rng-pack 0.2.5

use crate::rng32::Rng32;
use std::slice::from_raw_parts_mut;

/// A 32-bit Mersenne Twister (MT19937) random number generator.
///
/// # Examples
///
/// ```
/// use rng_pack::mt19937::Mt19937;
///
/// let mut rng = Mt19937::new(12345, 1024);
/// let val = rng.nextu();
/// ```
#[repr(C)]
pub struct Mt19937 {
    mt: [u32; N],
    mti: usize,
}

const N: usize = 624;
const M: usize = 397;
const MATRIX_A: u32 = 0x9908B0DF;
const UPPER_MASK: u32 = 0x80000000;
const LOWER_MASK: u32 = 0x7FFFFFFF;

impl Mt19937 {
    /// Creates a new `Mt19937` instance.
    ///
    /// # Arguments
    ///
    /// * `seed` - The initial seed value.
    /// * `warm` - The number of initial iterations to skip (warm-up).
    pub fn new(seed: u32, warm: usize) -> Self {
        let mut mt = [0u32; N];
        mt[0] = seed;
        for i in 1..N {
            let prev = mt[i - 1];
            mt[i] = (1812433253u32)
                .wrapping_mul(prev ^ (prev >> 30))
                .wrapping_add(i as u32);
        }
        let mut rng = Self { mt, mti: N };
        (0..warm).into_iter().for_each(|_| {
            let _ = rng.nextu();
        });
        rng
    }

    /// Generates the next random `u32` value.
    ///
    /// # Examples
    ///
    /// ```
    /// use rng_pack::mt19937::Mt19937;
    ///
    /// let mut rng = Mt19937::new(12345, 1024);
    /// let val = rng.nextu();
    /// ```
    #[inline]
    pub fn nextu(&mut self) -> u32 {
        if self.mti >= N {
            self.twist();
        }

        let mut y = self.mt[self.mti];
        self.mti += 1;

        y ^= y >> 11;
        y ^= (y << 7) & 0x9D2C5680;
        y ^= (y << 15) & 0xEFC60000;
        y ^= y >> 18;

        y
    }

    fn twist(&mut self) {
        for i in 0..N - M {
            let x = (self.mt[i] & UPPER_MASK) | (self.mt[i + 1] & LOWER_MASK);
            let mut x_a = x >> 1;
            if x & 1 != 0 {
                x_a ^= MATRIX_A;
            }
            self.mt[i] = self.mt[i + M] ^ x_a;
        }

        for i in N - M..N - 1 {
            let x = (self.mt[i] & UPPER_MASK) | (self.mt[i + 1] & LOWER_MASK);
            let mut x_a = x >> 1;
            if x & 1 != 0 {
                x_a ^= MATRIX_A;
            }
            self.mt[i] = self.mt[i + M - N] ^ x_a;
        }

        let x = (self.mt[N - 1] & UPPER_MASK) | (self.mt[0] & LOWER_MASK);
        let mut x_a = x >> 1;
        if x & 1 != 0 {
            x_a ^= MATRIX_A;
        }
        self.mt[N - 1] = self.mt[M - 1] ^ x_a;

        self.mti = 0;
    }

    /// Generates the next random `f32` value in the range [0, 1).
    ///
    /// # Examples
    ///
    /// ```
    /// use rng_pack::mt19937::Mt19937;
    ///
    /// let mut rng = Mt19937::new(12345, 1024);
    /// let val = rng.nextf();
    /// assert!(val >= 0.0 && val < 1.0);
    /// ```
    #[inline]
    pub fn nextf(&mut self) -> f32 {
        self.nextu() as f32 * (1.0 / (u32::MAX as f32 + 1.0))
    }

    /// Generates a random `i32` value in the range [min, max].
    ///
    /// # Arguments
    ///
    /// * `min` - The lower bound (inclusive).
    /// * `max` - The upper bound (inclusive).
    ///
    /// # Examples
    ///
    /// ```
    /// use rng_pack::mt19937::Mt19937;
    ///
    /// let mut rng = Mt19937::new(12345, 1024);
    /// let val = rng.randi(1, 10);
    /// assert!(val >= 1 && val <= 10);
    /// ```
    #[inline]
    pub fn randi(&mut self, min: i32, max: i32) -> i32 {
        let range = (max as i64 - min as i64 + 1) as u64;
        ((self.nextu() as u64 * range) >> 32) as i32 + min
    }

