Implemented sprite priority

Forgot to push this before merging gpu-rewrite

src/gpu/oam_search.rs

@@ -11,7 +11,9 @@ impl GameBoy {
             self.gpu.y = 0;
             self.gpu.sprites.clear();
 
-            for i in (0xFE00..0xFE9C).step_by(4) {
+            // We access sprites in reverse because the sprite with the lowest address has
+            // the most priority
+            for i in (0xFE00..0xFE9C).rev().step_by(4) {
                 self.gpu.sprites.push(self.get_sprite_data(i));
             }
         }
@@ -20,10 +22,10 @@ impl GameBoy {
     }
 
     fn get_sprite_data(&self, address: u16) -> SpriteData {
-        let y = self.bus[address];
-        let x = self.bus[address + 1];
-        let tile_number = self.bus[address + 2];
-        let flags = self.bus[address + 3];
+        let y = self.bus[address - 3];
+        let x = self.bus[address - 2];
+        let tile_number = self.bus[address - 1];
+        let flags = self.bus[address];
 
         SpriteData {
             y,

src/gpu/pixel_transfer/sprite.rs

@@ -78,7 +78,10 @@ impl Layer for SpriteLayer {
                     render_bottom_tall_sprite(&mut sprite_to_draw);
                 }
 
-                self.sprite_to_draw = Some(sprite_to_draw);
+                self.sprite_to_draw = Some(match self.sprite_to_draw {
+                    Some(existing_sprite) => sprite_priority(existing_sprite, sprite_to_draw),
+                    None => sprite_to_draw,
+                });
             }
         }
     }
@@ -180,6 +183,15 @@ fn render_bottom_tall_sprite(sprite: &mut SpriteData) {
     };
 }
 
+/// Returns the sprite with the highest priority
+fn sprite_priority(sprite1: SpriteData, sprite2: SpriteData) -> SpriteData {
+    if sprite1.x < sprite2.x {
+        return sprite1;
+    }
+
+    sprite2
+}
+
 #[derive(Clone, Copy)]
 pub(crate) struct SpriteData {
     pub(crate) y: u8,