gb/src/ppu.rs

use crate::Cycle;
use crate::GB_HEIGHT;
use crate::GB_WIDTH;
use bitfield::bitfield;
use std::collections::VecDeque;
use std::convert::TryInto;

const VRAM_SIZE: usize = 0x2000;
const OAM_SIZE: usize = 0xA0;
const PPU_START_ADDRESS: usize = 0x8000;

// OAM Scan
const OBJECT_LIMIT: usize = 10;

const WHITE: [u8; 4] = [0xFF, 0xFF, 0xFF, 0xFF];
const LIGHT_GRAY: [u8; 4] = [0xCC, 0xCC, 0xCC, 0xFF];
const DARK_GRAY: [u8; 4] = [0x77, 0x77, 0x77, 0xFF];
const BLACK: [u8; 4] = [0x00, 0x00, 0x00, 0x00];

#[derive(Debug, Clone)]
pub struct Ppu {
    pub int: Interrupt,
    pub control: LCDControl,
    pub monochrome: Monochrome,
    pub pos: ScreenPosition,
    pub vram: Box<[u8; VRAM_SIZE]>,
    pub stat: LCDStatus,
    pub oam: ObjectAttributeTable,
    fetcher: PixelFetcher,
    fifo: FifoRenderer,
    obj_buffer: ObjectBuffer,
    frame_buf: Box<[u8; GB_WIDTH * GB_HEIGHT * 4]>,
    x_pos: u8,
    cycles: Cycle,
}

impl Ppu {
    pub fn read_byte(&self, addr: u16) -> u8 {
        self.vram[addr as usize - PPU_START_ADDRESS]
    }

    pub fn write_byte(&mut self, addr: u16, byte: u8) {
        self.vram[addr as usize - PPU_START_ADDRESS] = byte;
    }
}

impl Ppu {
    pub fn step(&mut self, cycles: Cycle) {
        let start: u32 = self.cycles.into();
        let end: u32 = cycles.into();

        for _ in start..(start + end) {
            self.cycles += 1;

            match self.stat.mode() {
                Mode::OamScan => {
                    if self.cycles >= 80.into() {
                        self.stat.set_mode(Mode::Drawing);
                    }

                    self.scan_oam(self.cycles.into());
                }
                Mode::Drawing => {
                    if self.x_pos >= 160 {
                        if self.stat.hblank_int() {
                            // Enable HBlank LCDStat Interrupt
                            self.int.set_lcd_stat(true);
                        }

                        // Done with rendering this frame,
                        // we can reset the ppu x_pos and fetcher state now
                        self.x_pos = 0;
                        self.fetcher.hblank_reset();
                        self.obj_buffer.clear();

                        self.stat.set_mode(Mode::HBlank);
                    } else {
                        self.draw(self.cycles.into());
                    }
                }
                Mode::HBlank => {
                    // This mode will always end at 456 cycles

                    if self.cycles >= 456.into() {
                        self.cycles %= 456;
                        self.pos.line_y += 1;

                        // New Scanline is next, check for LYC=LY
                        if self.stat.coincidence_int() {
                            let are_equal = self.pos.line_y == self.pos.ly_compare;
                            self.stat.set_coincidence(are_equal);
                        }

                        let next_mode = if self.pos.line_y >= 144 {
                            // Request VBlank Interrupt
                            self.int.set_vblank(true);

                            // Reset Window Line Counter in Fetcher
                            self.fetcher.vblank_reset();

                            if self.stat.vblank_int() {
                                // Enable Vblank LCDStat Interrupt
                                self.int.set_lcd_stat(true);
                            }

                            Mode::VBlank
                        } else {
                            if self.stat.oam_int() {
                                // Enable OAM LCDStat Interrupt
                                self.int.set_lcd_stat(true);
                            }

                            Mode::OamScan
                        };

                        self.stat.set_mode(next_mode);
                    }
                }
                Mode::VBlank => {
                    if self.cycles > 456.into() {
                        self.cycles %= 456;
                        self.pos.line_y += 1;

                        // New Scanline is next, check for LYC=LY
                        if self.stat.coincidence_int() {
                            let are_equal = self.pos.line_y == self.pos.ly_compare;
                            self.stat.set_coincidence(are_equal);
                        }

                        if self.pos.line_y == 154 {
                            self.pos.line_y = 0;

