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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 Weibull distribution.

    use Rng;
    use distributions::{Distribution, OpenClosed01};

    /// Samples floating-point numbers according to the Weibull distribution
    ///
    /// # Example
    /// ```
    /// use rand::prelude::*;
    /// use rand::distributions::Weibull;
    ///
    /// let val: f64 = SmallRng::from_entropy().sample(Weibull::new(1., 10.));
    /// println!("{}", val);
    /// ```
    #[derive(Clone, Copy, Debug)]
    pub struct Weibull {
        inv_shape: f64,
        scale: f64,
    }

    impl Weibull {
        /// Construct a new `Weibull` distribution with given `scale` and `shape`.
        ///
        /// # Panics
        ///
        /// `scale` and `shape` have to be non-zero and positive.
        pub fn new(scale: f64, shape: f64) -> Weibull {
            assert!((scale > 0.) & (shape > 0.));
            Weibull { inv_shape: 1./shape, scale }
        }
    }

    impl Distribution<f64> for Weibull {
        fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> f64 {
            let x: f64 = rng.sample(OpenClosed01);
            self.scale * (-x.ln()).powf(self.inv_shape)
        }
    }

    #[cfg(test)]
    mod tests {
        use distributions::Distribution;
        use super::Weibull;

        #[test]
        #[should_panic]
        fn invalid() {
            Weibull::new(0., 0.);
        }

        #[test]
        fn sample() {
            let scale = 1.0;
            let shape = 2.0;
            let d = Weibull::new(scale, shape);
            let mut rng = ::test::rng(1);
            for _ in 0..1000 {
                let r = d.sample(&mut rng);
                assert!(r >= 0.);
            }
        }
    }

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