nesfuzz/src/apu/mod.rs

mod dmc;
mod envelope;
mod noise;
pub mod serialize;
mod square;
mod triangle;

use dmc::DMC;
use noise::Noise;
use square::Square;
use triangle::Triangle;

// APU clock ticks every other CPU cycle.
// Frame counter only ticks every 3728.5 APU ticks, and in audio frames of 4 or 5.
// Length counter controls note durations.

const FRAME_COUNTER_STEPS: [usize; 5] = [3728, 7456, 11185, 14914, 18640];
const LENGTH_COUNTER_TABLE: [u8; 32] = [
    10, 254, 20, 2, 40, 4, 80, 6, 160, 8, 60, 10, 14, 12, 26, 14, 12, 16, 24, 18, 48, 20, 96, 22,
    192, 24, 72, 26, 16, 28, 32, 30,
];

#[derive(serde::Serialize, serde::Deserialize, Clone)]
pub struct Apu {
    square1: Square,
    square2: Square,
    triangle: Triangle,
    noise: Noise,
    pub dmc: DMC,

    square_table: Vec<f32>,
    tnd_table: Vec<f32>,

    frame_sequence: u8,
    frame_counter: u8,
    interrupt_inhibit: bool,
    frame_interrupt: bool,
    cycle: usize,
    pub trigger_irq: bool,
}

impl Apu {
    pub fn new() -> Self {
        let square_table = (0..31)
            .map(|x| 95.52 / ((8128.0 / x as f32) + 100.0))
            .collect();
        let tnd_table = (0..203)
            .map(|x| 163.67 / ((24329.0 / x as f32) + 100.0))
            .collect();
        Apu {
            square1: Square::new(true),
            square2: Square::new(false),
            triangle: Triangle::new(),
            noise: Noise::new(),
            dmc: DMC::new(),

            square_table: square_table,
            tnd_table: tnd_table,

            frame_sequence: 0,
            frame_counter: 0,
            interrupt_inhibit: false,
            frame_interrupt: false,
            cycle: 0,
            trigger_irq: false,
        }
    }

    pub fn clock(&mut self, sample_byte: u8) -> f32 {
        // Clock each channel
        self.square1.clock();
        self.square2.clock();
        self.triangle.clock();
        self.triangle.clock(); // hacky. clocking triangle twice because it runs every CPU cycle
        self.noise.clock();
        self.dmc.clock(sample_byte);

        // Step frame counter if necessary
        if FRAME_COUNTER_STEPS.contains(&self.cycle) {
            self.clock_frame_counter();
        }
        self.cycle += 1;
        if (self.frame_sequence == 4 && self.cycle == 14915) || self.cycle == 18641 {
            self.cycle = 0;
        }

        // Send all samples to buffer, let the SDL2 audio callback take what it needs
        self.mix()
    }

    fn mix(&self) -> f32 {
        let square_out = self.square_table[(self.square1.sample + self.square2.sample) as usize];
        let tnd_out = self.tnd_table
            [((3 * self.triangle.sample) + (2 * self.noise.sample) + self.dmc.sample) as usize];
        square_out + tnd_out
    }

    pub fn write_reg(&mut self, address: usize, value: u8) {
        match address {
            0x4000 => self.square1.write_duty(value),
            0x4001 => self.square1.write_sweep(value),
            0x4002 => self.square1.write_timer_low(value),
            0x4003 => self.square1.write_timer_high(value),
            0x4004 => self.square2.write_duty(value),
            0x4005 => self.square2.write_sweep(value),
            0x4006 => self.square2.write_timer_low(value),
            0x4007 => self.square2.write_timer_high(value),
            0x4008 => self.triangle.write_counter(value),
            0x4009 => (),
            0x400A => self.triangle.write_timer_low(value),
            0x400B => self.triangle.write_timer_high(value),
            0x400C => self.noise.write_envelope(value),
            0x400D => (),
            0x400E => self.noise.write_loop_noise(value),
            0x400F => self.noise.write_length_counter(value),
            0x4010 => self.dmc.write_control(value),
            0x4011 => self.dmc.direct_load(value),
            0x4012 => self.dmc.write_sample_address(value),
            0x4013 => self.dmc.write_sample_length(value),
            0x4014 => (),
            0x4015 => self.write_control(value),
            0x4016 => (),
            0x4017 => self.write_frame_counter(value),
            _ => panic!("bad address written: 0x{:X}", address),
        }
    }

