[llvm] [FileCheck] Don't use regex to find prefixes (PR #72237)

[Matt Arsenault via llvm-commits]

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@@ -1634,6 +1634,59 @@ static size_t SkipWord(StringRef Str, size_t Loc) {
   return Loc;
 }
 
+static const char *DefaultCheckPrefixes[] = {"CHECK"};
+static const char *DefaultCommentPrefixes[] = {"COM", "RUN"};
+
+static void addDefaultPrefixes(FileCheckRequest &Req) {
+  if (Req.CheckPrefixes.empty()) {
+    for (const char *Prefix : DefaultCheckPrefixes)
+      Req.CheckPrefixes.push_back(Prefix);
+    Req.IsDefaultCheckPrefix = true;
+  }
+  if (Req.CommentPrefixes.empty())
+    for (const char *Prefix : DefaultCommentPrefixes)
+      Req.CommentPrefixes.push_back(Prefix);
+}
+
+struct PrefixMatcher {
+  /// Prefixes and their first occurrence past the current position.
+  SmallVector<std::pair<StringRef, size_t>> Prefixes;
+  StringRef Input;
+
+  PrefixMatcher(ArrayRef<StringRef> CheckPrefixes,
+                ArrayRef<StringRef> CommentPrefixes, StringRef Input)
+      : Input(Input) {
+    for (StringRef Prefix : CheckPrefixes)
+      Prefixes.push_back({Prefix, Input.find(Prefix)});
+    for (StringRef Prefix : CommentPrefixes)
+      Prefixes.push_back({Prefix, Input.find(Prefix)});
+
+    // Sort by descending length.
+    llvm::sort(Prefixes,
+               [](auto A, auto B) { return A.first.size() > B.first.size(); });
+  }
+
+  /// Find the next match of a prefix in Buffer.
+  std::optional<StringRef> match(StringRef Buffer) {
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arsenm wrote:

Don't see the point of the optional, the empty StringRef will naturally act like none 

https://github.com/llvm/llvm-project/pull/72237