Add MIDI note strumming operator

- Implement strum_notes functionality to apply progressive
  timing offsets to selected MIDI notes
- Provides guitar-style strumming for creating realistic
  note sequences

The feature applies 1/32 beat timing offsets progressively
to selected notes, creating a strummed effect where notes
play in sequence rather than simultaneously.

libs/ardour/ardour/strum.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2025
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#pragma once
+
+#include "ardour/libardour_visibility.h"
+#include "ardour/midi_model.h"
+#include "ardour/midi_operator.h"
+
+namespace ARDOUR {
+
+/** Strum notes (add progressive timing offset to notes).
+ *
+ * This operator applies a progressive timing offset to selected notes,
+ * creating a strumming effect where notes are offset by a specified
+ * amount in either forward or backward direction.
+ */
+class LIBARDOUR_API Strum : public MidiOperator {
+public:
+	typedef Evoral::Sequence<Temporal::Beats>::NotePtr     NotePtr;
+	typedef Evoral::Sequence<Temporal::Beats>::Notes       Notes;
+
+	Strum (bool forward, bool fine);
+
+	PBD::Command* operator() (std::shared_ptr<ARDOUR::MidiModel> model,
+	                     Temporal::Beats                      position,
+	                     std::vector<Notes>&                  seqs);
+
+	std::string name () const { return std::string ("strum"); }
+
+private:
+	bool _forward;
+	bool _fine;
+};
+
+} /* namespace */

libs/ardour/strum.cc

@@ -0,0 +1,88 @@
+/*
+ * Copyright (C) 2025
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "ardour/strum.h"
+#include "ardour/midi_model.h"
+
+namespace ARDOUR {
+
+Strum::Strum(bool forward, bool fine)
+	: _forward(forward)
+	, _fine(fine)
+{}
+
+PBD::Command*
+Strum::operator()(std::shared_ptr<ARDOUR::MidiModel> model,
+                  Temporal::Beats                      position,
+                  std::vector<Strum::Notes>&           seqs)
+{
+	typedef MidiModel::NoteDiffCommand Command;
+
+	Command* cmd = new Command(model, name());
+
+	if (seqs.empty()) {
+		return cmd;
+	}
+
+	// Get all notes from all sequences and sort them by start time
+	std::vector<NotePtr> all_notes;
+	for (std::vector<Notes>::iterator s = seqs.begin(); s != seqs.end(); ++s) {
+		for (Notes::const_iterator i = (*s).begin(); i != (*s).end(); ++i) {
+			all_notes.push_back(*i);
+		}
+	}
+
+	if (all_notes.size() < 2) {
+		return cmd;
+	}
+
+	// Sort notes by start time
+	std::sort(all_notes.begin(), all_notes.end(),
+	          [](const NotePtr& a, const NotePtr& b) {
+	              return a->time() < b->time();
+	          });
+
+	Temporal::Beats total_offset;
+	Temporal::Beats offset;
+
+	if (_fine) {
+		offset = Temporal::Beats::ticks(Temporal::ticks_per_beat / 128);
+	} else {
+		offset = Temporal::Beats::ticks(Temporal::ticks_per_beat / 32);
+	}
+
+	if (_forward) {
+		for (std::vector<NotePtr>::const_iterator i = all_notes.begin(); i != all_notes.end(); ++i) {
+			const NotePtr note = *i;
+			Temporal::Beats new_start = note->time() + total_offset;
+			cmd->change(note, MidiModel::NoteDiffCommand::StartTime, new_start);
+			total_offset += offset;
+		}
+	} else { // backward
+		for (std::vector<NotePtr>::const_reverse_iterator i = all_notes.rbegin(); i != all_notes.rend(); ++i) {
+			const NotePtr note = *i;
+			Temporal::Beats new_start = note->time() + total_offset;
+			cmd->change(note, MidiModel::NoteDiffCommand::StartTime, new_start);
+			total_offset += offset;
+		}
+	}
+
+	return cmd;
+}
+
+} /* namespace */

libs/ardour/wscript

@@ -250,6 +250,7 @@ libardour_sources = [
         'stripable.cc',
         # 'step_sequencer.cc',
         'strip_silence.cc',
+        'strum.cc',
         'surround_pannable.cc',
         'surround_return.cc',
         'surround_send.cc',