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<span class='comment'>// Copyright 2013 The Rust Project Developers. See the COPYRIGHT</span>
<span class='comment'>// file at the top-level directory of this distribution and at</span>
<span class='comment'>// http://rust-lang.org/COPYRIGHT.</span>
<span class='comment'>//</span>
<span class='comment'>// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class='comment'>// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class='comment'>// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class='comment'>// option. This file may not be copied, modified, or distributed</span>
<span class='comment'>// except according to those terms.</span>
<span class='doccomment'>//! The exponential distribution.</span>
<span class='kw'>use</span> {<span class='ident'>Rng</span>, <span class='ident'>Rand</span>};
<span class='kw'>use</span> <span class='ident'>distributions</span>::{<span class='ident'>ziggurat</span>, <span class='ident'>ziggurat_tables</span>, <span class='ident'>Sample</span>, <span class='ident'>IndependentSample</span>};
<span class='doccomment'>/// A wrapper around an `f64` to generate Exp(1) random numbers.</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// See `Exp` for the general exponential distribution.</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// Implemented via the ZIGNOR variant[1] of the Ziggurat method. The</span>
<span class='doccomment'>/// exact description in the paper was adjusted to use tables for the</span>
<span class='doccomment'>/// exponential distribution rather than normal.</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// [1]: Jurgen A. Doornik (2005). [*An Improved Ziggurat Method to</span>
<span class='doccomment'>/// Generate Normal Random</span>
<span class='doccomment'>/// Samples*](http://www.doornik.com/research/ziggurat.pdf). Nuffield</span>
<span class='doccomment'>/// College, Oxford</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// # Example</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// ```rust</span>
<span class='doccomment'>/// use rand::distributions::exponential::Exp1;</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// let Exp1(x) = rand::random();</span>
<span class='doccomment'>/// println!("{}", x);</span>
<span class='doccomment'>/// ```</span>
<span class='attribute'>#[<span class='ident'>derive</span>(<span class='ident'>Clone</span>, <span class='ident'>Copy</span>)]</span>
<span class='kw'>pub</span> <span class='kw'>struct</span> <span class='ident'>Exp1</span>(<span class='kw'>pub</span> <span class='ident'>f64</span>);
<span class='comment'>// This could be done via `-rng.gen::<f64>().ln()` but that is slower.</span>
<span class='kw'>impl</span> <span class='ident'>Rand</span> <span class='kw'>for</span> <span class='ident'>Exp1</span> {
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>fn</span> <span class='ident'>rand</span><span class='op'><</span><span class='ident'>R</span>:<span class='ident'>Rng</span><span class='op'>></span>(<span class='ident'>rng</span>: <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>R</span>) <span class='op'>-></span> <span class='ident'>Exp1</span> {
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>fn</span> <span class='ident'>pdf</span>(<span class='ident'>x</span>: <span class='ident'>f64</span>) <span class='op'>-></span> <span class='ident'>f64</span> {
(<span class='op'>-</span><span class='ident'>x</span>).<span class='ident'>exp</span>()
}
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>fn</span> <span class='ident'>zero_case</span><span class='op'><</span><span class='ident'>R</span>:<span class='ident'>Rng</span><span class='op'>></span>(<span class='ident'>rng</span>: <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>R</span>, <span class='ident'>_u</span>: <span class='ident'>f64</span>) <span class='op'>-></span> <span class='ident'>f64</span> {
<span class='ident'>ziggurat_tables</span>::<span class='ident'>ZIG_EXP_R</span> <span class='op'>-</span> <span class='ident'>rng</span>.<span class='ident'>gen</span>::<span class='op'><</span><span class='ident'>f64</span><span class='op'>></span>().<span class='ident'>ln</span>()
}
<span class='ident'>Exp1</span>(<span class='ident'>ziggurat</span>(<span class='ident'>rng</span>, <span class='bool-val'>false</span>,
<span class='kw-2'>&</span><span class='ident'>ziggurat_tables</span>::<span class='ident'>ZIG_EXP_X</span>,
<span class='kw-2'>&</span><span class='ident'>ziggurat_tables</span>::<span class='ident'>ZIG_EXP_F</span>,
<span class='ident'>pdf</span>, <span class='ident'>zero_case</span>))
}
}
<span class='doccomment'>/// The exponential distribution `Exp(lambda)`.</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// This distribution has density function: `f(x) = lambda *</span>
<span class='doccomment'>/// exp(-lambda * x)` for `x > 0`.</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// # Example</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// ```rust</span>
<span class='doccomment'>/// use rand::distributions::{Exp, IndependentSample};</span>
<span class='doccomment'>///</span>
<span class='doccomment'>/// let exp = Exp::new(2.0);</span>
<span class='doccomment'>/// let v = exp.ind_sample(&mut rand::thread_rng());</span>
