neser 1.2.0

/// Manages background rendering including shift registers, tile fetching, and pixel composition
pub struct Background {
    /// Background pattern shift register - low bit plane (16 bits)
    bg_pattern_shift_lo: u16,
    /// Background pattern shift register - high bit plane (16 bits)
    bg_pattern_shift_hi: u16,
    /// Background attribute shift register - low bit (16 bits)
    bg_attribute_shift_lo: u16,
    /// Background attribute shift register - high bit (16 bits)
    bg_attribute_shift_hi: u16,
    /// Nametable byte latch (tile index)
    nametable_latch: u8,
    /// Attribute table byte latch (palette selection)
    attribute_latch: u8,
    /// Pattern table low byte latch
    pattern_lo_latch: u8,
    /// Pattern table high byte latch
    pattern_hi_latch: u8,
    /// Latched tile address computed at nametable-byte fetch time.
    /// This captures the pattern table base + tile index + fine Y so that
    /// mid-tile $2000 writes do not affect the current tile's pattern fetch.
    tile_addr: u16,
}

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

impl Background {
    /// Create a new Background instance
    pub fn new() -> Self {
        Self {
            bg_pattern_shift_lo: 0,
            bg_pattern_shift_hi: 0,
            bg_attribute_shift_lo: 0,
            bg_attribute_shift_hi: 0,
            nametable_latch: 0,
            attribute_latch: 0,
            pattern_lo_latch: 0,
            pattern_hi_latch: 0,
            tile_addr: 0,
        }
    }

    /// Reset background state
    pub fn reset(&mut self) {
        self.bg_pattern_shift_lo = 0;
        self.bg_pattern_shift_hi = 0;
        self.bg_attribute_shift_lo = 0;
        self.bg_attribute_shift_hi = 0;
        self.nametable_latch = 0;
        self.attribute_latch = 0;
        self.pattern_lo_latch = 0;
        self.pattern_hi_latch = 0;
        self.tile_addr = 0;
    }

    // Debug: Get shift register state
    // pub fn debug_shift_registers(&self) -> (u16, u16) {
    //     (self.bg_pattern_shift_lo, self.bg_pattern_shift_hi)
    // }

    /// Fetch nametable byte from memory and latch the full tile address.
    ///
    /// The tile address is computed here (combining the nametable byte with
    /// the current pattern-table base and fine-Y from `v`) so that a
    /// mid-tile write to $2000 does not change the pattern table for the
    /// tile currently being fetched — matching real hardware and Mesen.
    pub fn fetch_nametable<F>(&mut self, v: u16, pattern_table_base: u16, read_nametable: F)
    where
        F: Fn(u16) -> u8,
    {
        let addr = 0x2000 | (v & 0x0FFF);
        self.nametable_latch = read_nametable(addr);

        // Latch tile address: pattern_table | (tile_index << 4) | fine_y
        let tile_index = self.nametable_latch as u16;
        let fine_y = (v >> 12) & 0x07;
        self.tile_addr = pattern_table_base | (tile_index << 4) | fine_y;
    }

    /// Fetch attribute byte from memory
    pub fn fetch_attribute<F>(&mut self, v: u16, read_nametable: F)
    where
        F: Fn(u16) -> u8,
    {
        let addr = 0x23C0 | (v & 0x0C00) | ((v >> 4) & 0x38) | ((v >> 2) & 0x07);
        self.attribute_latch = read_nametable(addr);
    }

    /// Fetch pattern table low byte from CHR ROM using the latched tile address.
    pub fn fetch_pattern_lo<F>(&mut self, read_chr: F)
    where
        F: Fn(u16) -> u8,
    {
        self.pattern_lo_latch = read_chr(self.tile_addr);
    }

    /// Fetch pattern table high byte from CHR ROM using the latched tile address.
    pub fn fetch_pattern_hi<F>(&mut self, read_chr: F)
    where
        F: Fn(u16) -> u8,
    {
        self.pattern_hi_latch = read_chr(self.tile_addr | 0x08);
    }