                            if self.stat.oam_int() {
                                // Enable OAM LCDStat Interrupt
                                self.int.set_lcd_stat(true);
                            }

                            self.stat.set_mode(Mode::OamScan);
                        }
                    }
                }
            }
        }
    }

    fn scan_oam(&mut self, cycle: u32) {
        if cycle % 2 != 0 {
            // This is run 50% of the time, or 40 times
            // which is the number of sprites in OAM

            let sprite_height = match self.control.obj_size() {
                ObjectSize::EightByEight => 8,
                ObjectSize::EightBySixteen => 16,
            };

            let attr = self.oam.attribute((cycle / 2) as usize);
            let line_y = self.pos.line_y + 16;

            if attr.x > 0
                && line_y >= attr.y
                && line_y < (attr.y + sprite_height)
                && !self.obj_buffer.full()
            {
                self.obj_buffer.add(attr);
            }
        }
    }

    fn draw(&mut self, cycle: u32) {
        use FetcherState::*;
        let control = &self.control;
        let pos = &self.pos;

        let line_y = self.pos.line_y;
        let window_y = self.pos.window_y;
        let is_window = self.control.window_enabled() && window_y <= line_y;

        // Determine whether we need to enable sprite fetching
        let mut obj_attr = None;

        for i in 0..self.obj_buffer.len() {
            if let Some(attr) = self.obj_buffer.get(i) {
                if attr.x <= (self.x_pos + 8) {
                    // self.fetcher.obj.resume(); TODO: Try running only when there's a sprite
                    self.fetcher.bg.reset();
                    self.fetcher.bg.pause();
                    self.fifo.pause();

                    obj_attr = Some(attr);
                }
            }
        }

        if let Some(attr) = obj_attr {
            match self.fetcher.obj.state {
                TileNumber => {
                    if cycle % 2 != 0 {
                        self.fetcher.obj.tile.with_id(attr.tile_index);

                        self.fetcher.obj.next(TileDataLow);
                    }
                }
                TileDataLow => {
                    if cycle % 2 != 0 {
                        let obj_size = match self.control.obj_size() {
                            ObjectSize::EightByEight => 8,
                            ObjectSize::EightBySixteen => 16,
                        };

                        let addr = PixelFetcher::get_obj_low_addr(attr, &self.pos, obj_size);

                        let byte = self.read_byte(addr);
                        self.fetcher.obj.tile.with_low_byte(byte);

                        self.fetcher.obj.next(TileDataHigh);
                    }
                }
                TileDataHigh => {
                    if cycle % 2 != 0 {
                        let obj_size = match self.control.obj_size() {
                            ObjectSize::EightByEight => 8,
                            ObjectSize::EightBySixteen => 16,
                        };

                        let addr = PixelFetcher::get_obj_low_addr(attr, &self.pos, obj_size);

                        let byte = self.read_byte(addr + 1);
                        self.fetcher.obj.tile.with_high_byte(byte);

                        self.fetcher.obj.next(SendToFifo);
                    }
                }
                SendToFifo => {
                    self.fetcher.obj.fifo_count += 1;

                    if self.fetcher.obj.fifo_count == 1 {
                        // Load into Fifo
                        let tile_bytes = self.fetcher.obj.tile.low.zip(self.fetcher.obj.tile.high);

                        if let Some(bytes) = tile_bytes {
                            let low = bytes.0;
                            let high = bytes.1;

                            let pixel = TwoBitsPerPixel::from_bytes(high, low);

                            let palette = match attr.flags.palette() {
                                ObjectPaletteId::Palette0 => self.monochrome.obj_palette_0,
                                ObjectPaletteId::Palette1 => self.monochrome.obj_palette_1,
                            };

                            let num_to_add = 8 - self.fifo.object.len();

                            for i in 0..num_to_add {
                                let bit = 7 - i;

                                let priority = attr.flags.priority();

                                let shade = palette.colour(pixel.pixel(bit));

                                let fifo_pixel = ObjectFifoPixel {
                                    shade,
                                    palette,
                                    priority,
                                };

                                self.fifo.object.push_back(fifo_pixel);
                            }

                            self.fetcher.bg.resume();
                            self.fifo.resume();
                        }
                    } else if self.fetcher.obj.fifo_count == 2 {
                        self.fetcher.obj.reset();
                    } else {
                        panic!("Object FIFO Logic Error has occurred :angry:");
                    }
                }
            }
        }

