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    // Copyright 2013 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.

    //! Generating numbers between two others.

    // this is surprisingly complicated to be both generic & correct

    use std::num::Wrapping as w;

    use Rng;
    use distributions::{Sample, IndependentSample};

    /// Sample values uniformly between two bounds.
    ///
    /// This gives a uniform distribution (assuming the RNG used to sample
    /// it is itself uniform & the `SampleRange` implementation for the
    /// given type is correct), even for edge cases like `low = 0u8`,
    /// `high = 170u8`, for which a naive modulo operation would return
    /// numbers less than 85 with double the probability to those greater
    /// than 85.
    ///
    /// Types should attempt to sample in `[low, high)`, i.e., not
    /// including `high`, but this may be very difficult. All the
    /// primitive integer types satisfy this property, and the float types
    /// normally satisfy it, but rounding may mean `high` can occur.
    ///
    /// # Example
    ///
    /// ```rust
    /// use rand::distributions::{IndependentSample, Range};
    ///
    /// fn main() {
    ///     let between = Range::new(10, 10000);
    ///     let mut rng = rand::thread_rng();
    ///     let mut sum = 0;
    ///     for _ in 0..1000 {
    ///         sum += between.ind_sample(&mut rng);
    ///     }
    ///     println!("{}", sum);
    /// }
    /// ```
    #[derive(Clone, Copy)]
    pub struct Range<X> {
        low: X,
        range: X,
        accept_zone: X
    }

    impl<X: SampleRange + PartialOrd> Range<X> {
        /// Create a new `Range` instance that samples uniformly from
        /// `[low, high)`. Panics if `low >= high`.
        pub fn new(low: X, high: X) -> Range<X> {
            assert!(low < high, "Range::new called with `low >= high`");
            SampleRange::construct_range(low, high)
        }
    }

    impl<Sup: SampleRange> Sample<Sup> for Range<Sup> {
        #[inline]
        fn sample<R: Rng>(&mut self, rng: &mut R) -> Sup { self.ind_sample(rng) }
    }
    impl<Sup: SampleRange> IndependentSample<Sup> for Range<Sup> {
        fn ind_sample<R: Rng>(&self, rng: &mut R) -> Sup {
            SampleRange::sample_range(self, rng)
        }
    }

    /// The helper trait for types that have a sensible way to sample
    /// uniformly between two values. This should not be used directly,
    /// and is only to facilitate `Range`.
    pub trait SampleRange : Sized {
        /// Construct the `Range` object that `sample_range`
        /// requires. This should not ever be called directly, only via
        /// `Range::new`, which will check that `low < high`, so this
        /// function doesn't have to repeat the check.
        fn construct_range(low: Self, high: Self) -> Range<Self>;

        /// Sample a value from the given `Range` with the given `Rng` as
        /// a source of randomness.
        fn sample_range<R: Rng>(r: &Range<Self>, rng: &mut R) -> Self;
    }

    macro_rules! integer_impl {
        ($ty:ty, $unsigned:ident) => {
            impl SampleRange for $ty {
                // we play free and fast with unsigned vs signed here
                // (when $ty is signed), but that's fine, since the
                // contract of this macro is for $ty and $unsigned to be
                // "bit-equal", so casting between them is a no-op & a
                // bijection.

                fn construct_range(low: $ty, high: $ty) -> Range<$ty> {
                    let range = (w(high as $unsigned) - w(low as $unsigned)).0;
                    let unsigned_max: $unsigned = ::std::$unsigned::MAX;

                    // this is the largest number that fits into $unsigned
                    // that `range` divides evenly, so, if we've sampled
                    // `n` uniformly from this region, then `n % range` is
                    // uniform in [0, range)
                    let zone = unsigned_max - unsigned_max % range;

                    Range {
                        low: low,
                        range: range as $ty,
                        accept_zone: zone as $ty
                    }
                }
                #[inline]
                fn sample_range<R: Rng>(r: &Range<$ty>, rng: &mut R) -> $ty {
                    loop {
                        // rejection sample
                        let v = rng.gen::<$unsigned>();
                        // until we find something that fits into the
                        // region which r.range evenly divides (this will
                        // be uniformly distributed)
                        if v < r.accept_zone as $unsigned {
                            // and return it, with some adjustments
                            return (w(r.low) + w((v % r.range as $unsigned) as $ty)).0;
                        }
                    }
                }
            }
        }
    }

    integer_impl! { i8, u8 }
    integer_impl! { i16, u16 }
    integer_impl! { i32, u32 }
    integer_impl! { i64, u64 }
    integer_impl! { isize, usize }
    integer_impl! { u8, u8 }
    integer_impl! { u16, u16 }
    integer_impl! { u32, u32 }
    integer_impl! { u64, u64 }
    integer_impl! { usize, usize }

    macro_rules! float_impl {
        ($ty:ty) => {
            impl SampleRange for $ty {
                fn construct_range(low: $ty, high: $ty) -> Range<$ty> {
                    Range {
                        low: low,
                        range: high - low,
                        accept_zone: 0.0 // unused
                    }
                }
                fn sample_range<R: Rng>(r: &Range<$ty>, rng: &mut R) -> $ty {
                    r.low + r.range * rng.gen::<$ty>()
                }
            }
        }
    }

    float_impl! { f32 }
    float_impl! { f64 }

    #[cfg(test)]
    mod tests {
        use distributions::{Sample, IndependentSample};
        use super::Range as Range;

        #[should_panic]
        #[test]
        fn test_range_bad_limits_equal() {
            Range::new(10, 10);
        }
        #[should_panic]
        #[test]
        fn test_range_bad_limits_flipped() {
            Range::new(10, 5);
        }

        #[test]
        fn test_integers() {
            let mut rng = ::test::rng();
            macro_rules! t {
                ($($ty:ident),*) => {{
                    $(
                       let v: &[($ty, $ty)] = &[(0, 10),
                                                (10, 127),
                                                (::std::$ty::MIN, ::std::$ty::MAX)];
                       for &(low, high) in v.iter() {
                            let mut sampler: Range<$ty> = Range::new(low, high);
                            for _ in 0..1000 {
                                let v = sampler.sample(&mut rng);
                                assert!(low <= v && v < high);
                                let v = sampler.ind_sample(&mut rng);
                                assert!(low <= v && v < high);
                            }
                        }
                     )*
                }}
            }
            t!(i8, i16, i32, i64, isize,
               u8, u16, u32, u64, usize)
        }

        #[test]
        fn test_floats() {
            let mut rng = ::test::rng();
            macro_rules! t {
                ($($ty:ty),*) => {{
                    $(
                       let v: &[($ty, $ty)] = &[(0.0, 100.0),
                                                (-1e35, -1e25),
                                                (1e-35, 1e-25),
                                                (-1e35, 1e35)];
                       for &(low, high) in v.iter() {
                            let mut sampler: Range<$ty> = Range::new(low, high);
                            for _ in 0..1000 {
                                let v = sampler.sample(&mut rng);
                                assert!(low <= v && v < high);
                                let v = sampler.ind_sample(&mut rng);
                                assert!(low <= v && v < high);
                            }
                        }
                     )*
                }}
            }

            t!(f32, f64)
        }

    }

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