    /// Load shift registers from latches
    pub fn load_shift_registers(&mut self, v: u16) {
        // Load pattern data into LOW 8 bits, then shift left to move to HIGH 8 bits
        // The NES hardware loads new data and existing data shifts left
        self.bg_pattern_shift_lo =
            (self.bg_pattern_shift_lo & 0xFF00) | (self.pattern_lo_latch as u16);
        self.bg_pattern_shift_hi =
            (self.bg_pattern_shift_hi & 0xFF00) | (self.pattern_hi_latch as u16);

        // Extract the correct 2-bit palette from the attribute byte
        let coarse_x = v & 0x1F;
        let coarse_y = (v >> 5) & 0x1F;
        let shift = ((coarse_y & 0x02) << 1) | (coarse_x & 0x02);
        let palette = (self.attribute_latch >> shift) & 0x03;

        // Load attribute data into LOW 8 bits (same as pattern data)
        let palette_lo_bits = if (palette & 0x01) != 0 { 0xFF } else { 0x00 };
        let palette_hi_bits = if (palette & 0x02) != 0 { 0xFF } else { 0x00 };

        self.bg_attribute_shift_lo =
            (self.bg_attribute_shift_lo & 0xFF00) | (palette_lo_bits as u16);
        self.bg_attribute_shift_hi =
            (self.bg_attribute_shift_hi & 0xFF00) | (palette_hi_bits as u16);
    }

    /// Shift all background rendering shift registers left by 1
    pub fn shift_registers(&mut self) {
        self.bg_pattern_shift_lo <<= 1;
        self.bg_pattern_shift_hi <<= 1;
        self.bg_attribute_shift_lo <<= 1;
        self.bg_attribute_shift_hi <<= 1;
    }

    /// Get the current background pixel value
    /// Returns a palette index (0-15) where 0 means transparent
    pub fn get_pixel(&self, fine_x: u8) -> u8 {
        // Read from HIGH 8 bits (bits 15-8), adjusted by fine_x
        // After loading into LOW byte and shifting left, data ends up in HIGH byte for reading

        let bit_position = 15 - fine_x;

        // Extract pattern bits from HIGH 8 bits of shift registers
        let pattern_lo_bit = ((self.bg_pattern_shift_lo >> bit_position) & 0x01) as u8;
        let pattern_hi_bit = ((self.bg_pattern_shift_hi >> bit_position) & 0x01) as u8;
        let pattern = (pattern_hi_bit << 1) | pattern_lo_bit;

        // If pattern is 0, pixel is transparent
        if pattern == 0 {
            return 0;
        }

        // Extract attribute bits
        let attr_lo_bit = ((self.bg_attribute_shift_lo >> bit_position) & 0x01) as u8;
        let attr_hi_bit = ((self.bg_attribute_shift_hi >> bit_position) & 0x01) as u8;
        let palette = (attr_hi_bit << 1) | attr_lo_bit;

        // Combine: palette * 4 + pattern
        palette * 4 + pattern
    }
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct BackgroundState {
    pub bg_pattern_shift_lo: u16,
    pub bg_pattern_shift_hi: u16,
    pub bg_attribute_shift_lo: u16,
    pub bg_attribute_shift_hi: u16,
    pub nametable_latch: u8,
    pub attribute_latch: u8,
    pub pattern_lo_latch: u8,
    pub pattern_hi_latch: u8,
}

impl Background {
    pub fn capture_state(&self) -> BackgroundState {
        BackgroundState {
            bg_pattern_shift_lo: self.bg_pattern_shift_lo,
            bg_pattern_shift_hi: self.bg_pattern_shift_hi,
            bg_attribute_shift_lo: self.bg_attribute_shift_lo,
            bg_attribute_shift_hi: self.bg_attribute_shift_hi,
            nametable_latch: self.nametable_latch,
            attribute_latch: self.attribute_latch,
            pattern_lo_latch: self.pattern_lo_latch,
            pattern_hi_latch: self.pattern_hi_latch,
        }
    }

    pub fn restore_state(&mut self, state: &BackgroundState) {
        self.bg_pattern_shift_lo = state.bg_pattern_shift_lo;
        self.bg_pattern_shift_hi = state.bg_pattern_shift_hi;
        self.bg_attribute_shift_lo = state.bg_attribute_shift_lo;
        self.bg_attribute_shift_hi = state.bg_attribute_shift_hi;
        self.nametable_latch = state.nametable_latch;
        self.attribute_latch = state.attribute_latch;
        self.pattern_lo_latch = state.pattern_lo_latch;
        self.pattern_hi_latch = state.pattern_hi_latch;
    }
}

