ppu.rs

        // calculates the offset that is going to be used in the update of the color buffer
        // which stores Game Boy colors from 0 to 3
        let mut color_offset = self.ly as usize * DISPLAY_WIDTH;

        // calculates the frame buffer offset position assuming the proper
        // Game Boy screen width and RGB pixel (3 bytes) size
        let mut frame_offset = self.ly as usize * DISPLAY_WIDTH * RGB_SIZE;

        // calculates both the current Y and X positions within the tiles
        // using the bitwise and operation as an effective modulus 8
        let y = (ld as usize + scy as usize) & 0x07;
        let mut x = (scx & 0x07) as usize;

        for index in 0..DISPLAY_WIDTH {
            // in case the current pixel to be drawn for the line
            // is visible within the window draws it an increments
            // the X coordinate of the tile
            if index as i16 >= wx as i16 - 7 {
                // obtains the current pixel data from the tile and
                // re-maps it according to the current palette
                let pixel = self.tiles[tile_index].get(x, y);
                let color = self.palette_bg[pixel as usize];

                // updates the pixel in the color buffer, which stores
                // the raw pixel color information (unmapped)
                self.color_buffer[color_offset] = pixel;

                // set the color pixel in the frame buffer
                self.frame_buffer[frame_offset] = color[0];
                self.frame_buffer[frame_offset + 1] = color[1];
                self.frame_buffer[frame_offset + 2] = color[2];

                // increments the current tile X position in drawing
                x += 1;

                // in case the end of tile width has been reached then
                // a new tile must be retrieved for plotting
                if x == TILE_WIDTH {
                    // resets the tile X position to the base value
                    // as a new tile is going to be drawn
                    x = 0;

                    // calculates the new line tile offset making sure that
                    // the maximum of 32 is not overflown
                    line_offset = (line_offset + 1) % 32;

                    // calculates the tile index and makes sure the value
                    // takes into consideration the bg tile value
                    tile_index = self.vram[map_offset + line_offset] as usize;
                    if !self.bg_tile && tile_index < 128 {
                        tile_index += 256;
                    }
                }
            }

            // increments the color offset by one, representing
            // the drawing of one pixel
            color_offset += 1;

            // increments the offset of the frame buffer by the
            // size of an RGB pixel (which is 3 bytes)
            frame_offset += RGB_SIZE;
        }
    }

    fn render_objects(&mut self) {
        let mut draw_count = 0u8;

        // allocates the buffer that is going to be used to determine
        // drawing priority for overlapping pixels between different
        // objects, in MBR mode the object that has the smallest X
        // coordinate takes priority in drawing the pixel
        let mut index_buffer = [-256i16; DISPLAY_WIDTH];

        for index in 0..OBJ_COUNT {
            // in case the limit on the number of objects to be draw per
            // line has been reached breaks the loop avoiding more draws
            if draw_count == 10 {
                break;
            }

            // obtains the meta data of the object that is currently
            // under iteration to be checked for drawing
            let obj = self.obj_data[index];

            let obj_height = if self.obj_size {
                TILE_DOUBLE_HEIGHT
            } else {
                TILE_HEIGHT
            };

            // verifies if the sprite is currently located at the
            // current line that is going to be drawn and skips it
            // in case it's not
            let is_contained =
                (obj.y <= self.ly as i16) && ((obj.y + obj_height as i16) > self.ly as i16);
            if !is_contained {
                continue;
            }

            let palette = if obj.palette == 0 {
                self.palette_obj_0
            } else {
                self.palette_obj_1
            };

            // calculates the offset in the color buffer (raw color information
            // from 0 to 3) for the sprit that is going to be drawn, this value
            // is kept as a signed integer to allow proper negative number math
            let mut color_offset = self.ly as i32 * DISPLAY_WIDTH as i32 + obj.x as i32;

            // calculates the offset in the frame buffer for the sprite
            // that is going to be drawn, this is going to be the starting
            // point for the draw operation to be performed
            let mut frame_offset =
                (self.ly as i32 * DISPLAY_WIDTH as i32 + obj.x as i32) * RGB_SIZE as i32;

            // the relative title offset should range from 0 to 7 in 8x8
            // objects and from 0 to 15 in 8x16 objects
            let mut tile_offset = self.ly as i16 - obj.y;

            // in case we're flipping the object we must recompute the
            // tile offset as an inverted value using the object's height
            if obj.yflip {
                tile_offset = obj_height as i16 - tile_offset - 1;
            }

            // saves some space for the reference to the tile that
            // is going to be used in the current operation
            let tile: &Tile;

            // in case we're facing a 8x16 object then we must
            // differentiate between the handling of the top tile
            // and the bottom tile through bitwise manipulation
            // of the tile index
            if self.obj_size {
                if tile_offset < 8 {
                    tile = &self.tiles[obj.tile as usize & 0xfe];
                } else {
                    tile = &self.tiles[obj.tile as usize | 0x01];
                    tile_offset -= 8;
                }
            }
            // otherwise we're facing a 8x8 sprite and we should grab
            // the tile directly from the object's tile index
            else {
                tile = &self.tiles[obj.tile as usize];
            }

            let tile_row = tile.get_row(tile_offset as usize);

            for tile_x in 0..TILE_WIDTH {
                let x = obj.x + tile_x as i16;
                let is_contained = (x >= 0) && (x < DISPLAY_WIDTH as i16);
                if is_contained {
                    // the object is only considered visible if no background or
                    // window should be drawn over or if the underlying pixel
                    // is transparent (zero value) meaning there's no background
                    // or window for the provided pixel
                    let is_visible = !obj.bg_over || self.color_buffer[color_offset as usize] == 0;