        // By only running on odd cycles, we can ensure that we draw every two T cycles
        if cycle % 2 != 0 {
            if self.fetcher.bg.is_enabled() {
                match self.fetcher.bg.state {
                    TileNumber => {
                        // Increment Window line counter if scanline had any window pixels on it
                        // only increment once per scanline though
                        if is_window && !self.fetcher.bg.window_line.checked() {
                            self.fetcher.bg.window_line.increment();
                        }

                        let x_pos = self.fetcher.x_pos;

                        let addr = self
                            .fetcher
                            .bg_tile_num_addr(control, pos, x_pos, is_window);

                        let id = self.read_byte(addr);
                        self.fetcher.bg.tile.with_id(id);

                        // Move on to the Next state in 2 T-cycles
                        self.fetcher.bg.next(TileDataLow);
                    }
                    TileDataLow => {
                        let addr = self.fetcher.bg_byte_low_addr(control, pos, is_window);

                        let low = self.read_byte(addr);
                        self.fetcher.bg.tile.with_low_byte(low);

                        self.fetcher.bg.next(TileDataHigh);
                    }
                    TileDataHigh => {
                        let addr = self.fetcher.bg_byte_low_addr(control, pos, is_window);

                        let high = self.read_byte(addr + 1);
                        self.fetcher.bg.tile.with_high_byte(high);

                        self.fetcher.bg.next(SendToFifo);
                    }
                    SendToFifo => {
                        let palette = &self.monochrome.bg_palette;
                        self.fetcher.send_to_fifo(&mut self.fifo, palette);

                        // FIXME: Should this be equivalent to a reset?
                        self.fetcher.bg.next(TileNumber);
                    }
                }
            }
        }

        if self.fifo.is_enabled() {
            // Handle Background Pixel and Sprite FIFO
            if let Some(bg_pixel) = self.fifo.background.pop_front() {
                if let Some(_object_pixel) = self.fifo.object.pop_front() {
                    todo!("Mix the pixels or whatever I'm supposed todo here");
                } else {
                    // Only Background Pixels will be rendered

                    let y = self.pos.line_y as usize;
                    let x = self.x_pos as usize;
                    let rgba = bg_pixel.0.into_rgba();

                    let i = (GB_WIDTH * 4) * y + (x * 4);
                    self.frame_buf[i..(i + rgba.len())].copy_from_slice(&rgba);
                }

                self.x_pos += 1;
            }
        }
    }

    pub fn copy_to_gui(&self, frame: &mut [u8]) {
        frame.copy_from_slice(self.frame_buf.as_ref());
    }
}

impl Default for Ppu {
    fn default() -> Self {
        Self {
            vram: Box::new([0u8; VRAM_SIZE]),
            cycles: 0.into(),
            frame_buf: Box::new([0; GB_WIDTH * GB_HEIGHT * 4]),
            int: Default::default(),
            control: Default::default(),
            monochrome: Default::default(),
            pos: Default::default(),
            stat: Default::default(),
            oam: Default::default(),
            fetcher: Default::default(),
            fifo: Default::default(),
            obj_buffer: Default::default(),
            x_pos: Default::default(),
        }
    }
}

#[derive(Debug, Clone, Copy, Default)]
pub struct Interrupt {
    _vblank: bool,
    _lcd_stat: bool,
}

impl Interrupt {
    pub fn vblank(&self) -> bool {
        self._vblank
    }

    pub fn set_vblank(&mut self, enabled: bool) {
        self._vblank = enabled;
    }

    pub fn lcd_stat(&self) -> bool {
        self._lcd_stat
    }

    pub fn set_lcd_stat(&mut self, enabled: bool) {
        self._lcd_stat = enabled;
    }
}

bitfield! {
    pub struct LCDStatus(u8);
    impl Debug;
    pub coincidence_int, set_coincidence_int: 6;
    pub oam_int, set_oam_int: 5;
    pub vblank_int, set_vblank_int: 4;
    pub hblank_int, set_hblank_int: 3;
    pub coincidence, set_coincidence: 2; // LYC == LY Flag
    from into Mode, _mode, set_mode: 1, 0;
}

impl LCDStatus {
    pub fn mode(&self) -> Mode {
        self._mode()
    }
}

impl Copy for LCDStatus {}
impl Clone for LCDStatus {
    fn clone(&self) -> Self {
        *self
    }
}

impl Default for LCDStatus {
    fn default() -> Self {
        Self(0x80) // bit 7 is always 1
    }
}

impl From<u8> for LCDStatus {
    fn from(byte: u8) -> Self {
        Self(byte)
    }
}

impl From<LCDStatus> for u8 {
    fn from(status: LCDStatus) -> Self {
        status.0
    }
}

