One place to pick a buffer's highlighter

Filetype detection (extension or shebang) was written out three times
(two open paths in Editor and the ImGui frontend's per-frame check) and
"highlighter for this filetype, else the null one" five times (Buffer's
copy constructor and assignment, Editor::SwitchTo, :set filetype).
Buffer::ApplyDetectedFiletype and InstallFiletypeHighlighter replace
them. The InvalidateFrom(0) calls after SetHighlighter are dropped:
SetHighlighter already clears the caches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

Buffer.cc

@@ -377,23 +377,8 @@ Buffer::Buffer(const Buffer &other)
 	undo_sys_  = std::make_unique<UndoSystem>(*this, *undo_tree_);
 
 	// Recreate a highlighter engine for this copy based on filetype/syntax state
-	if (syntax_enabled_) {
-		// Allocate engine and install an appropriate highlighter
-		highlighter_ = std::make_unique<kte::HighlighterEngine>();
-		if (!filetype_.empty()) {
-			auto hl = kte::HighlighterRegistry::CreateFor(filetype_);
-			if (hl) {
-				highlighter_->SetHighlighter(std::move(hl));
-			} else {
-				// Unsupported filetype -> NullHighlighter keeps syntax pipeline active
-				highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-			}
-		} else {
-			// No filetype -> keep syntax enabled but use NullHighlighter
-			highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-		}
-		// Fresh engine has empty caches; nothing to invalidate
-	}
+	if (syntax_enabled_)
+		InstallFiletypeHighlighter();
 }
 
 
@@ -428,19 +413,8 @@ Buffer::operator=(const Buffer &other)
 
 	// Recreate highlighter engine consistent with syntax settings
 	highlighter_.reset();
-	if (syntax_enabled_) {
-		highlighter_ = std::make_unique<kte::HighlighterEngine>();
-		if (!filetype_.empty()) {
-			auto hl = kte::HighlighterRegistry::CreateFor(filetype_);
-			if (hl) {
-				highlighter_->SetHighlighter(std::move(hl));
-			} else {
-				highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-			}
-		} else {
-			highlighter_->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-		}
-	}
+	if (syntax_enabled_)
+		InstallFiletypeHighlighter();
 	return *this;
 }
 
@@ -768,6 +742,29 @@ Buffer::insert_text(int row, int col, std::string_view text)
 }
 
 
+void
+Buffer::ApplyDetectedFiletype()
+{
+	EnsureHighlighter();
+	const std::string first = Nrows() > 0 ? GetLineString(0) : std::string();
+	const std::string ft    = kte::HighlighterRegistry::DetectForPath(filename_, first);
+	SetFiletype(ft);
+	SetSyntaxEnabled(true);
+	InstallFiletypeHighlighter();
+}
+
+
+void
+Buffer::InstallFiletypeHighlighter()
+{
+	EnsureHighlighter();
+	auto hl = filetype_.empty() ? nullptr : kte::HighlighterRegistry::CreateFor(filetype_);
+	if (!hl)
+		hl = std::make_unique<kte::NullHighlighter>();
+	highlighter_->SetHighlighter(std::move(hl));
+}
+
+
 // ===== Adapter helpers for PieceTable-backed Buffer =====
 std::string
 Buffer::Bytes() const

Buffer.h

@@ -472,6 +472,15 @@ public:
 	}
 
 
+	// Enable syntax with the highlighter the filename (or a shebang on the
+	// first line) calls for, or none for an unknown type.
+	void ApplyDetectedFiletype();
+
+	// Install the highlighter for Filetype() (the null highlighter for an
+	// empty or unsupported one), creating the engine if needed.
+	void InstallFiletypeHighlighter();
+
+
 	// Swap journal integration (set by Editor)
 	void SetSwapRecorder(kte::SwapRecorder *rec)
 	{

Command.cc

@@ -15,7 +15,6 @@
 #include "RegexEngine.h"
 #include "TermWidth.h"
 #include "syntax/HighlighterRegistry.h"
-#include "syntax/NullHighlighter.h"
 #include "Editor.h"
 #include "Buffer.h"
 #include "UndoSystem.h"
@@ -1641,29 +1640,12 @@ apply_filetype(Buffer &buf, const std::string &ft)
 		buf.SetSyntaxEnabled(false);
 		return;
 	}
-	if (val.empty()) {
-		// Empty means unknown/unspecified -> use NullHighlighter but keep syntax enabled
-		buf.SetFiletype("");
-		buf.SetSyntaxEnabled(true);
-		eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-		eng->InvalidateFrom(0);
-		return;
-	}
-	// Normalize and create via registry
-	std::string norm = kte::HighlighterRegistry::Normalize(val);
-	auto hl          = kte::HighlighterRegistry::CreateFor(norm);
-	if (hl) {
-		eng->SetHighlighter(std::move(hl));
-		buf.SetFiletype(norm);
-		buf.SetSyntaxEnabled(true);
-		eng->InvalidateFrom(0);
-	} else {
-		// Unknown -> install NullHighlighter and keep syntax enabled
-		eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-		buf.SetFiletype(val); // record what user asked even if unsupported
-		buf.SetSyntaxEnabled(true);
-		eng->InvalidateFrom(0);
-	}
+	// Empty means unspecified (no highlighting, syntax stays on); an
+	// unsupported type is recorded as asked, without highlighting.
+	const std::string norm = val.empty() ? val : kte::HighlighterRegistry::Normalize(val);
+	buf.SetFiletype(kte::HighlighterRegistry::CreateFor(norm) ? norm : val);
+	buf.SetSyntaxEnabled(true);
+	buf.InstallFiletypeHighlighter();
 }
 
