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    // Copyright 2018 Developers of the Rand project.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! The implementations of the `Standard` distribution for other built-in types.

    use core::char;
    use core::num::Wrapping;

    use crate::{Rng};
    use crate::distributions::{Distribution, Standard, Uniform};

    // ----- Sampling distributions -----

    /// Sample a `char`, uniformly distributed over ASCII letters and numbers:
    /// a-z, A-Z and 0-9.
    ///
    /// # Example
    ///
    /// ```
    /// use std::iter;
    /// use rand::{Rng, thread_rng};
    /// use rand::distributions::Alphanumeric;
    ///
    /// let mut rng = thread_rng();
    /// let chars: String = iter::repeat(())
    ///         .map(|()| rng.sample(Alphanumeric))
    ///         .take(7)
    ///         .collect();
    /// println!("Random chars: {}", chars);
    /// ```
    #[derive(Debug)]
    pub struct Alphanumeric;

    // ----- Implementations of distributions -----

    impl Distribution<char> for Standard {
        #[inline]
        fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> char {
            // A valid `char` is either in the interval `[0, 0xD800)` or
            // `(0xDFFF, 0x11_0000)`. All `char`s must therefore be in
            // `[0, 0x11_0000)` but not in the "gap" `[0xD800, 0xDFFF]` which is
            // reserved for surrogates. This is the size of that gap.
            const GAP_SIZE: u32 = 0xDFFF - 0xD800 + 1;

            // Uniform::new(0, 0x11_0000 - GAP_SIZE) can also be used but it
            // seemed slower.
            let range = Uniform::new(GAP_SIZE, 0x11_0000);

            let mut n = range.sample(rng);
            if n <= 0xDFFF {
                n -= GAP_SIZE;
            }
            unsafe { char::from_u32_unchecked(n) }
        }
    }

    impl Distribution<char> for Alphanumeric {
        fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> char {
            const RANGE: u32 = 26 + 26 + 10;
            const GEN_ASCII_STR_CHARSET: &[u8] =
                b"ABCDEFGHIJKLMNOPQRSTUVWXYZ\
                    abcdefghijklmnopqrstuvwxyz\
                    0123456789";
            // We can pick from 62 characters. This is so close to a power of 2, 64,
            // that we can do better than `Uniform`. Use a simple bitshift and
            // rejection sampling. We do not use a bitmask, because for small RNGs
            // the most significant bits are usually of higher quality.
            loop {
                let var = rng.next_u32() >> (32 - 6);
                if var < RANGE {
                    return GEN_ASCII_STR_CHARSET[var as usize] as char
                }
            }
        }
    }

    impl Distribution<bool> for Standard {
        #[inline]
        fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> bool {
            // We can compare against an arbitrary bit of an u32 to get a bool.
            // Because the least significant bits of a lower quality RNG can have
            // simple patterns, we compare against the most significant bit. This is
            // easiest done using a sign test.
            (rng.next_u32() as i32) < 0
        }
    }

    macro_rules! tuple_impl {
        // use variables to indicate the arity of the tuple
        ($($tyvar:ident),* ) => {
            // the trailing commas are for the 1 tuple
            impl< $( $tyvar ),* >
                Distribution<( $( $tyvar ),* , )>
                for Standard
                where $( Standard: Distribution<$tyvar> ),*
            {
                #[inline]
                fn sample<R: Rng + ?Sized>(&self, _rng: &mut R) -> ( $( $tyvar ),* , ) {
                    (
                        // use the $tyvar's to get the appropriate number of
                        // repeats (they're not actually needed)
                        $(
                            _rng.gen::<$tyvar>()
                        ),*
                        ,
                    )
                }
            }
        }
    }

    impl Distribution<()> for Standard {
        #[inline]
        fn sample<R: Rng + ?Sized>(&self, _: &mut R) -> () { () }
    }
    tuple_impl!{A}
    tuple_impl!{A, B}
    tuple_impl!{A, B, C}
    tuple_impl!{A, B, C, D}
    tuple_impl!{A, B, C, D, E}
    tuple_impl!{A, B, C, D, E, F}
    tuple_impl!{A, B, C, D, E, F, G}
    tuple_impl!{A, B, C, D, E, F, G, H}
    tuple_impl!{A, B, C, D, E, F, G, H, I}
    tuple_impl!{A, B, C, D, E, F, G, H, I, J}
    tuple_impl!{A, B, C, D, E, F, G, H, I, J, K}
    tuple_impl!{A, B, C, D, E, F, G, H, I, J, K, L}

    macro_rules! array_impl {
        // recursive, given at least one type parameter:
        {$n:expr, $t:ident, $($ts:ident,)*} => {
            array_impl!{($n - 1), $($ts,)*}

            impl<T> Distribution<[T; $n]> for Standard where Standard: Distribution<T> {
                #[inline]
                fn sample<R: Rng + ?Sized>(&self, _rng: &mut R) -> [T; $n] {
                    [_rng.gen::<$t>(), $(_rng.gen::<$ts>()),*]
                }
            }
        };
        // empty case:
        {$n:expr,} => {
            impl<T> Distribution<[T; $n]> for Standard {
                fn sample<R: Rng + ?Sized>(&self, _rng: &mut R) -> [T; $n] { [] }
            }
        };
    }

    array_impl!{32, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T, T,}

    impl<T> Distribution<Option<T>> for Standard where Standard: Distribution<T> {
        #[inline]
        fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Option<T> {
            // UFCS is needed here: https://github.com/rust-lang/rust/issues/24066
            if rng.gen::<bool>() {
                Some(rng.gen())
            } else {
                None
            }
        }
    }

    impl<T> Distribution<Wrapping<T>> for Standard where Standard: Distribution<T> {
        #[inline]
        fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Wrapping<T> {
            Wrapping(rng.gen())
        }
    }

    #[cfg(test)]
    mod tests {
        use crate::{Rng, RngCore, Standard};
        use crate::distributions::Alphanumeric;
        #[cfg(all(not(feature="std"), feature="alloc"))] use alloc::string::String;

        #[test]
        fn test_misc() {
            let rng: &mut dyn RngCore = &mut crate::test::rng(820);

            rng.sample::<char, _>(Standard);
            rng.sample::<bool, _>(Standard);
        }

        #[cfg(feature="alloc")]
        #[test]
        fn test_chars() {
            use core::iter;
            let mut rng = crate::test::rng(805);

            // Test by generating a relatively large number of chars, so we also
            // take the rejection sampling path.
            let word: String = iter::repeat(())
                    .map(|()| rng.gen::<char>()).take(1000).collect();
            assert!(word.len() != 0);
        }

        #[test]
        fn test_alphanumeric() {
            let mut rng = crate::test::rng(806);

            // Test by generating a relatively large number of chars, so we also
            // take the rejection sampling path.
            let mut incorrect = false;
            for _ in 0..100 {
                let c = rng.sample(Alphanumeric);
                incorrect |= !((c >= '0' && c <= '9') ||
                               (c >= 'A' && c <= 'Z') ||
                               (c >= 'a' && c <= 'z') );
            }
            assert!(incorrect == false);
        }
    }

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