Fix overwrite issues when adding scores

The add_to_player() function relied on parameters to be passed on the
'Globals::zp_tmpX' variables, but that's just plain stupid. First,
because these memory addresses are not meant for arguments; but most
importantly, because callers actually needed some of these values before
calling this function. Hence, everything went into chaos and sprites
began to act wrongly in all sorts of random ways.

Moreover, some of the callers also needed a guarantee on either the 'x'
or the 'y' registers. Since this wasn't always feasible, we needed to
store/load these registers' values case by case.

All in all, now add_to_player() accepts only one argument via the 'y'
register (which callers already knew it was going to be messed
with). The value on 'y' has to be set via some helper constants.

As a cherry on top, this not only fixes a nasty bug, but it also makes
things more performant and with less code.

Fixes: 63ef73de3bdd ("Update scores on screen")
Signed-off-by: Miquel Sabaté Solà <mssola@mssola.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/items.s b/src/items.s
index 084bd1b..164d5b7 100644
--- a/src/items.s
+++ b/src/items.s
@@ -527,7 +527,9 @@
         ;; game where the score was accounted on part/tank pickup, not
         ;; dropping. I find this more reliable and easier to code, and in the
         ;; end it's the same.
+        stx Globals::zp_tmp2
         ADD_PART_FUEL_SCORE
+        ldx Globals::zp_tmp2
 
         jmp @next
 
@@ -946,8 +948,6 @@
 
     ;; Collect an item as indexed by 'zp_pool_index'. This function assumes that
     ;; the item is already valid.
-    ;;
-    ;; NOTE: the 'y' register is preserved.
     .proc collect
         ldx Items::zp_pool_index
 
@@ -975,7 +975,9 @@
         sta Items::zp_pool_base, x
 
         ;; Account for this on the player's score.
+        stx Globals::zp_tmp2
         ADD_ITEM_SCORE
+        ldx Globals::zp_tmp2
 
         rts
     .endproc