Implemented sprite & background mixing

src/gpu/sprite_parser.rs

@@ -1,7 +1,8 @@
 use crate::{common::Bit, GameBoy};
 
-use super::tile_parser::Line;
+use super::{tile_parser::Line, Color};
 
+#[derive(PartialEq, Eq)]
 pub(crate) enum Priority {
     AlwaysAbove,
     AboveLightColor,
@@ -36,8 +37,8 @@ impl GameBoy {
             x,
             tile_number,
             priority: match flags.get_bit(7) {
-                false => Priority::AlwaysAbove,
-                true => Priority::AboveLightColor,
+                false => Priority::AboveLightColor,
+                true => Priority::AlwaysAbove,
             },
             palette: match flags.get_bit(4) {
                 false => Palette::OBP0,
@@ -82,3 +83,17 @@ impl GameBoy {
         sprite_fifo
     }
 }
+
+impl Line {
+    pub(crate) fn mix_with_background_tile(&mut self, background_tile: &Line, priority: &Priority) {
+        if *priority == Priority::AlwaysAbove {
+            return;
+        }
+
+        for (sprite_pixel, bg_pixel) in self.colors.iter_mut().zip(background_tile.colors) {
+            if *sprite_pixel == Color::Light {
+                *sprite_pixel = bg_pixel;
+            }
+        }
+    }
+}

src/gpu/states.rs

@@ -98,13 +98,15 @@ impl GameBoy {
         let background_fifo = self.get_line(self.bus[tile_map_address], self.gpu.y as u16 % 8);
         let mut sprite = self.get_sprite_fifo(self.gpu.x, self.gpu.y);
 
-        // TODO: Implement fifo mixing
         if let Some((sprite_fifo, sprite_data)) = &mut sprite {
-            self.apply_palette_to_sprite(sprite_fifo, &sprite_data.palette);
-
             if self.gpu.rendered_sprites_on_line < 10 {
+                self.apply_palette_to_sprite(sprite_fifo, &sprite_data.palette);
+                sprite_fifo.mix_with_background_tile(&background_fifo, &sprite_data.priority);
+
                 self.draw_line(&sprite_fifo, self.gpu.x as usize, self.gpu.y as usize);
                 self.gpu.rendered_sprites_on_line += 1;
+            } else {
+                self.draw_line(&background_fifo, self.gpu.x as usize, self.gpu.y as usize);
             }
         } else {
             self.draw_line(&background_fifo, self.gpu.x as usize, self.gpu.y as usize);