use {
    rand::Rng,
    std::{
        num::NonZero,
        ops::{Bound, RangeBounds},
    },
};

/// Compatibility uniform range `[0, limit)` sampler for `u64` numbers
///
/// Sampler uses provided `rand::Rng` reference to generate random `u64` numbers and
/// map them to desired range maintaining uniform distribution of generated numbers.
///
/// This utility exists only to provide compatibility of sampling algorithm with `rand`
/// library at versions <=0.8.5, since parts of the system rely on reproducible sequence
/// of numbers given stable seeded random number generator.
///
/// Two sampling algorithms are supported (they initialize internal `zone` value in different ways)
/// to provide compatibility with two ways `rand` sampling can be performed:
/// - `new_like_instance_sample`: reproduces values obtained from sampler instance created with
///   `rand::distributions::uniform::UniformSampler::new` and then used by calling `sample`
/// - `new_like_trait_sample`: reproduces values obtained from trait function calls
///   `rand::distributions::uniform::UniformSampler::sample_single`
#[derive(Debug)]
pub struct UniformU64Sampler {
    range_end: NonZero<u64>,
    zone: u64,
}

impl UniformU64Sampler {
    /// Create sampler reproducing `sample` calls on `UniformInt` instance
    ///
    /// The `zone` internal threshold is obtained by calculating modulo of `u64::MAX`'s difference
    /// with provided range's end to itself. See:
    /// https://github.com/rust-random/rand/blob/937320cbfeebd4352a23086d9c6e68f067f74644/src/distributions/uniform.rs#L458-L504
    #[allow(clippy::arithmetic_side_effects)]
    pub fn new_like_instance_sample(range_end: NonZero<u64>) -> Self {
        let ints_to_reject = (u64::MAX - range_end.get() + 1) % range_end.get();
        let zone = u64::MAX - ints_to_reject;
        Self { range_end, zone }
    }

    /// Create sampler reproducing direct `sample_single` calls on `UniformInt` trait
    ///
    /// The `zone` internal threshold is obtained by calculating 2^{number of leading zeros in provided range}. See
    /// https://github.com/rust-random/rand/blob/937320cbfeebd4352a23086d9c6e68f067f74644/src/distributions/uniform.rs#L534-L553
    pub fn new_like_trait_sample(range_end: NonZero<u64>) -> Self {
        let zone = (range_end.get() << range_end.leading_zeros()).wrapping_sub(1);
        Self { range_end, zone }
    }

    /// Obtain random number from `rng` and map it to the initialized range of this sampler
    pub fn sample(&self, rng: &mut impl Rng) -> u64 {
        loop {
            let (hi, lo) = Self::wmul(rng.random(), self.range_end);
            if lo <= self.zone {
                return hi;
            }
        }
    }

    #[allow(clippy::arithmetic_side_effects)]
    fn wmul(x: u64, y: NonZero<u64>) -> (u64, u64) {
        let tmp = (x as u128) * (y.get() as u128);
        ((tmp >> 64) as u64, tmp as u64)
    }
}

/// Sample a random number in `range` using `rng` as generator for random `u64` numbers
///
/// This utility exists only to provide compatibility of sampling algorithm with `rand`
/// library at versions <=0.8.5, it is equivalent to its `rng.gen_range(range)`.
///
/// Panics: when `range` is empty.
pub fn random_u64_range(rng: &mut impl Rng, range: impl RangeBounds<u64>) -> u64 {
    let start = match range.start_bound() {
        Bound::Unbounded => 0,
        Bound::Included(start) => *start,
        Bound::Excluded(&u64::MAX) => panic!("Cannot generate number in empty range (max..)"),
        Bound::Excluded(start) => start.wrapping_add(1),
    };
    let last = match range.end_bound() {
        Bound::Unbounded | Bound::Included(&u64::MAX) if start == 0 => return rng.random(),
        Bound::Unbounded => u64::MAX,
        Bound::Included(last) => *last,
        Bound::Excluded(0) => panic!("Cannot generate number in empty range (..0)"),
        Bound::Excluded(end) => end.wrapping_sub(1),
    };
    let zero_range_end = last
        .checked_sub(start)
        .expect("Range must not be empty")
        .wrapping_add(1);
    // last - start != u64::MAX after check calculating last above, so +1 won't overflow
    let zero_range_end = NonZero::new(zero_range_end).unwrap();
    let sampler = UniformU64Sampler::new_like_trait_sample(zero_range_end);
    sampler.sample(rng).wrapping_add(start)
}

