Fix ABSN backward playback

src/node/audio_buffer_source.rs

@@ -655,7 +655,7 @@ impl AudioProcessor for AudioBufferSourceRenderer {
                 if !self.render_state.started {
                     let delta = current_time - self.start_time;
                     // handle that start time may be between last sample and this one
-                    self.offset += delta;
+                    self.offset += delta * computed_playback_rate;
 
                     if is_looping && computed_playback_rate >= 0. && self.offset >= actual_loop_end
                     {
@@ -667,7 +667,7 @@ impl AudioProcessor for AudioBufferSourceRenderer {
                         self.offset = actual_loop_start;
                     }
 
-                    buffer_time = self.offset * computed_playback_rate;
+                    buffer_time = self.offset;
                     self.render_state.buffer_time_elapsed = delta * computed_playback_rate;
                     self.render_state.started = true;
                 }
@@ -1222,6 +1222,42 @@ mod tests {
         assert_float_eq!(channel[..], expected[..], abs_all <= 1e-6);
     }
 
+    #[test]
+    fn test_negative_playback_rate() {
+        let sample_rate = 44_100;
+        let mut context = OfflineAudioContext::new(1, sample_rate, sample_rate as f32);
+
+        let mut buffer = context.create_buffer(1, sample_rate, sample_rate as f32);
+        let mut sine = vec![];
+
+        // 1 Hz sine
+        for i in 0..sample_rate {
+            let phase = i as f32 / sample_rate as f32 * 2. * PI;
+            let sample = phase.sin();
+            sine.push(sample);
+        }
+
+        buffer.copy_to_channel(&sine[..], 0);
+
+        let mut src = context.create_buffer_source();
+        src.connect(&context.destination());
+        src.set_buffer(buffer.clone());
+        src.playback_rate.set_value(-1.);
+        src.start_at_with_offset(context.current_time(), buffer.duration());
+
+        let result = context.start_rendering_sync();
+        let channel = result.get_channel_data(0);
+
+        // -1 Hz sine
+        let mut expected: Vec<f32> = sine.into_iter().rev().collect();
+        // offset is at duration (after last sample), then result will start
+        // with a zero value
+        expected.pop();
+        expected.insert(0, 0.);
+
+        assert_float_eq!(channel[..], expected[..], abs_all <= 1e-6);
+    }
+
     #[test]
     fn test_detune() {
         let sample_rate = 44_100;