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    // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! The implementations of `Rand` for the built-in types.

    use core::{char, mem};

    use {Rand,Rng};

    impl Rand for isize {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> isize {
            if mem::size_of::<isize>() == 4 {
                rng.gen::<i32>() as isize
            } else {
                rng.gen::<i64>() as isize
            }
        }
    }

    impl Rand for i8 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> i8 {
            rng.next_u32() as i8
        }
    }

    impl Rand for i16 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> i16 {
            rng.next_u32() as i16
        }
    }

    impl Rand for i32 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> i32 {
            rng.next_u32() as i32
        }
    }

    impl Rand for i64 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> i64 {
            rng.next_u64() as i64
        }
    }

    #[cfg(feature = "i128_support")]
    impl Rand for i128 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> i128 {
            rng.gen::<u128>() as i128
        }
    }

    impl Rand for usize {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> usize {
            if mem::size_of::<usize>() == 4 {
                rng.gen::<u32>() as usize
            } else {
                rng.gen::<u64>() as usize
            }
        }
    }

    impl Rand for u8 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> u8 {
            rng.next_u32() as u8
        }
    }

    impl Rand for u16 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> u16 {
            rng.next_u32() as u16
        }
    }

    impl Rand for u32 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> u32 {
            rng.next_u32()
        }
    }

    impl Rand for u64 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> u64 {
            rng.next_u64()
        }
    }

    #[cfg(feature = "i128_support")]
    impl Rand for u128 {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> u128 {
            ((rng.next_u64() as u128) << 64) | (rng.next_u64() as u128)
        }
    }

    macro_rules! float_impls {
        ($mod_name:ident, $ty:ty, $mantissa_bits:expr, $method_name:ident) => {
            mod $mod_name {
                use {Rand, Rng, Open01, Closed01};

                const SCALE: $ty = (1u64 << $mantissa_bits) as $ty;

                impl Rand for $ty {
                    /// Generate a floating point number in the half-open
                    /// interval `[0,1)`.
                    ///
                    /// See `Closed01` for the closed interval `[0,1]`,
                    /// and `Open01` for the open interval `(0,1)`.
                    #[inline]
                    fn rand<R: Rng>(rng: &mut R) -> $ty {
                        rng.$method_name()
                    }
                }
                impl Rand for Open01<$ty> {
                    #[inline]
                    fn rand<R: Rng>(rng: &mut R) -> Open01<$ty> {
                        // add a small amount (specifically 2 bits below
                        // the precision of f64/f32 at 1.0), so that small
                        // numbers are larger than 0, but large numbers
                        // aren't pushed to/above 1.
                        Open01(rng.$method_name() + 0.25 / SCALE)
                    }
                }
                impl Rand for Closed01<$ty> {
                    #[inline]
                    fn rand<R: Rng>(rng: &mut R) -> Closed01<$ty> {
                        // rescale so that 1.0 - epsilon becomes 1.0
                        // precisely.
                        Closed01(rng.$method_name() * SCALE / (SCALE - 1.0))
                    }
                }
            }
        }
    }
    float_impls! { f64_rand_impls, f64, 53, next_f64 }
    float_impls! { f32_rand_impls, f32, 24, next_f32 }

    impl Rand for char {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> char {
            // a char is 21 bits
            const CHAR_MASK: u32 = 0x001f_ffff;
            loop {
                // Rejection sampling. About 0.2% of numbers with at most
                // 21-bits are invalid codepoints (surrogates), so this
                // will succeed first go almost every time.
                match char::from_u32(rng.next_u32() & CHAR_MASK) {
                    Some(c) => return c,
                    None => {}
                }
            }
        }
    }

    impl Rand for bool {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> bool {
            rng.gen::<u8>() & 1 == 1
        }
    }

    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 : Rand ),*
                > Rand for ( $( $tyvar ),* , ) {

                #[inline]
                fn rand<R: Rng>(_rng: &mut R) -> ( $( $tyvar ),* , ) {
                    (
                        // use the $tyvar's to get the appropriate number of
                        // repeats (they're not actually needed)
                        $(
                            _rng.gen::<$tyvar>()
                        ),*
                        ,
                    )
                }
            }
        }
    }

    impl Rand for () {
        #[inline]
        fn rand<R: Rng>(_: &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 {
        {$n:expr, $t:ident, $($ts:ident,)*} => {
            array_impl!{($n - 1), $($ts,)*}

            impl<T> Rand for [T; $n] where T: Rand {
                #[inline]
                fn rand<R: Rng>(_rng: &mut R) -> [T; $n] {
                    [_rng.gen::<$t>(), $(_rng.gen::<$ts>()),*]
                }
            }
        };
        {$n:expr,} => {
            impl<T> Rand for [T; $n] {
                fn rand<R: Rng>(_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:Rand> Rand for Option<T> {
        #[inline]
        fn rand<R: Rng>(rng: &mut R) -> Option<T> {
            if rng.gen() {
                Some(rng.gen())
            } else {
                None
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use {Rng, thread_rng, Open01, Closed01};

        struct ConstantRng(u64);
        impl Rng for ConstantRng {
            fn next_u32(&mut self) -> u32 {
                let ConstantRng(v) = *self;
                v as u32
            }
            fn next_u64(&mut self) -> u64 {
                let ConstantRng(v) = *self;
                v
            }
        }

        #[test]
        fn floating_point_edge_cases() {
            // the test for exact equality is correct here.
            assert!(ConstantRng(0xffff_ffff).gen::<f32>() != 1.0);
            assert!(ConstantRng(0xffff_ffff_ffff_ffff).gen::<f64>() != 1.0);
        }

        #[test]
        fn rand_open() {
            // this is unlikely to catch an incorrect implementation that
            // generates exactly 0 or 1, but it keeps it sane.
            let mut rng = thread_rng();
            for _ in 0..1_000 {
                // strict inequalities
                let Open01(f) = rng.gen::<Open01<f64>>();
                assert!(0.0 < f && f < 1.0);

                let Open01(f) = rng.gen::<Open01<f32>>();
                assert!(0.0 < f && f < 1.0);
            }
        }

        #[test]
        fn rand_closed() {
            let mut rng = thread_rng();
            for _ in 0..1_000 {
                // strict inequalities
                let Closed01(f) = rng.gen::<Closed01<f64>>();
                assert!(0.0 <= f && f <= 1.0);

                let Closed01(f) = rng.gen::<Closed01<f32>>();
                assert!(0.0 <= f && f <= 1.0);
            }
        }
    }

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