#[cfg(test)]
mod tests {
    use {
        super::*,
        rand::SeedableRng as _,
        rand_chacha::ChaChaRng,
        sha2::{Digest, Sha256},
        std::{array, ops::Range},
        test_case::test_case,
    };

    const CHACHA_SEED: [u8; 32] = [16; 32];

    #[test]
    fn test_uniform_sample_like_instance_sample_example() {
        let mut rng_compat = ChaChaRng::from_seed(CHACHA_SEED);
        let sampler_compat =
            UniformU64Sampler::new_like_instance_sample(NonZero::new(294_533).unwrap());
        let values: [u64; 10] = array::from_fn(|_| sampler_compat.sample(&mut rng_compat));
        assert_eq!(
            values,
            [
                280405, 7507, 84194, 272634, 52124, 190984, 8676, 230277, 223574, 126007
            ]
        );
    }

    #[test_case(10, "5p3DrG89DLrJotbMuZJUDHMrqcfoQiDQpa3FbNDbnND6")]
    #[test_case(2_729, "J65XfpzkppuxWEyvgVdDmGBsJSex6BuevYuDSRtaDNAK")]
    #[test_case(4_098, "EaRABrcPBw5LLNBmjn5ay5YN5yTPgv3GnQJLbpUarRkJ")]
    #[test_case(504_302_479, "HniJPVe7zir8XxHmtUuxzhPVvWjMcJxVhhj8QSs1PZTC")]
    #[test_case(1_000_346_000_000, "BghMy7yLe6BVzMbc4zNvAB4sz3ZSPYKJS5oLGvXYVzw2")]
    fn test_uniform_sampler_like_instance_sample_compat(range_end: u64, expected_hash: &str) {
        let mut rng_compat = ChaChaRng::from_seed(CHACHA_SEED);
        let sampler_compat =
            UniformU64Sampler::new_like_instance_sample(NonZero::new(range_end).unwrap());

        let mut hash = Sha256::new();
        (0..600_000).for_each(|_| {
            let compat = sampler_compat.sample(&mut rng_compat);
            hash.update(compat.to_le_bytes());
        });
        assert_eq!(bs58::encode(hash.finalize()).into_string(), expected_hash);
    }

    #[test]
    fn test_uniform_sample_like_trait_sample_example() {
        let mut rng_compat = ChaChaRng::from_seed(CHACHA_SEED);
        let sampler_compat =
            UniformU64Sampler::new_like_trait_sample(NonZero::new(294_533).unwrap());
        let values: [u64; 10] = array::from_fn(|_| sampler_compat.sample(&mut rng_compat));
        assert_eq!(
            values,
            [
                272634, 52124, 8676, 230277, 223574, 137788, 212533, 213080, 187008, 209168
            ]
        );
    }

    #[test_case(10, "2tsA6RuXekKvMMvAqjkMMipjYySpw8mrrwwoBeoKohD1")]
    #[test_case(1_000, "58SG5gnD5wR6ngrjxuTv7m1SEXiXejZvuJPUCsyrmqWx")]
    #[test_case(4098, "7w4TY1oaeEeqHmPw3iobD8WVq1BsL5eo7ZuNdvDkoSQo")]
    #[test_case(10_000_000_000, "ENe5A82Wq2nYsH17q9WkKAudXdLE9FVGRbySuuCpaR5k")]
    fn test_uniform_sampler_like_trait_sample_single(range_end: u64, expected_hash: &str) {
        let mut rng_compat = ChaChaRng::from_seed(CHACHA_SEED);
        let sampler_compat =
            UniformU64Sampler::new_like_trait_sample(NonZero::new(range_end).unwrap());

        let mut hash = Sha256::new();
        (0..1_000).for_each(|_| {
            let compat = sampler_compat.sample(&mut rng_compat);
            hash.update(compat.to_le_bytes());
        });
        assert_eq!(bs58::encode(hash.finalize()).into_string(), expected_hash);
    }