    //   mode 0:    mode 1:       function
    // ---------  -----------  -----------------------------
    // - - - f    - - - - -    IRQ (if bit 6 is clear)
    // - l - l    - l - - l    Length counter and sweep
    // e e e e    e e e - e    Envelope and linear counter
    fn clock_frame_counter(&mut self) {
        if !(self.frame_sequence == 5 && self.frame_counter == 3) {
            // step envelopes
            self.square1.envelope.clock();
            self.square2.envelope.clock();
            self.triangle.clock_linear_counter();
            self.noise.envelope.clock();
        }
        if (self.frame_counter == 1)
            || (self.frame_sequence == 4 && self.frame_counter == 3)
            || (self.frame_sequence == 5 && self.frame_counter == 4)
        {
            // step length counters and sweep units
            self.square1.clock_sweep();
            self.square2.clock_sweep();
            self.square1.clock_length_counter();
            self.square2.clock_length_counter();
            self.triangle.clock_length_counter();
            self.noise.clock_length_counter();
        }
        if self.frame_sequence == 4 && self.frame_counter == 3 && !self.interrupt_inhibit {
            self.trigger_irq = true;
        }
        // advance counter
        self.frame_counter = self.frame_counter.wrapping_add(1);
        if self.frame_counter == self.frame_sequence {
            self.frame_counter = 0;
        }
    }

    // CPU writes to $4015
    fn write_control(&mut self, value: u8) {
        // Writing to this register clears the DMC interrupt flag.
        self.dmc.interrupt = false;
        // Writing a zero to any of the channel enable bits will silence that channel and immediately set its length counter to 0.
        if value & (1 << 0) != 0 {
            self.square1.enabled = true;
        } else {
            self.square1.enabled = false;
            self.square1.length_counter = 0;
        }
        if value & (1 << 1) != 0 {
            self.square2.enabled = true;
        } else {
            self.square2.enabled = false;
            self.square2.length_counter = 0;
        }
        if value & (1 << 2) != 0 {
            self.triangle.enabled = true;
        } else {
            self.triangle.enabled = false;
            self.triangle.length_counter = 0;
        }
        if value & (1 << 3) != 0 {
            self.noise.enabled = true;
        } else {
            self.noise.enabled = false;
            self.noise.length_counter = 0;
        }
        if value & (1 << 4) != 0 {
            self.dmc.enabled = true;
            // If the DMC bit is set, the DMC sample will be restarted only if its bytes remaining is 0.
            // If there are bits remaining in the 1-byte sample buffer, these will finish playing before the next sample is fetched.
            if self.dmc.bytes_remaining == 0 {
                self.dmc.current_address = self.dmc.sample_address;
                self.dmc.bytes_remaining = self.dmc.sample_length;
            }
        } else {
            self.dmc.enabled = false;
            // If the DMC bit is clear, the DMC bytes remaining will be set to 0 and the DMC will silence when it empties.
            self.dmc.bytes_remaining = 0;
        }
    }

    // CPU reads from $4015
    pub fn read_status(&mut self) -> u8 {
        // IF-D NT21: 	DMC interrupt (I), frame interrupt (F), DMC active (D), length counter > 0 (N/T/2/1)
        let mut val = 0;
        // N/T/2/1 will read as 1 if the corresponding length counter is greater than 0. For the triangle channel, the status of the linear counter is irrelevant.
        if self.square1.length_counter != 0 {
            val |= 1 << 0;
        }
        if self.square2.length_counter != 0 {
            val |= 1 << 1;
        }
        if self.triangle.length_counter != 0 {
            val |= 1 << 2;
        }
        if self.noise.length_counter != 0 {
            val |= 1 << 3;
        }
        // D will read as 1 if the DMC bytes remaining is more than 0.
        if self.dmc.bytes_remaining != 0 {
            val |= 1 << 4;
        }
        if self.frame_interrupt {
            val |= 1 << 6;
        }
        if self.dmc.interrupt {
            val |= 1 << 7;
        }
        // Reading this register clears the frame interrupt flag (but not the DMC interrupt flag).
        self.frame_interrupt = false;
        // TODO: If an interrupt flag was set at the same moment of the read, it will read back as 1 but it will not be cleared.
        val
    }

    // $4017
    fn write_frame_counter(&mut self, value: u8) {
        // 0 selects 4-step sequence, 1 selects 5-step sequence
        self.frame_sequence = if value & (1 << 7) == 0 { 4 } else { 5 };
        // If set, the frame interrupt flag is cleared, otherwise it is unaffected.
        if value & (1 << 6) != 0 {
            self.interrupt_inhibit = false;
        }
        // If the mode flag is set, then both "quarter frame" and "half frame" signals are also generated.
        if self.frame_sequence == 5 {
            // Clock envelopes, length counters, and sweep units
            self.square1.envelope.clock();
            self.square1.clock_sweep();
            self.square1.clock_length_counter();
            self.square2.envelope.clock();
            self.square2.clock_sweep();
            self.square2.clock_length_counter();
            self.triangle.clock_linear_counter();
            self.triangle.clock_length_counter();
            self.noise.envelope.clock();
            self.noise.clock_length_counter();
        }
    }
}