#[derive(Debug, Clone, Copy)]
pub enum Mode {
    HBlank = 0,
    VBlank = 1,
    OamScan = 2,
    Drawing = 3,
}

impl From<u8> for Mode {
    fn from(byte: u8) -> Self {
        match byte {
            0b00 => Self::HBlank,
            0b01 => Self::VBlank,
            0b10 => Self::OamScan,
            0b11 => Self::Drawing,
            _ => unreachable!("{:#04X} is not a valid value for LCDMode", byte),
        }
    }
}

impl From<Mode> for u8 {
    fn from(mode: Mode) -> Self {
        mode as Self
    }
}

impl Default for Mode {
    fn default() -> Self {
        Self::HBlank
    }
}

#[derive(Debug, Clone, Copy, Default)]
pub struct ScreenPosition {
    pub scroll_y: u8,
    pub scroll_x: u8,
    pub line_y: u8,
    pub ly_compare: u8,
    pub window_y: u8,
    pub window_x: u8,
}

bitfield! {
    pub struct LCDControl(u8);
    impl Debug;
    lcd_enabled, set_lcd_enabled: 7;
    from into TileMapAddress, win_tile_map_addr, set_win_tile_map_addr: 6, 6;
    window_enabled, set_window_enabled: 5;
    from into TileDataAddress, tile_data_addr, set_tile_data_addr: 4, 4;
    from into TileMapAddress, bg_tile_map_addr, set_bg_tile_map_addr: 3, 3;
    from into ObjectSize, obj_size, set_obj_size: 2, 2;
    obj_enabled, set_obj_enabled: 1;
    bg_win_enabled, set_bg_win_enabled: 0;
}

impl Copy for LCDControl {}
impl Clone for LCDControl {
    fn clone(&self) -> Self {
        *self
    }
}

impl Default for LCDControl {
    fn default() -> Self {
        Self(0)
    }
}

impl From<u8> for LCDControl {
    fn from(byte: u8) -> Self {
        Self(byte)
    }
}

impl From<LCDControl> for u8 {
    fn from(ctrl: LCDControl) -> Self {
        ctrl.0
    }
}

#[derive(Debug, Clone, Copy)]
enum TileMapAddress {
    X9800 = 0,
    X9C00 = 1,
}

impl TileMapAddress {
    pub fn into_address(self) -> u16 {
        match self {
            TileMapAddress::X9800 => 0x9800,
            TileMapAddress::X9C00 => 0x9C00,
        }
    }
}

impl From<u8> for TileMapAddress {
    fn from(byte: u8) -> Self {
        match byte {
            0b00 => Self::X9800,
            0b01 => Self::X9C00,
            _ => unreachable!("{:#04X} is not a valid value for TileMapRegister", byte),
        }
    }
}

impl From<TileMapAddress> for u8 {
    fn from(reg: TileMapAddress) -> Self {
        reg as Self
    }
}

impl Default for TileMapAddress {
    fn default() -> Self {
        Self::X9800
    }
}

#[derive(Debug, Clone, Copy)]
enum TileDataAddress {
    X8800 = 0,
    X8000 = 1,
}

impl From<u8> for TileDataAddress {
    fn from(byte: u8) -> Self {
        match byte {
            0b00 => Self::X8800,
            0b01 => Self::X8000,
            _ => unreachable!("{:#04X} is not a valid value for TileDataRegister", byte),
        }
    }
}

impl From<TileDataAddress> for u8 {
    fn from(reg: TileDataAddress) -> Self {
        reg as Self
    }
}

impl Default for TileDataAddress {
    fn default() -> Self {
        Self::X8800
    }
}

#[derive(Debug, Clone, Copy)]
enum ObjectSize {
    EightByEight = 0,
    EightBySixteen = 1,
}

impl From<u8> for ObjectSize {
    fn from(byte: u8) -> Self {
        match byte {
            0b00 => Self::EightByEight,
            0b01 => Self::EightBySixteen,
            _ => unreachable!("{:#04X} is not a valid value for ObjSize", byte),
        }
    }
}

impl From<ObjectSize> for u8 {
    fn from(size: ObjectSize) -> Self {
        size as Self
    }
}

impl Default for ObjectSize {
    fn default() -> Self {
        Self::EightByEight
    }
}