    #[test_case(0..100, &[95, 2, 28, 92, 17, 78, 72, 72, 21, 65])]
    #[test_case(4..=100, &[96, 6, 31, 21, 77, 45, 49, 74, 24, 62])]
    #[test_case(4..101, &[96, 6, 31, 21, 77, 45, 49, 74, 24, 62])]
    #[test_case(77..=77, &[77, 77, 77, 77, 77, 77, 77, 77, 77, 77])]
    #[test_case((u64::MAX - 1)..,
                &[18446744073709551614, 18446744073709551614, 18446744073709551615, 18446744073709551614,
                  18446744073709551615, 18446744073709551615, 18446744073709551614, 18446744073709551615,
                  18446744073709551615, 18446744073709551615])]
    #[test_case((u64::MAX - 1)..=u64::MAX,
                &[18446744073709551614, 18446744073709551614, 18446744073709551615, 18446744073709551614,
                  18446744073709551615, 18446744073709551615, 18446744073709551614, 18446744073709551615,
                  18446744073709551615, 18446744073709551615])]
    #[test_case(1_000..3_463, &[1704, 2597, 1072, 2152, 2777, 1526, 2619, 3318, 1824, 2749])]
    #[test_case(.., &[17561962876765024395, 470213581521912695, 5273148429961811548, 17075204616871931816,
                      3264552321098222474, 11961477208003522799, 543394791199312464, 14422393090876740590,
                      14002568482199435789, 7891933133259177431])]
    #[test_case(0..=u64::MAX,
                &[17561962876765024395, 470213581521912695, 5273148429961811548, 17075204616871931816,
                  3264552321098222474, 11961477208003522799, 543394791199312464, 14422393090876740590,
                  14002568482199435789, 7891933133259177431])]
    fn test_random_range_example(range: impl RangeBounds<u64> + Clone, expected: &[u64]) {
        let mut rng_compat = ChaChaRng::from_seed(CHACHA_SEED);
        let result: Vec<u64> = (0..10)
            .map(|_| random_u64_range(&mut rng_compat, range.clone()))
            .collect();
        assert_eq!(result, expected);
    }

    #[test_case(0..100, "9qrc9Atiwy7f5vYs6UK4fk4WMtRqjLF8Zd8WsG7MFsYX")]
    #[test_case(1_000..9_999, "6qRfSYk55bPZty3qc3jLaSxASXgDRLaw8AWnXKEhiW6M")]
    #[test_case(4..4098, "7rKu65HwouY8sr3o7jZrdRPCtGwVXjZDTXsjcKg3VVr")]
    #[test_case(1_000..20_000_000_000, "J5vwi9DrgXrTNinMQmDw1zSq6ogeUencayoqEn4r64gH")]
    fn test_random_range_reproducibility(range: Range<u64>, expected_hash: &str) {
        let mut rng_compat = ChaChaRng::from_seed(CHACHA_SEED);

        let mut hash = Sha256::new();
        (0..10_000).for_each(|_| {
            let compat = random_u64_range(&mut rng_compat, range.clone());
            hash.update(compat.to_le_bytes());
        });
        assert_eq!(bs58::encode(hash.finalize()).into_string(), expected_hash);
    }

    #[test]
    #[should_panic]
    fn test_random_range_panic_empty() {
        let mut rng = ChaChaRng::from_seed(CHACHA_SEED);
        let _ = random_u64_range(&mut rng, 100..100);
    }
}