<span class='doccomment'>/// println!("{} is from a Exp(2) distribution", v);</span>
<span class='doccomment'>/// ```</span>
<span class='attribute'>#[<span class='ident'>derive</span>(<span class='ident'>Clone</span>, <span class='ident'>Copy</span>)]</span>
<span class='kw'>pub</span> <span class='kw'>struct</span> <span class='ident'>Exp</span> {
<span class='doccomment'>/// `lambda` stored as `1/lambda`, since this is what we scale by.</span>
<span class='ident'>lambda_inverse</span>: <span class='ident'>f64</span>
}
<span class='kw'>impl</span> <span class='ident'>Exp</span> {
<span class='doccomment'>/// Construct a new `Exp` with the given shape parameter</span>
<span class='doccomment'>/// `lambda`. Panics if `lambda <= 0`.</span>
<span class='attribute'>#[<span class='ident'>inline</span>]</span>
<span class='kw'>pub</span> <span class='kw'>fn</span> <span class='ident'>new</span>(<span class='ident'>lambda</span>: <span class='ident'>f64</span>) <span class='op'>-></span> <span class='ident'>Exp</span> {
<span class='macro'>assert</span><span class='macro'>!</span>(<span class='ident'>lambda</span> <span class='op'>></span> <span class='number'>0.0</span>, <span class='string'>"Exp::new called with `lambda` <= 0"</span>);
<span class='ident'>Exp</span> { <span class='ident'>lambda_inverse</span>: <span class='number'>1.0</span> <span class='op'>/</span> <span class='ident'>lambda</span> }
}
}
<span class='kw'>impl</span> <span class='ident'>Sample</span><span class='op'><</span><span class='ident'>f64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>Exp</span> {
<span class='kw'>fn</span> <span class='ident'>sample</span><span class='op'><</span><span class='ident'>R</span>: <span class='ident'>Rng</span><span class='op'>></span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='self'>self</span>, <span class='ident'>rng</span>: <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>R</span>) <span class='op'>-></span> <span class='ident'>f64</span> { <span class='self'>self</span>.<span class='ident'>ind_sample</span>(<span class='ident'>rng</span>) }
}
<span class='kw'>impl</span> <span class='ident'>IndependentSample</span><span class='op'><</span><span class='ident'>f64</span><span class='op'>></span> <span class='kw'>for</span> <span class='ident'>Exp</span> {
<span class='kw'>fn</span> <span class='ident'>ind_sample</span><span class='op'><</span><span class='ident'>R</span>: <span class='ident'>Rng</span><span class='op'>></span>(<span class='kw-2'>&</span><span class='self'>self</span>, <span class='ident'>rng</span>: <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>R</span>) <span class='op'>-></span> <span class='ident'>f64</span> {
<span class='kw'>let</span> <span class='ident'>Exp1</span>(<span class='ident'>n</span>) <span class='op'>=</span> <span class='ident'>rng</span>.<span class='ident'>gen</span>::<span class='op'><</span><span class='ident'>Exp1</span><span class='op'>></span>();
<span class='ident'>n</span> <span class='op'>*</span> <span class='self'>self</span>.<span class='ident'>lambda_inverse</span>
}
}
<span class='attribute'>#[<span class='ident'>cfg</span>(<span class='ident'>test</span>)]</span>
<span class='kw'>mod</span> <span class='ident'>test</span> {
<span class='kw'>use</span> <span class='ident'>distributions</span>::{<span class='ident'>Sample</span>, <span class='ident'>IndependentSample</span>};
<span class='kw'>use</span> <span class='kw'>super</span>::<span class='ident'>Exp</span>;
<span class='attribute'>#[<span class='ident'>test</span>]</span>
<span class='kw'>fn</span> <span class='ident'>test_exp</span>() {
<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>exp</span> <span class='op'>=</span> <span class='ident'>Exp</span>::<span class='ident'>new</span>(<span class='number'>10.0</span>);
<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>rng</span> <span class='op'>=</span> ::<span class='ident'>test</span>::<span class='ident'>rng</span>();
<span class='kw'>for</span> _ <span class='kw'>in</span> <span class='number'>0</span>..<span class='number'>1000</span> {
<span class='macro'>assert</span><span class='macro'>!</span>(<span class='ident'>exp</span>.<span class='ident'>sample</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>rng</span>) <span class='op'>>=</span> <span class='number'>0.0</span>);
<span class='macro'>assert</span><span class='macro'>!</span>(<span class='ident'>exp</span>.<span class='ident'>ind_sample</span>(<span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>rng</span>) <span class='op'>>=</span> <span class='number'>0.0</span>);
}
}
<span class='attribute'>#[<span class='ident'>test</span>]</span>
<span class='attribute'>#[<span class='ident'>should_panic</span>]</span>
<span class='kw'>fn</span> <span class='ident'>test_exp_invalid_lambda_zero</span>() {
<span class='ident'>Exp</span>::<span class='ident'>new</span>(<span class='number'>0.0</span>);
}
<span class='attribute'>#[<span class='ident'>test</span>]</span>
<span class='attribute'>#[<span class='ident'>should_panic</span>]</span>
<span class='kw'>fn</span> <span class='ident'>test_exp_invalid_lambda_neg</span>() {
<span class='ident'>Exp</span>::<span class='ident'>new</span>(<span class='op'>-</span><span class='number'>10.0</span>);
}
}
</pre>
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