From 240d3c5718f78580f62479d7aede930d4a2e30f6 Mon Sep 17 00:00:00 2001
From: tildearrow <me@tildearrow.org>
Date: Sat, 27 Dec 2025 17:30:19 -0500
Subject: [PATCH] GUI: use an accumulator for wave synth preview

issue #2735
---
 src/gui/gui.cpp     |  1 +
 src/gui/gui.h       |  1 +
 src/gui/insEdit.cpp | 15 ++++++++++++++-
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/gui/gui.cpp b/src/gui/gui.cpp
index 27693d56c..9cab5c014 100644
--- a/src/gui/gui.cpp
+++ b/src/gui/gui.cpp
@@ -9010,6 +9010,7 @@ FurnaceGUI::FurnaceGUI():
   wavePreviewHeight(255),
   wavePreviewInit(true),
   wavePreviewPaused(false),
+  wavePreviewAccum(0.0f),
   pgSys(0),
   pgAddr(0),
   pgVal(0),
diff --git a/src/gui/gui.h b/src/gui/gui.h
index ce2aaec8c..008e1a40d 100644
--- a/src/gui/gui.h
+++ b/src/gui/gui.h
@@ -2470,6 +2470,7 @@ class FurnaceGUI {
   DivWaveSynth wavePreview;
   int wavePreviewLen, wavePreviewHeight;
   bool wavePreviewInit, wavePreviewPaused;
+  float wavePreviewAccum;
 
   // bit 31: ctrl
   // bit 30: reserved for SDL scancode mask
diff --git a/src/gui/insEdit.cpp b/src/gui/insEdit.cpp
index 3ea1ce6f7..db3ddd7d2 100644
--- a/src/gui/insEdit.cpp
+++ b/src/gui/insEdit.cpp
@@ -3289,6 +3289,7 @@ void FurnaceGUI::insTabWavetable(DivInstrument* ins)
         DivWavetable* wave2=e->getWave(ins->ws.wave2);
         if (wavePreviewInit) {
           wavePreview.init(ins,wavePreviewLen,wavePreviewHeight,true);
+          wavePreviewAccum=0.0f;
           wavePreviewInit=false;
         }
         float wavePreview1[257];
@@ -3315,7 +3316,19 @@ void FurnaceGUI::insTabWavetable(DivInstrument* ins)
           wavePreview2[wave2->len]=wave2->data[wave2->len-1];
         }
         if (ins->ws.enabled && (!wavePreviewPaused || wavePreviewInit)) {
-          wavePreview.tick(true);
+          if (wavePreviewInit) {
+            wavePreview.tick(true);
+          } else {
+            wavePreviewAccum+=ImGui::GetIO().DeltaTime;
+            double accumRate=e->getCurHz();
+            if (accumRate<1.0) accumRate=1.0;
+            if (accumRate>1023.0) accumRate=1023.0;
+            accumRate=1.0/accumRate;
+            while (wavePreviewAccum>=accumRate) {
+              wavePreviewAccum-=accumRate;
+              wavePreview.tick(true);
+            }
+          }
           WAKE_UP;
         }
         for (int i=0; i<wavePreviewLen; i++) {