use rand::SeedableRng;
use rand_xoshiro::Xoshiro256StarStar;

/// Convert an RNG state buffer to an instance of [`Xoshiro256StarStar`].
///
/// While [`SeedableRng::from_seed`] is an infallible function,
/// it relies on some methods that can't be compiled by the SPIR-V
/// backend (specifically, formatting functions in the core crate).
///
/// In practice, the xoshiro RNG state is entirely defined by its seed,
/// so this function does the work of [`SeedableRng::from_seed`] by
/// transmuting the seed value to an RNG instance.
///
/// This function assumes a properly-initialized state array;
/// output may silently degenerate if the initial state is all zeros,
/// so this module is private to the crate.
pub(crate) fn xoshiro256starstar_from_seed(
    rng_state: <Xoshiro256StarStar as SeedableRng>::Seed,
) -> Xoshiro256StarStar {
    let mut rng_state_actual = [0u64; 4];

    // NOTE: Bit shifting is tedious, but we don't have great alternatives:
    //  - `chunks_exact` has issues with pointer casting
    //  - `u64::from_le_bytes` has issues with `OpBitcast` in SPIR-V validation
    for i in 0..rng_state_actual.len() {
        for j in 0..size_of::<u64>() {
            rng_state_actual[i] |= (rng_state[i * size_of::<u64>() + j] as u64) << j * 8;
        }
    }

    unsafe { core::mem::transmute(rng_state_actual) }
}

#[cfg(test)]
mod test {
    use crate::rng::xoshiro256starstar_from_seed;
    use core::iter::zip;
    use rand::{RngExt, SeedableRng};
    use rand_xoshiro::Xoshiro256StarStar;

    #[test]
    fn match_seeded() {
        let mut seed: <Xoshiro256StarStar as SeedableRng>::Seed = [0u8; 32];
        for i in 0..seed.len() {
            seed[i] = i as u8;
        }

        let rng1 = Xoshiro256StarStar::from_seed(seed).random_iter::<u64>();
        let rng2 = xoshiro256starstar_from_seed(seed).random_iter::<u64>();

        zip(rng1, rng2)
            .take(100)
            .for_each(|(rng1_value, rng2_value)| assert_eq!(rng1_value, rng2_value));
    }
}