#[derive(Debug, Clone, Copy)]
pub enum GrayShade {
    White = 0,
    LightGray = 1,
    DarkGray = 2,
    Black = 3,
}

impl GrayShade {
    pub fn into_rgba(self) -> [u8; 4] {
        match self {
            GrayShade::White => WHITE,
            GrayShade::LightGray => LIGHT_GRAY,
            GrayShade::DarkGray => DARK_GRAY,
            GrayShade::Black => BLACK,
        }
    }

    pub fn from_rgba(slice: &[u8]) -> Self {
        let rgba: [u8; 4] = slice
            .try_into()
            .expect("Unable to interpret &[u8] as [u8; 4]");

        match rgba {
            WHITE => GrayShade::White,
            LIGHT_GRAY => GrayShade::LightGray,
            DARK_GRAY => GrayShade::DarkGray,
            BLACK => GrayShade::Black,
            _ => panic!("{:#04X?} is not a colour the DMG-01 supports", rgba),
        }
    }
}

impl Default for GrayShade {
    fn default() -> Self {
        Self::White
    }
}

impl From<u8> for GrayShade {
    fn from(byte: u8) -> Self {
        match byte {
            0b00 => GrayShade::White,
            0b01 => GrayShade::LightGray,
            0b10 => GrayShade::DarkGray,
            0b11 => GrayShade::Black,
            _ => unreachable!("{:#04X} is not a valid value for GrayShade", byte),
        }
    }
}

impl From<GrayShade> for u8 {
    fn from(shade: GrayShade) -> Self {
        shade as Self
    }
}

#[derive(Debug, Clone, Copy, Default)]
pub struct Monochrome {
    pub bg_palette: BackgroundPalette,
    pub obj_palette_0: ObjectPalette,
    pub obj_palette_1: ObjectPalette,
}

bitfield! {
    pub struct BackgroundPalette(u8);
    impl Debug;
    pub from into GrayShade, i3_colour, set_i3_colour: 7, 6;
    pub from into GrayShade, i2_colour, set_i2_colour: 5, 4;
    pub from into GrayShade, i1_colour, set_i1_colour: 3, 2;
    pub from into GrayShade, i0_colour, set_i0_colour: 1, 0;
}

impl BackgroundPalette {
    pub fn colour(&self, id: u8) -> GrayShade {
        match id {
            0b00 => self.i0_colour(),
            0b01 => self.i1_colour(),
            0b10 => self.i2_colour(),
            0b11 => self.i3_colour(),
            _ => unreachable!("{:#04X} is not a valid BG colour id", id),
        }
    }
}

impl Copy for BackgroundPalette {}
impl Clone for BackgroundPalette {
    fn clone(&self) -> Self {
        *self
    }
}

impl Default for BackgroundPalette {
    fn default() -> Self {
        Self(0)
    }
}

impl From<u8> for BackgroundPalette {
    fn from(byte: u8) -> Self {
        Self(byte)
    }
}

impl From<BackgroundPalette> for u8 {
    fn from(palette: BackgroundPalette) -> Self {
        palette.0
    }
}

bitfield! {
    pub struct ObjectPalette(u8);
    impl Debug;
    pub from into GrayShade, i3_colour, set_i3_colour: 7, 6;
    pub from into GrayShade, i2_colour, set_i2_colour: 5, 4;
    pub from into GrayShade, i1_colour, set_i1_colour: 3, 2;
}

impl ObjectPalette {
    pub fn colour(&self, id: u8) -> Option<GrayShade> {
        match id {
            0b00 => None,
            0b01 => Some(self.i1_colour()),
            0b10 => Some(self.i2_colour()),
            0b11 => Some(self.i3_colour()),
            _ => unreachable!("{:#04X} is not a valid OBJ colour id", id),
        }
    }
}

impl Copy for ObjectPalette {}
impl Clone for ObjectPalette {
    fn clone(&self) -> Self {
        *self
    }
}

impl Default for ObjectPalette {
    fn default() -> Self {
        Self(0)
    }
}

impl From<u8> for ObjectPalette {
    fn from(byte: u8) -> Self {
        Self(byte)
    }
}

impl From<ObjectPalette> for u8 {
    fn from(palette: ObjectPalette) -> Self {
        palette.0
    }
}

struct TwoBitsPerPixel(u8, u8);

impl TwoBitsPerPixel {
    pub fn from_bytes(higher: u8, lower: u8) -> Self {
        Self(higher, lower)
    }

    pub fn pixel(&self, bit: usize) -> u8 {
        let higher = self.0 >> bit;
        let lower = self.1 >> bit;

