use std::slice::from_raw_parts_mut;

/// A Philox 4x32 random number generator.
///
/// This is a counter-based RNG suitable for parallel applications.
///
/// # Examples
///
/// ```
/// use rng_pack::philox32::Philox32;
///
/// let mut rng = Philox32::new([123, 456]);
/// ```
#[repr(C)]
pub struct Philox32 {
    c: [u32; 4],
    k: [u32; 2],
}

impl Philox32 {
    const fn chunk_size() -> usize {
        4
    }

    const fn m0() -> u32 {
        0xD2511F53
    }

    const fn m1() -> u32 {
        0xCD9E8D57
    }

    /// Creates a new `Philox32` instance.
    pub fn new(seed: [u32; 2]) -> Self {
        Self {
            c: [1, 0, 0, 0],
            k: seed,
        }
    }

    /// Advances the generator state by `count` steps.
    pub fn warm(&mut self, count: usize) {
        for _i in 0..count {
            let _ = self.nextu();
        }
    }

    /// Generates the next block of random numbers.
    ///
    /// # Examples
    ///
    /// ```
    /// use rng_pack::philox32::Philox32;
    ///
    /// let mut rng = Philox32::new([123, 456]);
    /// let val = rng.nextu();
    /// ```
    #[inline]
    pub fn nextu(&mut self) -> [u32; 4] {
        let mut out = [0u32; 4];

        let p0 = self.c[0].wrapping_mul(Self::m0());
        let p1 = self.c[2].wrapping_mul(Self::m1());
        out[0] = p0 ^ self.c[1] ^ self.k[0];
        out[1] = p0;
        out[2] = p1 ^ self.c[3] ^ self.k[1];
        out[3] = p1;

        self.c[0] = self.c[0].wrapping_add(1);
        self.c[1] = self.c[1].wrapping_add(1);
        self.c[2] = self.c[2].wrapping_add(1);
        self.c[3] = self.c[3].wrapping_add(1);

        out
    }

    /// Generates the next random `f32` value in the range [0, 1).
    ///
    /// # Examples
    ///
    /// ```
    /// use rng_pack::philox32::Philox32;
    ///
    /// let mut rng = Philox32::new([123, 456]);
    /// let val = rng.nextf();
    /// assert!(val >= 0.0 && val < 1.0);
    /// ```
    #[inline]
    pub fn nextf(&mut self) -> f32 {
        self.nextu()[0] 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::philox32::Philox32;
    ///
    /// let mut rng = Philox32::new([123, 456]);
    /// 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;
        let x = self.nextu()[0];
        ((x 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::philox32::Philox32;
    ///
    /// let mut rng = Philox32::new([123, 456]);
    /// 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()[0] as f32 * scale) + min
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn philox32_new(seed1: u32, seed2: u32) -> *mut Philox32 {
    Box::into_raw(Box::new(Philox32::new([seed1, seed2])))
}

#[unsafe(no_mangle)]
pub extern "C" fn philox32_warm(ptr: *mut Philox32, count: usize) {
    unsafe {
        let rng = &mut *ptr;
        rng.warm(count);
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn philox32_free(ptr: *mut Philox32) {
    if !ptr.is_null() {
        unsafe { drop(Box::from_raw(ptr)) }
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn philox32_next_u32s(ptr: *mut Philox32, out: *mut u32, count: usize) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        let mut i = 0;
        while i < count {
            let chunk = rng.nextu();
            let take = (count - i).min(Philox32::chunk_size());
            buffer[i..i + take].copy_from_slice(&chunk[..take]);
            i += take;
        }
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn philox32_next_f32s(ptr: *mut Philox32, out: *mut f32, count: usize) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        let mut i = 0;
        while i < count {
            let chunk = rng.nextu();
            let take = (count - i).min(Philox32::chunk_size());
            for j in 0..take {
                buffer[i + j] = chunk[j] as f32 * (1.0 / (u32::MAX as f32 + 1.0));
            }
            i += take;
        }
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn philox32_rand_i32s(
    ptr: *mut Philox32,
    out: *mut i32,
    count: usize,
    min: i32,
    max: i32,
) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        let mut i = 0;
        while i < count {
            let chunk = rng.nextu();
            let take = (count - i).min(Philox32::chunk_size());
            for j in 0..take {
                let range = (max as i64 - min as i64 + 1) as u64;
                buffer[i + j] = ((chunk[j] as u64 * range) >> 32) as i32 + min;
            }
            i += take;
        }
    }
}

#[unsafe(no_mangle)]
pub extern "C" fn philox32_rand_f32s(
    ptr: *mut Philox32,
    out: *mut f32,
    count: usize,
    min: f32,
    max: f32,
) {
    unsafe {
        let rng = &mut *ptr;
        let buffer = from_raw_parts_mut(out, count);
        let mut i = 0;
        while i < count {
            let chunk = rng.nextu();
            let take = (count - i).min(Philox32::chunk_size());
            for j in 0..take {
                let scale = (max - min) * (1.0 / (u32::MAX as f32 + 1.0));
                buffer[i + j] = (chunk[j] as f32 * scale) + min;
            }
            i += take;
        }
    }
}

#[cfg(test)]
mod tests {
    use super::Philox32;

    #[test]
    fn test_philox32() {
        let rng = &mut Philox32::new([1, 2]);
        assert_eq!(rng.nextu(), [3528531794, 3528531795, 2, 0]);
    }

    #[test]
    fn test_philox32_methods() {
        let mut rng = Philox32::new([1, 2]);
        let _ = rng.nextu();
        let f = rng.nextf();
        assert!(f >= 0.0 && f < 1.0);
        let i = rng.randi(10, 20);
        assert!(i >= 10 && i <= 20);
        let rf = rng.randf(10.0, 20.0);
        assert!(rf >= 10.0 && rf < 20.0);
    }
}
