Convert enemy vs bullet collision into a loop

The fact that unrolling the loop was easier or faster was a plain lie
from my lazyness. Convert it into a proper loop just so we can
experiment with adding more enemies in one screen.

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

3 files changed, 26 insertions, 24 deletions

diff --git a/.nasm/segments.txt b/.nasm/segments.txt
index 5bd1c28..f09170c 100644
--- a/.nasm/segments.txt
+++ b/.nasm/segments.txt
@@ -1,4 +1,4 @@
 - HEADER: 16/16 (100%)
-- ROM0: 6551/32762 (20.00%)
+- ROM0: 6538/32762 (19.96%)
 - ROMV: 6/6 (100%)
 - ROM2: 8192/8192 (100%)
diff --git a/src/bullets.s b/src/bullets.s
index 8b93bac..8bbbeaa 100644
--- a/src/bullets.s
+++ b/src/bullets.s
@@ -287,29 +287,33 @@
         inx
         jmp @move_loop
 
-        ;; Enemy collision for this bullet. It's actually easier/faster to just
-        ;; unroll the loop.
+        ;; Enemy collision for this bullet.
     @check_enemy_collision:
-        lda #Enemies::ENEMY_0_IDX
-        sta Enemies::zp_pool_index
-        jsr Enemies::collides
-        beq @enemy_1
-        jsr Enemies::bite_the_dust
-        jmp @save_bullet_move
-    @enemy_1:
-        lda #Enemies::ENEMY_1_IDX
+        ldx #0
         sta Enemies::zp_pool_index
+
+    @enemy_collision_loop:
+        ;; Does it collide?
         jsr Enemies::collides
-        beq @enemy_2
+        beq @next_enemy_collision
+
+        ;; Yes! Kill the enemy and break the loop.
         jsr Enemies::bite_the_dust
         jmp @save_bullet_move
-    @enemy_2:
-        lda #Enemies::ENEMY_2_IDX
-        sta Enemies::zp_pool_index
-        jsr Enemies::collides
+
+    @next_enemy_collision:
+        ;; Advance by the size of each pool item.
+        lda Enemies::zp_pool_index
+        clc
+        adc #Enemies::SIZEOF_POOL_ITEM
+
+        ;; Are we at the end of the enemies' pool?
+        cmp #Enemies::ENEMIES_POOL_CAPACITY_BYTES
         beq @save_bullet_move
-        jsr Enemies::bite_the_dust
-        __fallthrough__ @save_bullet_move
+
+        ;; Nope! Save the index and fetch the next enemy.
+        sta Enemies::zp_pool_index
+        jmp @enemy_collision_loop
 
     @save_bullet_move:
         ;; Restore back the old value from the 'x' register.
diff --git a/src/enemies.s b/src/enemies.s
index b6005fc..33b4c33 100644
--- a/src/enemies.s
+++ b/src/enemies.s
@@ -17,13 +17,11 @@
     ;; change it there.
     ENEMIES_POOL_CAPACITY = 3
 
-    ;; The capacity of the enemies pool in bytes.
-    ENEMIES_POOL_CAPACITY_BYTES = ENEMIES_POOL_CAPACITY * 4
+    ;; The amount of bytes each pool item takes.
+    SIZEOF_POOL_ITEM = 4
 
-    ;; Indeces where each enemy definition starts on the pool.
-    ENEMY_0_IDX = 0
-    ENEMY_1_IDX = 4
-    ENEMY_2_IDX = 8
+    ;; The capacity of the enemies pool in bytes.
+    ENEMIES_POOL_CAPACITY_BYTES = ENEMIES_POOL_CAPACITY * SIZEOF_POOL_ITEM
 
     ;; Initial X coordinates for enemies depending on if they appear on the
     ;; left/right edge of the screen.