1 // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! The exponential distribution.
13 use core::kinds::Copy;
17 use distributions::{ziggurat, ziggurat_tables, Sample, IndependentSample};
19 /// A wrapper around an `f64` to generate Exp(1) random numbers.
21 /// See `Exp` for the general exponential distribution.Note that this
22 // has to be unwrapped before use as an `f64` (using either
23 /// `*` or `mem::transmute` is safe).
25 /// Implemented via the ZIGNOR variant[1] of the Ziggurat method. The
26 /// exact description in the paper was adjusted to use tables for the
27 /// exponential distribution rather than normal.
29 /// [1]: Jurgen A. Doornik (2005). [*An Improved Ziggurat Method to
30 /// Generate Normal Random
31 /// Samples*](http://www.doornik.com/research/ziggurat.pdf). Nuffield
33 pub struct Exp1(pub f64);
37 // This could be done via `-rng.gen::<f64>().ln()` but that is slower.
40 fn rand<R:Rng>(rng: &mut R) -> Exp1 {
42 fn pdf(x: f64) -> f64 {
46 fn zero_case<R:Rng>(rng: &mut R, _u: f64) -> f64 {
47 ziggurat_tables::ZIG_EXP_R - rng.gen::<f64>().ln()
50 Exp1(ziggurat(rng, false,
51 &ziggurat_tables::ZIG_EXP_X,
52 &ziggurat_tables::ZIG_EXP_F,
57 /// The exponential distribution `Exp(lambda)`.
59 /// This distribution has density function: `f(x) = lambda *
60 /// exp(-lambda * x)` for `x > 0`.
66 /// use std::rand::distributions::{Exp, IndependentSample};
68 /// let exp = Exp::new(2.0);
69 /// let v = exp.ind_sample(&mut rand::task_rng());
70 /// println!("{} is from a Exp(2) distribution", v);
73 /// `lambda` stored as `1/lambda`, since this is what we scale by.
80 /// Construct a new `Exp` with the given shape parameter
81 /// `lambda`. Panics if `lambda <= 0`.
82 pub fn new(lambda: f64) -> Exp {
83 assert!(lambda > 0.0, "Exp::new called with `lambda` <= 0");
84 Exp { lambda_inverse: 1.0 / lambda }
88 impl Sample<f64> for Exp {
89 fn sample<R: Rng>(&mut self, rng: &mut R) -> f64 { self.ind_sample(rng) }
91 impl IndependentSample<f64> for Exp {
92 fn ind_sample<R: Rng>(&self, rng: &mut R) -> f64 {
93 let Exp1(n) = rng.gen::<Exp1>();
94 n * self.lambda_inverse
102 use distributions::{Sample, IndependentSample};
107 let mut exp = Exp::new(10.0);
108 let mut rng = ::test::rng();
109 for _ in range(0u, 1000) {
110 assert!(exp.sample(&mut rng) >= 0.0);
111 assert!(exp.ind_sample(&mut rng) >= 0.0);
116 fn test_exp_invalid_lambda_zero() {
121 fn test_exp_invalid_lambda_neg() {
132 use self::test::Bencher;
133 use std::mem::size_of;
135 use distributions::Sample;
138 fn rand_exp(b: &mut Bencher) {
139 let mut rng = ::test::weak_rng();
140 let mut exp = Exp::new(2.71828 * 3.14159);
143 for _ in range(0, ::RAND_BENCH_N) {
144 exp.sample(&mut rng);
147 b.bytes = size_of::<f64>() as u64 * ::RAND_BENCH_N;