                    // determines if the current pixel has priority over a possible
                    // one that has been drawn by a previous object, this happens
                    // in case the current object has a small X coordinate according
                    // to the MBR algorithm
                    let has_priority =
                        index_buffer[x as usize] == -256 || obj.x < index_buffer[x as usize];

                    let pixel = tile_row[if obj.xflip { 7 - tile_x } else { tile_x }];
                    if is_visible && has_priority && pixel != 0 {
                        // marks the current pixel in iteration as "owned"
                        // by the object with the defined X base position,
                        // to be used in priority calculus
                        index_buffer[x as usize] = obj.x;

                        // obtains the current pixel data from the tile row and
                        // re-maps it according to the object palette
                        let color = palette[pixel as usize];

                        // updates the pixel in the color buffer, which stores
                        // the raw pixel color information (unmapped)
                        self.color_buffer[color_offset as usize] = pixel;

                        // sets the color pixel in the frame buffer
                        self.frame_buffer[frame_offset as usize] = color[0];
                        self.frame_buffer[frame_offset as usize + 1] = color[1];
                        self.frame_buffer[frame_offset as usize + 2] = color[2];
                    }
                }

                // increment the color offset by one as this represents
                // the advance of one color pixel
                color_offset += 1;

                // increments the offset of the frame buffer by the
                // size of an RGB pixel (which is 3 bytes)
                frame_offset += RGB_SIZE as i32;
            }

            // increments the counter so that we're able to keep
            // track on the number of object drawn
            draw_count += 1;
        }
    }

    /// Runs an update operation on the LCD STAT interrupt meaning
    /// that the flag that control will be updated in case the conditions
    /// required for the LCD STAT interrupt to be triggered are met.
    fn update_stat(&mut self) {
        self.int_stat = self.stat_level();
    }

    /// Obtains the current level of the LCD STAT interrupt by
    /// checking the current PPU state in various sections.
    fn stat_level(&self) -> bool {
        self.stat_lyc && self.lyc == self.ly
            || self.stat_oam && self.mode == PpuMode::OamRead
            || self.stat_vblank && self.mode == PpuMode::VBlank
            || self.stat_hblank && self.mode == PpuMode::HBlank
    }

    /// Computes the values for all of the palettes, this method
    /// is useful to "flush" color computation whenever the base
    /// palette colors are changed.
    fn compute_palettes(&mut self) {
        // re-computes the complete set of palettes according to
        // the currently set palette colors (that may have changed)
        Self::compute_palette(&mut self.palette_bg, &self.palette_colors, self.palettes[0]);
        Self::compute_palette(
            &mut self.palette_obj_0,
            &self.palette_colors,
            self.palettes[1],
        );
        Self::compute_palette(
            &mut self.palette_obj_1,
            &self.palette_colors,
            self.palettes[2],
        );

        // clears the frame buffer to allow the new background
        // color to be used
        self.clear_frame_buffer();
    }

    /// Static method used for the base logic of computation of RGB
    /// based palettes from the internal Game Boy color indexes.
    /// This method should be called whenever the palette indexes
    /// are changed.
    fn compute_palette(palette: &mut Palette, palette_colors: &Palette, value: u8) {
        for (index, palette_item) in palette.iter_mut().enumerate() {
            let color_index: usize = (value as usize >> (index * 2)) & 3;
            match color_index {
                0..=3 => *palette_item = palette_colors[color_index],
                color_index => panic!("Invalid palette color index {:04x}", color_index),
            }
        }
    }

    /// Static method that computes an RGB888 color palette ready to
    /// be used for frame buffer operations from 4 (colors) x 2 bytes (RGB555)
    /// that represent an RGB555 set of colors. This method should be
    /// used for CGB mode where colors are represented using RGB555.
    fn compute_palette_color(palette: &mut Palette, palette_color: &[u8; 64], palette_index: u8) {
        let palette_offset = palette_index * 4 * 2;
        for (index, palette_item) in palette.iter_mut().enumerate() {
            let color_index: usize = palette_offset as usize + index * 2;
            *palette_item =
                Self::rgb555_to_rgb888(palette_color[color_index], palette_color[color_index + 1]);
        }
    }

    fn rgb555_to_rgb888(first: u8, second: u8) -> Pixel {
        let r = first & 0x1f << 3;
        let g = (first & 0xe0 >> 5 | (second & 0x03) << 3) << 3;
        let b = ((second & 0x7c) >> 2) << 3;
        [r as u8, g as u8, b as u8]
    }
}

impl Default for Ppu {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(test)]
mod tests {
    use super::Ppu;

    #[test]
    fn test_update_tile_simple() {
        let mut ppu = Ppu::new();
        ppu.vram[0x0000] = 0xff;
        ppu.vram[0x0001] = 0xff;

        let result = ppu.tiles()[0].get(0, 0);
        assert_eq!(result, 0);

        ppu.update_tile(0x8000, 0x00);
        let result = ppu.tiles()[0].get(0, 0);
        assert_eq!(result, 3);
    }

    #[test]
    fn test_update_tile_upper() {
        let mut ppu = Ppu::new();
        ppu.vram[0x1000] = 0xff;
        ppu.vram[0x1001] = 0xff;

        let result = ppu.tiles()[256].get(0, 0);
        assert_eq!(result, 0);

        ppu.update_tile(0x9000, 0x00);
        let result = ppu.tiles()[256].get(0, 0);
        assert_eq!(result, 3);
    }
}