#[cfg(test)]
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct BackgroundDebugState {
    pub bg_pattern_shift_lo: u16,
    pub bg_pattern_shift_hi: u16,
    pub bg_attribute_shift_lo: u16,
    pub bg_attribute_shift_hi: u16,
    pub nametable_latch: u8,
    pub attribute_latch: u8,
    pub pattern_lo_latch: u8,
    pub pattern_hi_latch: u8,
}

#[cfg(test)]
impl Background {
    pub fn debug_state(&self) -> BackgroundDebugState {
        BackgroundDebugState {
            bg_pattern_shift_lo: self.bg_pattern_shift_lo,
            bg_pattern_shift_hi: self.bg_pattern_shift_hi,
            bg_attribute_shift_lo: self.bg_attribute_shift_lo,
            bg_attribute_shift_hi: self.bg_attribute_shift_hi,
            nametable_latch: self.nametable_latch,
            attribute_latch: self.attribute_latch,
            pattern_lo_latch: self.pattern_lo_latch,
            pattern_hi_latch: self.pattern_hi_latch,
        }
    }

    pub fn set_debug_state(&mut self, state: BackgroundDebugState) {
        self.bg_pattern_shift_lo = state.bg_pattern_shift_lo;
        self.bg_pattern_shift_hi = state.bg_pattern_shift_hi;
        self.bg_attribute_shift_lo = state.bg_attribute_shift_lo;
        self.bg_attribute_shift_hi = state.bg_attribute_shift_hi;
        self.nametable_latch = state.nametable_latch;
        self.attribute_latch = state.attribute_latch;
        self.pattern_lo_latch = state.pattern_lo_latch;
        self.pattern_hi_latch = state.pattern_hi_latch;
    }
}

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

    #[test]
    fn test_background_new() {
        let bg = Background::new();
        assert_eq!(bg.get_pixel(0), 0);
    }

    #[test]
    fn test_background_reset() {
        let mut bg = Background::new();
        bg.nametable_latch = 42;
        bg.reset();
        assert_eq!(bg.nametable_latch, 0);
    }

    #[test]
    fn test_fetch_nametable() {
        let mut bg = Background::new();
        // v=0x2000: fine_y = (0x2000 >> 12) & 7 = 2
        bg.fetch_nametable(0x2000, 0x0000, |_| 0x42);
        assert_eq!(bg.nametable_latch, 0x42);
        // tile_addr = pattern_table(0) | (0x42 << 4) | fine_y(2) = 0x0422
        assert_eq!(bg.tile_addr, 0x0422);
    }

    #[test]
    fn test_shift_registers() {
        let mut bg = Background::new();
        bg.bg_pattern_shift_lo = 0x8000;
        bg.shift_registers();
        assert_eq!(bg.bg_pattern_shift_lo, 0);
    }

    #[test]
    fn test_load_shift_registers() {
        let mut bg = Background::new();
        bg.pattern_lo_latch = 0xFF;
        bg.pattern_hi_latch = 0xFF;
        bg.attribute_latch = 0x03;
        bg.load_shift_registers(0);
        assert_eq!(bg.bg_pattern_shift_lo & 0xFF, 0xFF);
        assert_eq!(bg.bg_pattern_shift_hi & 0xFF, 0xFF);
    }

    #[test]
    fn test_get_pixel_transparent() {
        let bg = Background::new();
        assert_eq!(bg.get_pixel(0), 0);
    }

    #[test]
    fn test_get_pixel_with_pattern() {
        let mut bg = Background::new();
        bg.bg_pattern_shift_lo = 0x8000;
        bg.bg_pattern_shift_hi = 0x8000;
        let pixel = bg.get_pixel(0);
        assert_eq!(pixel, 3); // Pattern 3, palette 0
    }
}