Fix volume zero and power of 2 (#375)

Fixes #371.

If the volume was zero, the calculation for leading zeros was incorrect.
So both reduce the amount of work done for 0 volume and also count the
leading zeros correctly in the fast path.

- [x] Changelog updated / no changelog update needed

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Text renderer can now be re-used which is useful for rpg style character/word at a time text boxes.
 
+### Fixed
+- Zero volume now plays no sound.
+- Fixed issue where volume was incorrect for volumes which were powers of 2.
+
 ## [0.12.2] - 2022/10/22
 
 This is a minor release to fix an alignment issue with background tiles.

agb/src/sound/mixer/mixer.s

@@ -64,6 +64,8 @@ same_modification:
     lsrs r7, r7, #1
     bne 1b
 
+    sub r3, r3, #1
+
     mov r5, #0                   @ current index we're reading from
     ldr r8, =agb_rs__buffer_size @ the number of steps left
     ldr r8, [r8]

agb/src/sound/mixer/sw_mixer.rs

@@ -446,6 +446,7 @@ impl MixerBuffer {
                 }
             }
 
+            if channel.volume != 0.into() {
                 if channel.is_stereo {
                     unsafe {
                         agb_rs__mixer_add_stereo(
@@ -468,6 +469,7 @@ impl MixerBuffer {
                         );
                     }
                 }
+            }
 
             channel.pos += playback_speed * self.frequency.buffer_size();
         }