        (higher & 0x01) << 1 | lower & 0x01
    }
}

#[derive(Debug, Clone)]
pub struct ObjectAttributeTable {
    buf: Box<[u8; OAM_SIZE]>,
}

impl ObjectAttributeTable {
    pub fn read_byte(&self, addr: u16) -> u8 {
        let index = (addr - 0xFE00) as usize;
        self.buf[index]
    }

    pub fn write_byte(&mut self, addr: u16, byte: u8) {
        let index = (addr - 0xFE00) as usize;
        self.buf[index] = byte;
    }

    pub fn attribute(&self, index: usize) -> ObjectAttribute {
        let slice: &[u8; 4] = self.buf[index..(index + 4)]
            .try_into()
            .expect("Could not interpret &[u8] as a &[u8; 4]");

        slice.into()
    }
}

impl Default for ObjectAttributeTable {
    fn default() -> Self {
        Self {
            buf: Box::new([0; OAM_SIZE]),
        }
    }
}

#[derive(Debug, Clone, Copy, Default)]
pub struct ObjectAttribute {
    y: u8,
    x: u8,
    tile_index: u8,
    flags: ObjectFlags,
}

impl From<[u8; 4]> for ObjectAttribute {
    fn from(bytes: [u8; 4]) -> Self {
        Self {
            y: bytes[0],
            x: bytes[1],
            tile_index: bytes[2],
            flags: bytes[3].into(),
        }
    }
}

impl<'a> From<&'a [u8; 4]> for ObjectAttribute {
    fn from(bytes: &'a [u8; 4]) -> Self {
        Self {
            y: bytes[0],
            x: bytes[1],
            tile_index: bytes[2],
            flags: bytes[3].into(),
        }
    }
}

bitfield! {
    pub struct ObjectFlags(u8);
    impl Debug;

    from into RenderPriority, priority, set_priority: 7, 7;
    y_flip, set_y_flip: 6;
    x_flip, set_x_flip: 5;
    from into ObjectPaletteId, palette, set_palette: 4, 4;
}

impl Copy for ObjectFlags {}
impl Clone for ObjectFlags {
    fn clone(&self) -> Self {
        *self
    }
}

impl From<u8> for ObjectFlags {
    fn from(byte: u8) -> Self {
        Self(byte)
    }
}

impl From<ObjectFlags> for u8 {
    fn from(flags: ObjectFlags) -> Self {
        flags.0
    }
}

impl Default for ObjectFlags {
    fn default() -> Self {
        Self(0)
    }
}

#[derive(Debug, Clone, Copy)]
pub enum RenderPriority {
    Object = 0,
    BackgroundAndWindow = 1,
}

impl From<u8> for RenderPriority {
    fn from(byte: u8) -> Self {
        match byte {
            0b00 => Self::Object,
            0b01 => Self::BackgroundAndWindow,
            _ => unreachable!("{:#04X} is not a valid value for RenderPriority", byte),
        }
    }
}

impl From<RenderPriority> for u8 {
    fn from(priority: RenderPriority) -> Self {
        priority as u8
    }
}

impl Default for RenderPriority {
    fn default() -> Self {
        Self::Object
    }
}

#[derive(Debug, Clone, Copy)]
pub enum ObjectPaletteId {
    Palette0 = 0,
    Palette1 = 1,
}

impl From<u8> for ObjectPaletteId {
    fn from(byte: u8) -> Self {
        match byte {
            0b00 => ObjectPaletteId::Palette0,
            0b01 => ObjectPaletteId::Palette1,
            _ => unreachable!("{:#04X} is not a valid value for BgPaletteNumber", byte),
        }
    }
}

impl From<ObjectPaletteId> for u8 {
    fn from(palette_num: ObjectPaletteId) -> Self {
        palette_num as u8
    }
}

