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extern crate num_traits;
extern crate rand;
use std::fmt;
use std::iter::{FromIterator, Sum};
use std::vec::Vec;
use num_traits::{Float, NumCast, One, Zero};
use rand::Rng;
use rand::distributions::range::{Range, SampleRange};
use rand::distributions::IndependentSample;
#[derive(Debug)]
enum AliasEntry<F> {
Aliased {
threshold: F,
value: usize,
alias: usize,
},
Unaliased(usize),
}
use AliasEntry::*;
pub struct AliasTable<T, F> {
table: Vec<AliasEntry<F>>,
objs: Vec<T>,
range: Range<usize>,
float: Range<F>,
}
#[derive(Clone)]
pub struct AliasTableIterator<'a, T: 'a, F: 'a, R>
where R: Rng + Sized
{
rng: R,
table: &'a AliasTable<T, F>
}
impl<T, F> fmt::Debug for AliasTable<T, F>
where F: fmt::Debug
{
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
write!(fmt, "AliasTable {{ table: {:?} }}", self.table)
}
}
impl<T, F> AliasTable<T, F>
where F: PartialOrd + SampleRange
{
pub fn pick<'a, R: Rng>(&'a self, rng: &mut R) -> &'a T {
let idx = self.range.ind_sample(rng);
let entry = &self.table[idx];
match *entry {
Aliased { ref threshold, value, alias } => {
if &self.float.ind_sample(rng) < threshold {
&self.objs[value]
} else {
&self.objs[alias]
}
}
Unaliased(idx) => &self.objs[idx],
}
}
pub fn iter<R: Rng>(&self, rng: R) -> AliasTableIterator<T, F, R> {
AliasTableIterator {
rng: rng,
table: self
}
}
}
impl<'a, T, F: 'a> FromIterator<(T, F)> for AliasTable<T, F>
where F: Float + NumCast + One + SampleRange + Sum<F> + Zero
{
fn from_iter<I: IntoIterator<Item = (T, F)>>(iter: I) -> Self {
let (objs, ps): (Vec<_>, Vec<_>) = iter.into_iter().unzip();
let psum: F = ps.iter().cloned().sum();
let pn = F::from(ps.len())
.expect("Error casting usize to generic parameter F of AliasTable<T, F>");
let pcoeff = pn / psum;
let (mut small, mut large): (Vec<_>, Vec<_>) =
ps.into_iter().map(|p| pcoeff * p).enumerate().partition(|&(_, p)| p < F::one());
let mut table = Vec::new();
while !(small.is_empty() || large.is_empty()) {
let (l, p_l) = small.pop().unwrap();
let (g, p_g) = large.pop().unwrap();
table.push(Aliased {
threshold: p_l,
value: l,
alias: g,
});
let p_g = (p_g + p_l) - F::one();
if p_g < F::one() {
&mut small
} else {
&mut large
}
.push((g, p_g));
}
table.extend(large.iter().map(|&(g, _)| Unaliased(g)));
table.extend(small.iter().map(|&(l, _)| Unaliased(l)));
AliasTable {
range: Range::new(0, table.len()),
float: Range::new(F::zero(), F::one()),
table: table,
objs: objs,
}
}
}
impl<'a, T: 'a, F, R> Iterator for AliasTableIterator<'a, T, F, R>
where F: PartialOrd + SampleRange,
R: Rng
{
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
Some(self.table.pick(&mut self.rng))
}
fn size_hint(&self) -> (usize, Option<usize>) {
(std::usize::MAX, None)
}
}
impl<'a, T, F> IntoIterator for &'a AliasTable<T, F>
where F: Sized + PartialOrd + SampleRange
{
type Item = &'a T;
type IntoIter = AliasTableIterator<'a, T, F, rand::ThreadRng>;
fn into_iter(self) -> Self::IntoIter {
self.iter(rand::thread_rng())
}
}