Initial commit: NES emulator with GTK4 desktop frontend

Full NES emulation: CPU, PPU, APU, 47 mappers, iNES/NES 2.0 parsing.
GTK4 desktop client with HeaderBar, pixel-perfect Cairo rendering,
drag-and-drop ROM loading, and keyboard shortcuts.
187 tests covering core emulation, mappers, and runtime.

src/native_core/bus/state.rs

@@ -0,0 +1,158 @@
+use super::*;
+use crate::native_core::apu::ApuStateTail;
+use crate::native_core::state_io as sio;
+
+const BUS_STATE_CTX: &str = "bus state";
+
+impl NativeBus {
+    pub fn save_state(&self, out: &mut Vec<u8>) {
+        out.extend_from_slice(&self.cpu_ram);
+        self.ppu.save_state(out);
+        out.extend_from_slice(self.apu.registers());
+        out.push(self.cpu_open_bus);
+        out.push(self.joypad_state);
+        out.push(self.joypad_shift);
+        out.push(self.joypad2_state);
+        out.push(self.joypad2_shift);
+        out.push(u8::from(self.joypad_strobe));
+        out.push(u8::from(self.nmi_pending));
+        out.extend_from_slice(&self.ppu_dot.to_le_bytes());
+        out.push(u8::from(self.odd_frame));
+        out.push(u8::from(self.in_vblank));
+        out.push(u8::from(self.mmc3_a12_prev_high));
+        out.extend_from_slice(&self.mmc3_a12_low_dots.to_le_bytes());
+        out.extend_from_slice(&self.mmc3_last_irq_scanline.to_le_bytes());
+        self.apu.save_state_tail(out);
+
+        let mut mapper_state = Vec::new();
+        self.mapper.save_state(&mut mapper_state);
+        out.extend_from_slice(&(mapper_state.len() as u32).to_le_bytes());
+        out.extend_from_slice(&mapper_state);
+    }
+
+    pub fn load_state(&mut self, data: &[u8]) -> Result<(), String> {
+        let mut cursor = 0usize;
+
+        self.cpu_ram.copy_from_slice(sio::take_exact(
+            data,
+            &mut cursor,
+            CPU_RAM_SIZE,
+            BUS_STATE_CTX,
+        )?);
+        let ppu_consumed = self.ppu.load_state(&data[cursor..])?;
+        cursor = cursor.saturating_add(ppu_consumed);
+        let mut apu_regs = [0u8; 0x20];
+        apu_regs.copy_from_slice(sio::take_exact(data, &mut cursor, 0x20, BUS_STATE_CTX)?);
+        self.apu.set_registers(apu_regs);
+        self.frame_complete = false;
+
+        self.cpu_open_bus = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        self.joypad_state = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        self.joypad_shift = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        self.joypad2_state = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        self.joypad2_shift = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        self.joypad_strobe = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        self.nmi_pending = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        self.suppress_vblank_this_frame = false;
+        self.ppu_dot = sio::take_u32(data, &mut cursor, BUS_STATE_CTX)?;
+        self.ppu_dot %= PPU_DOTS_PER_FRAME;
+        self.odd_frame = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        self.in_vblank = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        self.mmc3_a12_prev_high = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        self.mmc3_a12_low_dots = u16::from_le_bytes([
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+        ]);
+        self.mmc3_last_irq_scanline = u32::from_le_bytes([
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+        ]);
+        let frame_cycle = sio::take_u32(data, &mut cursor, BUS_STATE_CTX)?;
+        let frame_mode_5step = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let frame_irq_inhibit = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let frame_irq_pending = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let channel_enable_mask = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let mut length_counters = [0u8; 4];
+        length_counters.copy_from_slice(sio::take_exact(data, &mut cursor, 4, BUS_STATE_CTX)?);
+        let dmc_bytes_remaining = u16::from_le_bytes([
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+        ]);
+        let dmc_irq_enabled = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let dmc_irq_pending = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let dmc_cycle_counter = u16::from_le_bytes([
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+        ]);
+        let dmc_current_addr = u16::from_le_bytes([
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+            sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?,
+        ]);
+        let dmc_sample_buffer = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let dmc_sample_buffer_valid = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let dmc_shift_reg = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let dmc_bits_remaining = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let dmc_silence = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let dmc_output_level = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let dmc_dma_request = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let mut envelope_divider = [0u8; 3];
+        envelope_divider.copy_from_slice(sio::take_exact(data, &mut cursor, 3, BUS_STATE_CTX)?);
+        let mut envelope_decay = [0u8; 3];
+        envelope_decay.copy_from_slice(sio::take_exact(data, &mut cursor, 3, BUS_STATE_CTX)?);
+        let envelope_start_flags = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let triangle_linear_counter = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let triangle_linear_reload_flag = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let mut sweep_divider = [0u8; 2];
+        sweep_divider.copy_from_slice(sio::take_exact(data, &mut cursor, 2, BUS_STATE_CTX)?);
+        let sweep_reload_flags = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let cpu_cycle_parity = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let frame_reset_pending = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let frame_reset_delay = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)?;
+        let pending_frame_mode_5step = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        let pending_frame_irq_inhibit = sio::take_u8(data, &mut cursor, BUS_STATE_CTX)? != 0;
+        self.apu.load_state_tail(ApuStateTail {
+            frame_cycle,
+            frame_mode_5step,
+            frame_irq_inhibit,
+            frame_irq_pending,
+            channel_enable_mask,
+            length_counters,
+            dmc_bytes_remaining,
+            dmc_irq_enabled,
+            dmc_irq_pending,
+            dmc_cycle_counter,
+            dmc_current_addr,
+            dmc_sample_buffer,
+            dmc_sample_buffer_valid,
+            dmc_shift_reg,
+            dmc_bits_remaining,
+            dmc_silence,
+            dmc_output_level,
+            dmc_dma_request,
+            envelope_divider,
+            envelope_decay,
+            envelope_start_flags,
+            triangle_linear_counter,
+            triangle_linear_reload_flag,
+            sweep_divider,
+            sweep_reload_flags,
+            cpu_cycle_parity,
+            frame_reset_pending,
+            frame_reset_delay,
+            pending_frame_mode_5step,
+            pending_frame_irq_inhibit,
+        });
+        let mapper_len = sio::take_u32(data, &mut cursor, BUS_STATE_CTX)? as usize;
+        let mapper_state = sio::take_exact(data, &mut cursor, mapper_len, BUS_STATE_CTX)?;
+
+        self.ppu.set_vblank(self.in_vblank);
+        self.mapper.load_state(mapper_state)?;
+
+        if cursor != data.len() {
+            return Err("bus state: trailing bytes in payload".to_string());
+        }
+        Ok(())
+    }
+}