From a20a4c0029506763f9dbfcc7cdd5e860b99dcafd Mon Sep 17 00:00:00 2001
From: Jean Pierre Cimalando <jp-dev@inbox.ru>
Date: Sun, 27 Sep 2020 11:25:43 +0200
Subject: [PATCH 1/2] flexEG: If stage is of zero duration, immediate
 transition to level

---
 src/sfizz/FlexEnvelope.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/sfizz/FlexEnvelope.cpp b/src/sfizz/FlexEnvelope.cpp
index 70dc0948..c58d3ae5 100644
--- a/src/sfizz/FlexEnvelope.cpp
+++ b/src/sfizz/FlexEnvelope.cpp
@@ -144,6 +144,9 @@ void FlexEnvelope::Impl::process(absl::Span<float> out)
         const bool isReleased = isReleased_;
         while ((!stageSustained_ && currentTime_ >= stageTime_) ||
                (stageSustained_ && isReleased)) {
+            // If stage is of zero duration, immediate transition to level
+            if (!stageSustained_ && stageTime_ == 0)
+                currentLevel_ = stageTargetLevel_;
             if (!advanceToNextStage()) {
                 out.remove_prefix(frameIndex);
                 fill(out, 0.0f);

From 6694e8b85e45ff9cdc4a8639990f6e27b1d3d38e Mon Sep 17 00:00:00 2001
From: Jean Pierre Cimalando <jp-dev@inbox.ru>
Date: Mon, 28 Sep 2020 14:43:58 +0200
Subject: [PATCH 2/2] Add test

---
 tests/FlexEGT.cpp | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tests/FlexEGT.cpp b/tests/FlexEGT.cpp
index db16f93c..229bee47 100644
--- a/tests/FlexEGT.cpp
+++ b/tests/FlexEGT.cpp
@@ -9,6 +9,7 @@
 #include "sfizz/FlexEnvelope.h"
 #include "catch2/catch.hpp"
 #include "TestHelpers.h"
+#include <array>
 using namespace Catch::literals;
 using namespace sfz::literals;
 
@@ -261,3 +262,29 @@ TEST_CASE("[FlexEG] Detailed numerical envelope test (with shapes)")
     envelope.process(absl::MakeSpan(output));
     REQUIRE( approxEqual<float>(output, expected, 0.01f) );
 }
+
+TEST_CASE("[FlexEG] Zero delay transitions")
+{
+    sfz::Synth synth;
+
+    synth.loadSfzString(fs::current_path(), R"(
+        <region> sample=*sine
+        eg1_time1=0  eg1_level1=1
+        eg1_time2=1  eg1_level2=0
+        eg1_time3=1  eg1_level3=.5 eg1_sustain=3
+        eg1_time4=1  eg1_level4=1
+    )");
+    sfz::FlexEnvelope envelope;
+    REQUIRE(synth.getNumRegions() == 1);
+    REQUIRE(synth.getRegionView(0)->flexEGs.size() == 1);
+    envelope.configure(&synth.getRegionView(0)->flexEGs[0]);
+    envelope.setSampleRate(10);
+    envelope.start(1);
+
+    std::array<float, 2> output;
+    envelope.process(absl::MakeSpan(output));
+    REQUIRE(output[0] == 0.0f);
+    REQUIRE(output[1] == Approx(0.9f).margin(0.01f));
+    // Note(jpc): 0.9 is because EG pre-increments the time counter, slope is
+    //            1 frame off into the future
+}