MP3File: Fix stereo playback on samd AudioOut

There were several problems with the way this worked -- the read_count
approach was too complicated and I made a mistake "simplifying" it from
WaveFile.  And when the right channel was returned, it was off by 1 byte,
making it into static.

Instead, directly track which is the "other" channel that has data
available, and by using the right data type make the "+ channel"
arithmetic give the right result.

This requires a double cast (int16_t*)(void*) due to an alignment warning;
the alignment is now ensured manually, but the compiler doesn't make the
necessary inference that the low address bit must be clear.

1 files changed, 3 insertions, 3 deletions

diff --git a/shared-module/audiomp3/MP3File.h b/shared-module/audiomp3/MP3File.h
index 12649ac1a..9d99e8d1f 100644
--- a/shared-module/audiomp3/MP3File.h
+++ b/shared-module/audiomp3/MP3File.h
@@ -39,7 +39,7 @@ typedef struct {
     uint8_t* inbuf;
     uint32_t inbuf_length;
     uint32_t inbuf_offset;
-    uint8_t* buffers[2];
+    int16_t* buffers[2];
     uint32_t len;
     uint32_t frame_buffer_size;
 
@@ -50,8 +50,8 @@ typedef struct {
     uint8_t channel_count;
     bool eof;
 
-    uint16_t read_count;
-    uint16_t channel_read_count[2];
+    int8_t other_channel;
+    int8_t other_buffer_index;
 } audiomp3_mp3file_obj_t;
 
 // These are not available from Python because it may be called in an interrupt.