nesfuzz/src/apu/square.rs

const DUTY_CYCLE_SEQUENCES: [[u8; 8]; 4] = [
    [0, 1, 0, 0, 0, 0, 0, 0],
    [0, 1, 1, 0, 0, 0, 0, 0],
    [0, 1, 1, 1, 1, 0, 0, 0],
    [1, 0, 0, 1, 1, 1, 1, 1],
];

pub struct Square {
    pub sample: u16,
    duty_cycle: [u8; 8],
    duty_counter: u8,
    length_counter_halt: bool, // (this bit is also the envelope's loop flag)
    constant_volume_flag: bool, // (0: use volume from envelope; 1: use constant volume)
    timer: u16,
    pub length_counter: u8,
    envelope: u8,
    sweep: u8,
    pub enabled: bool,
    decay_counter: u8,
    start: bool,
    divider: u8,
}

impl Square {
    pub fn new() -> Self {
        Square {
            duty_cycle: DUTY_CYCLE_SEQUENCES[0],
            duty_counter: 0,
            length_counter_halt: false,
            constant_volume_flag: false,
            timer: 0,
            length_counter: 0,
            envelope: 0,
            sweep: 0,
            sample: 0,
            enabled: false,
            decay_counter: 0,
            start: false,
            divider: 0,
        }
    }

    pub fn clock(&mut self) {

    }

    pub fn clock_frame_counter(&mut self) {

    }

    pub fn clock_envelope(&mut self) {
        // When clocked by the frame counter, one of two actions occurs:
        // if the start flag is clear, the divider is clocked,
        if !self.start {
            self.clock_divider();
        } else {
            self.start = false; // otherwise the start flag is cleared,
            self.decay_counter = 15; // the decay level counter is loaded with 15,
            self.divider = self.envelope; // and the divider's period is immediately reloaded
        }
    }
    
    pub fn clock_length_counter(&mut self) {
        
    }

    fn clock_divider(&mut self) {
        // When the divider is clocked while at 0, it is loaded with V and clocks the decay level counter.
        if self.divider == 0 {
            self.divider = self.envelope;
            // Then one of two actions occurs: If the counter is non-zero, it is decremented,
            if self.decay_counter != 0 {
                self.decay_counter -= 1;
            } else if self.length_counter_halt {
                // otherwise if the loop flag is set, the decay level counter is loaded with 15. 
                self.decay_counter = 15;
            }
        } else {
            self.divider -= 1;
        }
    }

    // $4000/$4004
    pub fn duty(&mut self, value: u8) {
        self.duty_cycle = DUTY_CYCLE_SEQUENCES[(value >> 6) as usize];
        self.length_counter_halt = value & (1<<5) != 0;
        self.constant_volume_flag = value & (1<<4) != 0;
        if self.constant_volume_flag {
            self.envelope = value & 0b1111;
        } else {
            self.envelope = self.decay_counter;
        }
    }

    // $4001/$4005
    pub fn sweep(&mut self, value: u8) {
        
    }

    // $4002/$4006
    pub fn timer_low(&mut self, value: u8) {

    }

    // $4003/$4007
    pub fn timer_high(&mut self, value: u8) {
        
    }
}

struct EnvelopeGenerator {

}

struct SweepUnit {

}

struct Timer {

}

struct Sequencer {

}

struct LengthCounter {

}