//! Random number generator support

use super::Limb;

use crate::{Encoding, NonZero, Random, RandomMod};

use rand_core::CryptoRngCore;

use subtle::ConstantTimeLess;

impl Random for Limb {

    #[cfg(target_pointer_width = "32")]

    fn random(rng: &mut impl CryptoRngCore) -> Self {

        Self(rng.next_u32())

    }

    #[cfg(target_pointer_width = "64")]

    fn random(rng: &mut impl CryptoRngCore) -> Self {

        Self(rng.next_u64())

    }

}

impl RandomMod for Limb {

    fn random_mod(rng: &mut impl CryptoRngCore, modulus: &NonZero<Self>) -> Self {

        let mut bytes = <Self as Encoding>::Repr::default();

        let n_bits = modulus.bits();

        let n_bytes = (n_bits + 7) / 8;

        let mask = 0xff >> (8 * n_bytes - n_bits);

        loop {

            rng.fill_bytes(&mut bytes[..n_bytes]);

            bytes[n_bytes - 1] &= mask;

            let n = Limb::from_le_bytes(bytes);

            if n.ct_lt(modulus).into() {

                return n;

            }

        }

    }

}