 

Editor.cc

@@ -6,9 +6,7 @@
 
 #include "Editor.h"
 #include "ErrorHandler.h"
-#include "syntax/HighlighterRegistry.h"
 #include "syntax/CppHighlighter.h"
-#include "syntax/NullHighlighter.h"
 
 
 namespace {
@@ -285,27 +283,8 @@ Editor::OpenFile(const std::string &path, std::string &err)
 				cur.SetSwapRecorder(Swap()->RecorderFor(&cur));
 				Swap()->NotifyFilenameChanged(cur);
 			}
-			// Setup highlighting using registry (extension + shebang)
-			cur.EnsureHighlighter();
-			std::string first = "";
-			if (cur.Nrows() > 0)
-				first = cur.GetLineString(0);
-			std::string ft = kte::HighlighterRegistry::DetectForPath(path, first);
-			if (!ft.empty()) {
-				cur.SetFiletype(ft);
-				cur.SetSyntaxEnabled(true);
-				if (auto *eng = cur.Highlighter()) {
-					eng->SetHighlighter(kte::HighlighterRegistry::CreateFor(ft));
-					eng->InvalidateFrom(0);
-				}
-			} else {
-				cur.SetFiletype("");
-				cur.SetSyntaxEnabled(true);
-				if (auto *eng = cur.Highlighter()) {
-					eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-					eng->InvalidateFrom(0);
-				}
-			}
+			// Highlighting by extension or shebang
+			cur.ApplyDetectedFiletype();
 			// Defensive: ensure any active prompt is closed after a successful open
 			CancelPrompt();
 			return true;
@@ -318,27 +297,8 @@ Editor::OpenFile(const std::string &path, std::string &err)
 	}
 	// NOTE: swap recorder/attach must happen after the buffer is stored in its
 	// final location (vector) because swap manager keys off Buffer*.
-	// Initialize syntax highlighting by extension + shebang via registry (v2)
-	b.EnsureHighlighter();
-	std::string first = "";
-	if (b.Nrows() > 0)
-		first = b.GetLineString(0);
-	std::string ft = kte::HighlighterRegistry::DetectForPath(path, first);
-	if (!ft.empty()) {
-		b.SetFiletype(ft);
-		b.SetSyntaxEnabled(true);
-		if (auto *eng = b.Highlighter()) {
-			eng->SetHighlighter(kte::HighlighterRegistry::CreateFor(ft));
-			eng->InvalidateFrom(0);
-		}
-	} else {
-		b.SetFiletype("");
-		b.SetSyntaxEnabled(true);
-		if (auto *eng = b.Highlighter()) {
-			eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-			eng->InvalidateFrom(0);
-		}
-	}
+	// Highlighting by extension or shebang
+	b.ApplyDetectedFiletype();
 	// Add as a new buffer and switch to it
 	std::size_t idx = AddBuffer(std::move(b));
 	if (Swap()) {
@@ -650,25 +610,8 @@ Editor::SwitchTo(std::size_t index)
 	curbuf_ = index;
 	// Robustness: ensure a valid highlighter is installed when switching buffers
 	Buffer &b = bufs[curbuf_];
-	if (b.SyntaxEnabled()) {
-		b.EnsureHighlighter();
-		if (auto *eng = b.Highlighter()) {
-			if (!eng->HasHighlighter()) {
-				// Try to set based on existing filetype; fall back to NullHighlighter
-				if (!b.Filetype().empty()) {
-					auto hl = kte::HighlighterRegistry::CreateFor(b.Filetype());
-					if (hl) {
-						eng->SetHighlighter(std::move(hl));
-					} else {
-						eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-					}
-				} else {
-					eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-				}
-				eng->InvalidateFrom(0);
-			}
-		}
-	}
+	if (b.SyntaxEnabled() && !(b.Highlighter() && b.Highlighter()->HasHighlighter()))
+		b.InstallFiletypeHighlighter();
 	return true;
 }
 

ImGuiFrontend.cc

@@ -19,8 +19,6 @@
 #include "fonts/Font.h" // embedded default font (DefaultFont)
 #include "fonts/FontRegistry.h"
 #include "fonts/IosevkaExtended.h"
-#include "syntax/HighlighterRegistry.h"
-#include "syntax/NullHighlighter.h"
 
 
 #ifndef KTE_FONT_SIZE
@@ -54,22 +52,8 @@ apply_syntax_to_buffer(Buffer *b, const GUIConfig &cfg)
 	if (cfg.syntax && b->GetEditMode() != EditMode::Writing) {
 		b->SetSyntaxEnabled(true);
 		b->EnsureHighlighter();
-		if (auto *eng = b->Highlighter()) {
-			if (!eng->HasHighlighter()) {
-				const std::string first_line = b->Nrows() > 0 ? b->GetLineString(0) : std::string();
-				std::string ft = kte::HighlighterRegistry::DetectForPath(
-					b->Filename(), first_line);
-				if (!ft.empty()) {
-					eng->SetHighlighter(kte::HighlighterRegistry::CreateFor(ft));
-					b->SetFiletype(ft);
-					eng->InvalidateFrom(0);
-				} else {
-					eng->SetHighlighter(std::make_unique<kte::NullHighlighter>());
-					b->SetFiletype("");
-					eng->InvalidateFrom(0);
-				}
-			}
-		}
+		if (!b->Highlighter()->HasHighlighter())
+			b->ApplyDetectedFiletype();
 	} else {
 		b->SetSyntaxEnabled(false);
 	}