Fixed incorrect SCY handling

And also made a lot of code better, removing the ambiguosly
named variable "self.gpu.i", SCX is still wrong tho

src/gpu/sprite_parser.rs

@@ -98,7 +98,12 @@ impl GameBoy {
 
             if x_condition && y_condition {
                 sprite_fifo = Some((
-                    self.get_line_rotation(tile_number, y as u16 % 8, sprite.x_flip, sprite.y_flip),
+                    self.get_line_from_tile_number_with_rotation(
+                        tile_number,
+                        y % 8,
+                        sprite.x_flip,
+                        sprite.y_flip,
+                    ),
                     sprite,
                 ));
             }

src/gpu/states.rs

@@ -38,9 +38,6 @@ pub struct Gpu {
     x: u8,
     y: u8,
 
-    /// This is the offset to add to the tile map address
-    i: u16,
-
     /// A dot is 1/4 of a CPU cycle
     dots: u16,
 }
@@ -55,7 +52,6 @@ impl Gpu {
             rendered_sprites_on_line: 0,
             x: 0,
             y: 0,
-            i: 0,
             dots: 0,
         }
     }
@@ -90,11 +86,6 @@ impl GameBoy {
     }
 
     fn pixel_transfer(&mut self) {
-        let mut tile_map_address: u16 = match self.bus[0xFF40].get_bit(3) {
-            false => 0x9800,
-            true => 0x9C00,
-        };
-
         let sprite_height = match self.bus[LCDC].get_bit(2) {
             false => SpriteHeight::Short,
             true => SpriteHeight::Tall,
@@ -106,31 +97,20 @@ impl GameBoy {
             self.gpu.dots += 12;
         }
 
-        // Y Scrolling
-        // The gameboy tilemap is 32x32 tiles, both `SCX` and `SCY` use pixels, not tiles
-        // so we have to divide them by 8, skipping 32 tiles just means to set the
-        // "cursor" on the line below
-        for _ in 0..(self.bus[SCY] / 8) {
-            tile_map_address += 32;
-        }
-
-        // X Scrolling
-        // We add the number of tiles to skip to the adress
-        tile_map_address += self.bus[SCX] as u16 / 8;
+        // And we get the background fifo and the sprite fifo
+        let background_fifo = self.get_line_from_coordinates(
+            self.gpu.x.wrapping_add(self.bus[SCX]),
+            self.gpu.y.wrapping_add(self.bus[SCY]),
+        );
 
-        // Adding i
-        tile_map_address += self.gpu.i;
-
-        // And we get both background/window fifo and the sprite fifo
-        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, &sprite_height);
 
         if let Some((sprite_fifo, sprite_data)) = &mut sprite {
             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.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);
@@ -139,20 +119,9 @@ impl GameBoy {
             self.draw_line(&background_fifo, self.gpu.x as usize, self.gpu.y as usize);
         }
 
-        self.gpu.i += 1;
         self.gpu.x += 8;
 
         if self.gpu.x == (DISPLAY_SIZE_X as u8) {
-            // If we finished rendering all the 20 tiles, and we want to go to the next
-            // set of tile, we skip 12, because the tile map is 32x32, and the viewport
-            // is 20x18 (32 - 20), and if we haven't rendered the 20 tiles, we go back
-            // to the first one
-            if self.gpu.y % 8 == 7 {
-                self.gpu.i += 12;
-            } else {
-                self.gpu.i -= 20;
-            }
-
             self.gpu.y += 1;
         }
 
@@ -200,7 +169,6 @@ impl GameBoy {
 
         if self.gpu.dots == 0 {
             self.gpu.y = 0;
-            self.gpu.i = 0;
         }
 
         // After every line

src/gpu/tile_parser.rs

@@ -43,7 +43,13 @@ impl Line {
 }
 
 impl GameBoy {
-    pub(crate) fn get_line(&self, tile_number: u8, y: u16) -> Line {
+    /// Returns the given line of a tile
+    /// # Parameters
+    /// - *tile_number*: The tile of the number, it will follow LCDC.4's tile data area
+    /// specification
+    /// - *line_y*: The line of the tile that we want to get, as a tile is 8x8, this
+    /// number can go from 0 to 7
+    pub(crate) fn get_line_from_tile_number(&self, tile_number: u8, line_y: u8) -> Line {
         let mut tile = Line::new_blank();
 
         // To calculate the address to fetch we calculate
@@ -65,21 +71,37 @@ impl GameBoy {
 
         let tile_number: u16 = (tile_number as u16) << 4;
 
-        let low = self.bus[tile_data_address + tile_number + y * 2];
-        let high = self.bus[tile_data_address + tile_number + y * 2 + 1];
+        let low = self.bus[tile_data_address + tile_number + line_y as u16 * 2];
+        let high = self.bus[tile_data_address + tile_number + line_y as u16 * 2 + 1];
 
         tile.draw_line(high, low);
         tile
     }
 
-    pub(crate) fn get_line_rotation(
+    /// Returns the line that should be at the given coordinates of the screen (excluding
+    /// horizontal and vertical scroll)
+    pub(crate) fn get_line_from_coordinates(&self, x: u8, y: u8) -> Line {
+        let tile_map_address: u16 = match self.bus[0xFF40].get_bit(3) {
+            false => 0x9800,
+            true => 0x9C00,
+        };
+
+        // TODO: This gets tricky when the X Scrolling is not a multiplier of 8
+        let tiled_y = y / 8; // The Y tile number
+        let tiled_x = x / 8; // The X tile number
+
+        let tile_number = self.bus[tile_map_address + (tiled_y as u16 * 0x20) + tiled_x as u16];
+        self.get_line_from_tile_number(tile_number, y % 8)
+    }
+
+    pub(crate) fn get_line_from_tile_number_with_rotation(
         &self,
         tile_number: u8,
-        y: u16,
+        y: u8,
         x_flip: bool,
         y_flip: bool,
     ) -> Line {
-        let mut line = self.get_line(
+        let mut line = self.get_line_from_tile_number(
             tile_number,
             match y_flip {
                 false => y,