fix(apu): resolve off-by-one error

src/apu.rs

@@ -101,8 +101,6 @@ impl Apu {
         if self.falling_edge(12, div) {
             use FrameSequencerState::*;
 
-            self.sequencer.next();
-
             match self.sequencer.state() {
                 Length => self.handle_length(),
                 LengthAndSweep => {
@@ -112,6 +110,8 @@ impl Apu {
                 Envelope => self.handle_envelope(),
                 Nothing => {}
             }
+
+            self.sequencer.next();
         }
 
         self.div_prev = Some(div);

src/apu/types.rs

@@ -531,13 +531,9 @@ pub(super) mod fs {
 
             self.step = (self.step + 1) % 8;
             self.state = match self.step {
-                0 => Length,
+                1 | 3 | 5 => Nothing,
-                1 => Nothing,
+                0 | 4 => Length,
-                2 => LengthAndSweep,
+                2 | 6 => LengthAndSweep,
-                3 => Nothing,
-                4 => Length,
-                5 => Nothing,
-                6 => LengthAndSweep,
                 7 => Envelope,
                 _ => unreachable!("Step {} is invalid for the Frame Sequencer", self.step),
             };