Fix a cycling bug when shooting carelessly

When shooting like crazy and killing random enemies, after a while a
cycling bug would appear which would make enemies to turn in weird
directions, die unexpectedly, and other weird shenanigans.

This comes back to commit 7c493ba3f338 ("Improve a bit the performance
on enemy death"), which removed the saving/restoring of the value on the
'y' register. That being said, this register was actually being needed
at least by Bullets::update(), so in certain situations this register
might have a bad value (e.g. via the Explosions::create() call).

Fixes: 7c493ba3f338 ("Improve a bit the performance on enemy death")

Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/enemies.s b/src/enemies.s
index facb255..7097af5 100644
--- a/src/enemies.s
+++ b/src/enemies.s
@@ -613,7 +613,16 @@
     .endproc
 
     ;; The enemy has been set to dust, remove it.
+    ;;
+    ;; NOTE: the 'x' register is modified, the 'y' register is _preserved_.
     .proc bite_the_dust
+        ;; This function might be called by loops which abuse index
+        ;; registers. Luckily that's not the case for the 'x' register, but at
+        ;; least the loop on Bullets::update() does heavy use of the 'y'
+        ;; register. Preserve it now on the stack.
+        tya
+        pha
+
         ;; Invalidate this enemy.
         lda #$FF
         ldx Enemies::zp_pool_index
@@ -634,6 +643,10 @@
         ldx Enemies::zp_pool_index
         sta Enemies::zp_enemies_pool_base + 3, x
 
+        ;; Restore back the value for the 'y' register.
+        pla
+        tay
+
         rts
     .endproc