    /// Generates a random `f32` value in the range [min, max).
    ///
    /// # Arguments
    ///
    /// * `min` - The lower bound (inclusive).
    /// * `max` - The upper bound (exclusive).
    ///
    /// # Examples
    ///
    /// ```
    /// use rng_pack::mt19937::Mt19937;
    ///
    /// let mut rng = Mt19937::new(12345, 1024);
    /// let val = rng.randf(1.0, 10.0);
    /// assert!(val >= 1.0 && val < 10.0);
    /// ```
    #[inline]
    pub fn randf(&mut self, min: f32, max: f32) -> f32 {
        let range = max - min;
        let scale = range * (1.0 / (u32::MAX as f32 + 1.0));
        (self.nextu() as f32 * scale) + min
    }

    /// Returns a random element from a slice.
    ///
    /// # Arguments
    ///
    /// * `choices` - The slice to choose from.
    ///
    /// # Examples
    ///
    /// ```
    /// use rng_pack::mt19937::Mt19937;
    ///
    /// let mut rng = Mt19937::new(12345, 1024);
    /// let choices = [1, 2, 3, 4, 5];
    /// let val = rng.choice(&choices);
    /// assert!(choices.contains(val));
    /// ```
    #[inline]
    pub fn choice<'a, T>(&mut self, choices: &'a [T]) -> &'a T {
        let index = self.randi(0, choices.len() as i32 - 1);
        &choices[index as usize]
    }
}

impl Rng32 for Mt19937 {
    #[inline]
    fn randi(&mut self, min: i32, max: i32) -> i32 {
        self.randi(min, max)
    }

    #[inline]
    fn randf(&mut self, min: f32, max: f32) -> f32 {
        self.randf(min, max)
    }

    #[inline]
    fn choice<'a, T>(&'a mut self, choices: &'a [T]) -> &'a T {
        self.choice(choices)
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn mt19937_new(seed: u32, warm: usize) -> *mut Mt19937 {
    Box::into_raw(Box::new(Mt19937::new(seed, warm)))
}

#[unsafe(no_mangle)]
pub extern "C" fn mt19937_free(ptr: *mut Mt19937) {
    if !ptr.is_null() {
        unsafe { drop(Box::from_raw(ptr)) };
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn mt19937_next_u32s(ptr: *mut Mt19937, out: *mut u32, count: usize) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        for v in buffer {
            *v = rng.nextu();
        }
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn mt19937_next_f32s(ptr: *mut Mt19937, out: *mut f32, count: usize) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        for v in buffer {
            *v = rng.nextf();
        }
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn mt19937_rand_i32s(
    ptr: *mut Mt19937,
    out: *mut i32,
    count: usize,
    min: i32,
    max: i32,
) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        for v in buffer {
            *v = rng.randi(min, max);
        }
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn mt19937_rand_f32s(
    ptr: *mut Mt19937,
    out: *mut f32,
    count: usize,
    min: f32,
    max: f32,
) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        for v in buffer {
            *v = rng.randf(min, max);
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::mt19937::Mt19937;

    #[test]
    fn it_works() {
        let mut rng = Mt19937::new(1, 1024);

        assert_eq!(rng.nextu(), 244660247);
        assert_eq!(rng.nextf(), 0.2754702);
        assert_eq!(rng.randi(10, 20), 12);
        assert_eq!(rng.randf(10.0, 20.0), 10.516413);
        assert_eq!(*rng.choice(&[0, 1, 2, 3, 4]), 3);
        assert_eq!(*rng.choice(&[1, 2, 3, 4, 5]), 1);
        assert_eq!(*rng.choice(&[2, 3, 4, 5, 6]), 4);
        assert_eq!(*rng.choice(&[3, 4, 5, 6, 7]), 6);
    }
}