#[derive(Debug, Clone, Copy)]
struct ObjectBuffer {
    buf: [ObjectAttribute; 10],
    len: usize,
}

impl ObjectBuffer {
    pub fn full(&self) -> bool {
        self.len == self.buf.len()
    }

    pub fn clear(&mut self) {
        self.buf = [Default::default(); 10];
        self.len = 0;
    }

    pub fn add(&mut self, attr: ObjectAttribute) {
        self.buf[self.len] = attr;
        self.len += 1;
    }

    pub fn len(&self) -> usize {
        self.len
    }

    pub fn get(&self, index: usize) -> Option<&ObjectAttribute> {
        if index < self.len {
            Some(&self.buf[index])
        } else {
            None
        }
    }
}

impl Default for ObjectBuffer {
    fn default() -> Self {
        Self {
            buf: [Default::default(); OBJECT_LIMIT],
            len: 0,
        }
    }
}

#[derive(Debug, Clone, Copy, Default)]
struct PixelFetcher {
    x_pos: u8,
    bg: BackgroundFetcher,
    obj: ObjectFetcher,
}

impl PixelFetcher {
    pub fn hblank_reset(&mut self) {
        self.bg.window_line.hblank_reset();

        self.bg.tile = Default::default();
        self.bg.state = Default::default();
        self.x_pos = 0;
    }

    pub fn vblank_reset(&mut self) {
        self.bg.window_line.vblank_reset();
    }

    fn bg_tile_num_addr(
        &self,
        control: &LCDControl,
        pos: &ScreenPosition,
        x_pos: u8,
        window: bool,
    ) -> u16 {
        let line_y = pos.line_y;
        let scroll_y = pos.scroll_y;
        let scroll_x = pos.scroll_x;

        // Determine which tile map is being used
        let tile_map = if window {
            control.win_tile_map_addr()
        } else {
            control.bg_tile_map_addr()
        };
        let tile_map_addr = tile_map.into_address();

        // Both Offsets are used to offset the tile map address we found above
        // Offsets are ANDed wih 0x3FF so that we stay in bounds of tile map memory
        // TODO: Is this necessary / important in other fetcher modes?
        let x_offset = (x_pos + scroll_x) as u16 & 0x03FF;
        let y_offset = (line_y.wrapping_add(scroll_y)) as u16 & 0x03FF;

        // Scroll X Offset is only used when we're rendering the background;
        let scx_offset = if window { 0 } else { scroll_x / 8 } & 0x1F;

        let offset = if window {
            32 * (self.bg.window_line.count() as u16 / 8)
        } else {
            32 * (((y_offset) & 0x00FF) / 8)
        };

        // Determine Address
        tile_map_addr + offset + x_offset + scx_offset as u16
    }

    fn bg_byte_low_addr(
        &mut self,
        control: &LCDControl,
        pos: &ScreenPosition,
        window: bool,
    ) -> u16 {
        let line_y = pos.line_y;
        let scroll_y = pos.scroll_y;

        let id = self.bg.tile.id.expect("Tile Number unexpectedly missing");

        let tile_data_addr = match control.tile_data_addr() {
            TileDataAddress::X8800 => (0x9000_i32 + (id as i32 * 16)) as u16,
            TileDataAddress::X8000 => 0x8000 + (id as u16 * 16),
        };

        let offset = if window {
            2 * (self.bg.window_line.count() % 8)
        } else {
            2 * ((line_y + scroll_y) % 8)
        };

        tile_data_addr + offset as u16
    }

    fn send_to_fifo(&mut self, fifo: &mut FifoRenderer, palette: &BackgroundPalette) {
        let tile_bytes = self.bg.tile.low.zip(self.bg.tile.high);

        if let Some(bytes) = tile_bytes {
            let low = bytes.0;
            let high = bytes.1;

            let pixel = TwoBitsPerPixel::from_bytes(high, low);

            if fifo.background.is_empty() {
                for i in 0..8 {
                    // Horizontally flip pixels
                    let bit = 7 - i;

                    let shade = palette.colour(pixel.pixel(bit));

                    let fifo_pixel = BackgroundFifoPixel(shade);
                    fifo.background.push_back(fifo_pixel);
                }
            }
        }

        self.x_pos += 1;
    }

    pub fn get_obj_low_addr(attr: &ObjectAttribute, pos: &ScreenPosition, obj_size: u8) -> u16 {
        let line_y = pos.line_y;

        // FIXME: Should we subtract 16 from attr.y?
        let y = attr.y.wrapping_sub(16);

        let line = if attr.flags.y_flip() {
            (obj_size - (line_y - y)) * 2
        } else {
            (line_y - y) * 2
        };

