existence.

Not sure whether this is enough to close the radar, but I believe it is
a valuable performance improvement anyway.

Author: nkcsgexi

lib/IDE/SyntaxModel.cpp

@@ -297,7 +297,7 @@ class ModelASTWalker : public ASTWalker {
   unsigned BufferID;
   std::vector<StructureElement> SubStructureStack;
   SourceLoc LastLoc;
-  static const std::regex &getURLRegex(unsigned Index);
+  static const std::regex &getURLRegex(StringRef Protocol);
   static const std::regex &getDocCommentRegex(unsigned Index);
 
   Optional<SyntaxNode> parseFieldNode(StringRef Text, StringRef OrigText,
@@ -366,13 +366,27 @@ class ModelASTWalker : public ASTWalker {
   }
 };
 
-const std::regex &ModelASTWalker::getURLRegex(unsigned Index) {
+const std::regex &ModelASTWalker::getURLRegex(StringRef Pro) {
   static const std::regex Regexes[3] =  {
     std::regex{ RegexStrURL, std::regex::ECMAScript | std::regex::nosubs },
     std::regex{ RegexStrMailURL, std::regex::ECMAScript | std::regex::nosubs },
     std::regex{ RegexStrRadarURL, std::regex::ECMAScript | std::regex::nosubs }
   };
-  return Regexes[Index];
+
+  static const auto MailToPosition = std::find(URLProtocols.begin(),
+                                               URLProtocols.end(),
+                                               "mailto");
+  static const auto RadarPosition = std::find(URLProtocols.begin(),
+                                              URLProtocols.end(),
+                                              "radar");
+  auto Found = std::find(URLProtocols.begin(), URLProtocols.end(), Pro);
+  assert(Found != URLProtocols.end() && "bad protocol name");
+  if (Found < MailToPosition)
+    return Regexes[0];
+  else if (Found < RadarPosition)
+    return Regexes[1];
+  else
+    return Regexes[2];
 }
 
 const std::regex &ModelASTWalker::getDocCommentRegex(unsigned Index) {
@@ -1369,24 +1383,32 @@ bool ModelASTWalker::findUrlStartingLoc(StringRef Text,
   static const auto MailToPosition = std::find(URLProtocols.begin(),
                                                URLProtocols.end(),
                                                "mailto");
-  static const auto RadarPosition = std::find(URLProtocols.begin(),
-                                              URLProtocols.end(),
-                                              "radar");
   auto Index = Text.find(":");
   if (Index == StringRef::npos)
     return false;
 
-  for (auto It = URLProtocols.begin(); It != URLProtocols.end(); ++ It) {
-    if (Index >= It->size() &&
-        Text.substr(Index - It->size(), It->size()) == *It) {
-      Start = Index - It->size();
-      if (It < MailToPosition)
-        Regex = getURLRegex(0);
-      else if (It < RadarPosition)
-        Regex = getURLRegex(1);
-      else
-        Regex = getURLRegex(2);
-      return true;
+  auto Lookback = [Text](unsigned Index, StringRef Name) {
+    return Index >= Name.size() &&
+      Text.substr(Index - Name.size(), Name.size()) == Name;
+  };
+
+  auto HasSlash = Text.substr(Index).startswith("://");
+
+  if (HasSlash) {
+    for (auto It = URLProtocols.begin(); It < URLProtocols.end(); ++ It) {
+      if (Lookback(Index, *It)) {
+        Regex = getURLRegex(*It);
+        Start = Index - It->size();
+        return true;
+      }
+    }
+  } else {
+    for (auto It = MailToPosition; It < URLProtocols.end(); ++ It) {
+      if (Lookback(Index, *It)) {
+        Regex = getURLRegex(*It);
+        Start = Index - It->size();
+        return true;
+      }
     }
   }
 #endif