Check for all corners on the 'basic' algorithm

There were still some gaps where the check was failing, and trying to be
smart about it was going the "it's too complex for what I need here"
route. Hence, just go to the good old dumb algorithm of "check all
corners". For now the performance has not dropped, so let's hope that we
don't have to re-visit this one.

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

1 files changed, 25 insertions, 17 deletions

diff --git a/src/enemies.s b/src/enemies.s
index 9bda83d..b2b9ca6 100644
--- a/src/enemies.s
+++ b/src/enemies.s
@@ -512,25 +512,17 @@
         ora Globals::zp_tmp0
         sta Enemies::zp_enemies_pool_base + 3, x
 
-        ;; Check collisions with the background.
+        ;; Check collisions with the background. The check is pretty dumb and we
+        ;; just check all four corners, as trying to be smart about it became
+        ;; too complex for what I needed (and the dumb approach is actually not
+        ;; that slow).
 
-        ;; Remember that background checks are done in tile coordinates, not
-        ;; screen ones. So we have to do the translation to it (3 x
-        ;; 'lsr'). After that, for the X coordinate, depending if the enemy is
-        ;; facing left/right, we have to increment this coordinate (i.e. twice
-        ;; if facing right as an enemy of this type is always 2x2 sprites).
+        ;; Translate the X coordinate into tile ones.
         lda Enemies::zp_enemies_pool_base + 2, x
         lsr
         lsr
         lsr
-        tay
-        lda Enemies::zp_enemies_pool_base, x
-        and #$80
-        beq @after_x
-        iny
-        iny
-    @after_x:
-        sty Globals::zp_arg1
+        sta Globals::zp_arg1
 
         ;; Translate the Y coordinate into tile ones.
         lda Enemies::zp_enemies_pool_base + 1, x
@@ -539,14 +531,20 @@
         lsr
         sta Globals::zp_arg0
 
-        ;; Perform a collision check with the upper boundary.
+        ;; Perform a collision check with the upper left boundary.
+        jsr Background::collides
+        beq @check_up_right
+        JAL bite_the_dust
+
+    @check_up_right:
+        ;; Increase the X tile coordinate to check for the upper right boundary.
+        inc Globals::zp_arg1
         jsr Background::collides
         beq @check_down
         JAL bite_the_dust
 
     @check_down:
-        ;; If that failed, then increment the vertical tile coordinate to get
-        ;; the bottom boundary and check again. Note that the "bottom" is
+        ;; Now let's go for bottom boundaries. The notion of "bottom" is
         ;; different if it's the regular 'basic' enemy or it's the fighter jet
         ;; re-using this algorithm. That's why if the level kind is 3 the bottom
         ;; is increased by one and not twice.
@@ -555,7 +553,17 @@
         cpy #3
         beq @skip_second_inc
         inc Globals::zp_arg0
+
     @skip_second_inc:
+        ;; And the actual check.
+        jsr Background::collides
+        beq @check_down_left
+        JAL bite_the_dust
+
+    @check_down_left:
+        ;; So now the only corner left is the bottom left one. Adjust the X tile
+        ;; coordinate and try again.
+        dec Globals::zp_arg1
         jsr Background::collides
         beq @end
         JAL bite_the_dust