        0x8000 + (attr.tile_index as u16 * 16) + line as u16
    }
}

trait Fetcher {
    fn next(&mut self, state: FetcherState);
    fn reset(&mut self);
    fn pause(&mut self);
    fn resume(&mut self);
    fn is_enabled(&self) -> bool;
}

#[derive(Debug, Clone, Copy)]
struct BackgroundFetcher {
    state: FetcherState,
    tile: TileBuilder,
    window_line: WindowLineCounter,
    enabled: bool,
}

impl Fetcher for BackgroundFetcher {
    fn next(&mut self, state: FetcherState) {
        self.state = state
    }

    fn reset(&mut self) {
        self.state = FetcherState::TileNumber;
    }

    fn pause(&mut self) {
        self.enabled = false;
    }

    fn resume(&mut self) {
        self.enabled = true;
    }

    fn is_enabled(&self) -> bool {
        self.enabled
    }
}

impl Default for BackgroundFetcher {
    fn default() -> Self {
        Self {
            state: Default::default(),
            tile: Default::default(),
            window_line: Default::default(),
            enabled: true,
        }
    }
}

#[derive(Debug, Clone, Copy, Default)]
struct ObjectFetcher {
    state: FetcherState,
    tile: TileBuilder,
    fifo_count: u8,
    enabled: bool,
}

impl Fetcher for ObjectFetcher {
    fn next(&mut self, state: FetcherState) {
        self.state = state
    }

    fn reset(&mut self) {
        self.fifo_count = 0;
        self.state = FetcherState::TileNumber;
    }

    fn pause(&mut self) {
        self.enabled = false;
    }

    fn resume(&mut self) {
        self.enabled = true;
    }

    fn is_enabled(&self) -> bool {
        self.enabled
    }
}

#[derive(Debug, Clone, Copy, Default)]
struct WindowLineCounter {
    count: u8,
    checked: bool,
}

impl WindowLineCounter {
    pub fn checked(&self) -> bool {
        self.checked
    }

    pub fn increment(&mut self) {
        self.count += 1;
        self.checked = true;
    }

    pub fn hblank_reset(&mut self) {
        self.checked = false;
    }

    pub fn vblank_reset(&mut self) {
        self.count = 0;
        self.checked = false;
    }

    pub fn count(&self) -> u8 {
        self.count
    }
}

#[derive(Debug, Clone, Copy)]
pub enum FetcherState {
    TileNumber,
    TileDataLow,
    TileDataHigh,
    SendToFifo,
}

impl Default for FetcherState {
    fn default() -> Self {
        Self::TileNumber
    }
}

#[derive(Debug, Clone, Copy)]
enum FifoPixelKind {
    Background,
    Object,
}

impl Default for FifoPixelKind {
    fn default() -> Self {
        Self::Background
    }
}

#[derive(Debug, Clone, Copy, Default)]
struct BackgroundFifoPixel(GrayShade);

#[derive(Debug, Clone, Copy, Default)]
struct ObjectFifoPixel {
    shade: Option<GrayShade>,
    palette: ObjectPalette,
    priority: RenderPriority,
}

// FIXME: Fifo Registers have a known size. Are heap allocations
// really necessary here?
#[derive(Debug, Clone)]
struct FifoRenderer {
    background: VecDeque<BackgroundFifoPixel>,
    object: VecDeque<ObjectFifoPixel>,
    enabled: bool,
}

impl FifoRenderer {
    pub fn is_enabled(&self) -> bool {
        self.enabled
    }

    pub fn pause(&mut self) {
        self.enabled = false;
    }

    pub fn resume(&mut self) {
        self.enabled = true;
    }
}

impl Default for FifoRenderer {
    fn default() -> Self {
        Self {
            background: VecDeque::with_capacity(8),
            object: VecDeque::with_capacity(8),
            enabled: true,
        }
    }
}

#[derive(Debug, Clone, Copy, Default)]
struct TileBuilder {
    id: Option<u8>,
    low: Option<u8>,
    high: Option<u8>,
}

impl TileBuilder {
    pub fn with_id(&mut self, id: u8) {
        self.id = Some(id);
    }

    pub fn with_low_byte(&mut self, data: u8) {
        self.low = Some(data);
    }

    pub fn with_high_byte(&mut self, data: u8) {
        self.high = Some(data);
    }
}