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Binomial sampling: limit max n and use powi #752

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Merged
dhardy merged 1 commit into from
Mar 7, 2019
Merged

Binomial sampling: limit max n and use powi #752

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29 changes: 2 additions & 27 deletions src/distributions/binomial.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -47,31 +47,6 @@ impl Binomial {
}
}

/// Raise a `base` to the power of `exp`, using exponentiation by squaring.
///
/// This implementation is based on the one in the `num_traits` crate. It is
/// slightly modified to accept `u64` exponents.
fn pow(mut base: f64, mut exp: u64) -> f64 {
if exp == 0 {
return 1.;
}

while exp & 1 == 0 {
base *= base;
exp >>= 1;
}

let mut acc = base;
while exp > 1 {
exp >>= 1;
base *= base;
if exp & 1 == 1 {
acc *= base;
}
}
acc
}

impl Distribution<u64> for Binomial {
fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> u64 {
// Handle these values directly.
Expand All @@ -98,11 +73,11 @@ impl Distribution<u64> for Binomial {
// Voratas Kachitvichyanukul and Bruce W. Schmeiser. 1988. Binomial
// random variate generation. Commun. ACM 31, 2 (February 1988),
// 216-222. http://dx.doi.org/10.1145/42372.42381
if (self.n as f64) * p < 10. {
if (self.n as f64) * p < 10. && self.n <= (::std::i32::MAX as u64) {
let q = 1. - p;
let s = p / q;
let a = ((self.n + 1) as f64) * s;
let mut r = pow(q, self.n);
let mut r = q.powi(self.n as i32);
let mut u: f64 = rng.gen();
let mut x = 0;
while u > r as f64 {
Expand Down