soundchip 0.4.61

Software sinth with configurable channels for authentic sounding virtual sound chips.
Documentation 
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//! A simple, "old school" LFSR with configurable bit count.

#[derive(Debug)]
pub struct Rng {
    state: u32,
    mask: u32,
    tap: u32,
    f32_max: f32,
}

const DEFAULT_VALUE: u32 = 0b01100010101011110110101010101011;

impl Rng {
    /// Creates a new LFSR with `n` bits and an initial state.
    pub fn new(bit_count: u32, initial_state: u32) -> Self {
        let bit_count = bit_count.clamp(3, 32);
        let mask = ((1u64 << bit_count) - 1) as u32;
        let state = if (initial_state & mask) == 0 {
            DEFAULT_VALUE & mask
        } else {
            initial_state & mask
        };
        Self {
            state,
            mask,
            tap: get_tap(bit_count),
            f32_max: libm::powf(2.0, bit_count as f32 ),
        }
    }

    /// Next random u32 value in the sequence
    pub fn next_u32(&mut self) -> u32 {
        let lsb = self.state & 1; // Store least significant bit
        self.state = xor_with_tap(self.state >> 1, self.tap, lsb);
        self.state & self.mask
    }

    /// Converts next random u32 value to range (0.0 .. 1.0)
    pub fn next_f32(&mut self) -> f32 {
        self.next_u32() as f32 / self.f32_max
    }
}

#[inline(always)]
//  Bit can only be 0 or 1, or will overflow!
fn xor_with_tap(value: u32, tap: u32, bit: u32) -> u32 {
    value ^ (tap & (bit * 0xFFFFFFFF))
}

fn get_tap(bit_count: u32) -> u32 {
    // Maximal-length tap configurations.
    match bit_count {
        3 => 0b110,
        4 => 0b1100,
        5 => 0b10100,
        6 => 0b110000,
        7 => 0b1100000,
        8 => 0b10111000,
        9 => 0b100010000,
        10 => 0b1001000000,
        11 => 0b10100000000,
        12 => 0b111000001000,
        13 => 0b1110010000000,
        14 => 0b11100000000010,
        15 => 0b110000000000000,
        16 => 0b1101000000001000,
        17 => 0b10010000000000000,
        18 => 0b100000010000000000,
        19 => 0b1110010000000000000,
        20 => 0b10010000000000000000,
        21 => 0b101000000000000000000,
        22 => 0b1100000000000000000000,
        23 => 0b10000100000000000000000,
        24 => 0b111000010000000000000000,
        25 => 0b1000100000000000000000000,
        26 => 0b10010000000000000000000000,
        27 => 0b101000000000000000000000000,
        28 => 0b1011100000000000000000000000,
        29 => 0b11000000000000000000000000000,
        30 => 0b110010000000000000000000000000,
        31 => 0b1101000000000000000000000000000,
        32 => 0b11100000000000000000000000000000,
        _ => 0b101101,
    }
}