Merge pull request #2555 from paulkaplan/fix-reverse-undo

Fix the "reverse" audio effect not working with the undo stack.

src/lib/audio/audio-effects.js

@@ -34,9 +34,6 @@ class AudioEffects {
             playbackRate = 1 / pitchRatio;
             sampleCount = Math.floor(buffer.length / playbackRate);
             break;
-        case effectTypes.REVERSE:
-            buffer.getChannelData(0).reverse();
-            break;
         }
         if (window.OfflineAudioContext) {
             this.audioContext = new window.OfflineAudioContext(1, sampleCount, buffer.sampleRate);
@@ -46,7 +43,24 @@ class AudioEffects {
             const sampleScale = 44100 / buffer.sampleRate;
             this.audioContext = new window.webkitOfflineAudioContext(1, sampleScale * sampleCount, 44100);
         }
-        this.buffer = buffer;
+
+        // For the reverse effect we need to manually reverse the data into a new audio buffer
+        // to prevent overwriting the original, so that the undo stack works correctly.
+        // Doing buffer.reverse() would mutate the original data.
+        if (name === effectTypes.REVERSE) {
+            const originalBufferData = buffer.getChannelData(0);
+            const newBuffer = this.audioContext.createBuffer(1, buffer.length, buffer.sampleRate);
+            const newBufferData = newBuffer.getChannelData(0);
+            const bufferLength = buffer.length;
+            for (let i = 0; i < bufferLength; i++) {
+                newBufferData[i] = originalBufferData[bufferLength - i - 1];
+            }
+            this.buffer = newBuffer;
+        } else {
+            // All other effects use the original buffer because it is not modified.
+            this.buffer = buffer;
+        }
+
         this.source = this.audioContext.createBufferSource();
         this.source.buffer = this.buffer;
         this.source.playbackRate.value = playbackRate;