diff --git a/src/distributions/mod.rs b/src/distributions/mod.rs index f6dccc1bbf7..4a2a4f2e046 100644 --- a/src/distributions/mod.rs +++ b/src/distributions/mod.rs @@ -158,6 +158,7 @@ //! [`Normal`]: struct.Normal.html //! [`Open01`]: struct.Open01.html //! [`OpenClosed01`]: struct.OpenClosed01.html +//! [`Pareto`]: struct.Pareto.html //! [`Poisson`]: struct.Poisson.html //! [`Standard`]: struct.Standard.html //! [`StandardNormal`]: struct.StandardNormal.html @@ -177,6 +178,8 @@ pub use self::uniform::Uniform as Range; #[doc(inline)] pub use self::normal::{Normal, LogNormal, StandardNormal}; #[cfg(feature="std")] #[doc(inline)] pub use self::exponential::{Exp, Exp1}; +#[cfg(feature="std")] +#[doc(inline)] pub use self::pareto::Pareto; #[cfg(feature = "std")] #[doc(inline)] pub use self::poisson::Poisson; #[cfg(feature = "std")] @@ -192,6 +195,8 @@ pub mod uniform; #[doc(hidden)] pub mod normal; #[cfg(feature="std")] #[doc(hidden)] pub mod exponential; +#[cfg(feature="std")] +#[doc(hidden)] pub mod pareto; #[cfg(feature = "std")] #[doc(hidden)] pub mod poisson; #[cfg(feature = "std")] diff --git a/src/distributions/pareto.rs b/src/distributions/pareto.rs new file mode 100644 index 00000000000..ba628e061d5 --- /dev/null +++ b/src/distributions/pareto.rs @@ -0,0 +1,76 @@ +// Copyright 2018 The Rust Project Developers. See the COPYRIGHT +// file at the top-level directory of this distribution and at +// https://rust-lang.org/COPYRIGHT. +// +// Licensed under the Apache License, Version 2.0 or the MIT license +// , at your +// option. This file may not be copied, modified, or distributed +// except according to those terms. + +//! The Pareto distribution. + +use Rng; +use distributions::{Distribution, OpenClosed01}; + +/// Samples floating-point numbers according to the Pareto distribution +/// +/// # Example +/// ``` +/// use rand::prelude::*; +/// use rand::distributions::Pareto; +/// +/// let val: f64 = SmallRng::from_entropy().sample(Pareto::new(1., 2.)); +/// println!("{}", val); +/// ``` +#[derive(Clone, Copy, Debug)] +pub struct Pareto { + scale: f64, + inv_neg_shape: f64, +} + +impl Pareto { + /// Construct a new Pareto distribution with given `scale` and `shape`. + /// + /// In the literature, `scale` is commonly written as xm or k and + /// `shape` is often written as α. + /// + /// # Panics + /// + /// `scale` and `shape` have to be non-zero and positive. + pub fn new(scale: f64, shape: f64) -> Pareto { + assert!((scale > 0.) & (shape > 0.)); + Pareto { scale, inv_neg_shape: -1.0 / shape } + } +} + +impl Distribution for Pareto { + fn sample(&self, rng: &mut R) -> f64 { + let u: f64 = rng.sample(OpenClosed01); + self.scale * u.powf(self.inv_neg_shape) + } +} + +#[cfg(test)] +mod tests { + use distributions::Distribution; + use super::Pareto; + + #[test] + #[should_panic] + fn invalid() { + Pareto::new(0., 0.); + } + + #[test] + fn sample() { + let scale = 1.0; + let shape = 2.0; + let d = Pareto::new(scale, shape); + let mut rng = ::test::rng(1); + for _ in 0..1000 { + let r = d.sample(&mut rng); + assert!(r >= scale); + } + } +}