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```
```use super::Weight;
use std::{collections::HashMap, hash::Hash};

#[derive(Copy, Clone, Debug)]
struct RandWeightItem<T> {
item: T,
weight: isize,
}

// Use the random algorithm to select next item.
#[derive(Default)]
pub struct RandWeight<T> {
items: Vec<RandWeightItem<T>>,
n: usize,
sum_of_weights: isize,
}

impl<T: Copy + PartialEq + Eq + Hash> RandWeight<T> {
pub fn new() -> Self {
RandWeight {
items: Vec::new(),
n: 0,
sum_of_weights: 0,
}
}
}

impl<T: Copy + PartialEq + Eq + Hash> Weight for RandWeight<T> {
type Item = T;

fn next(&mut self) -> Option<T> {
if self.n == 0 {
return None;
}
if self.n == 1 {
return Some(self.items[0].item);
}

let mut index = self.r.gen_range(0, self.sum_of_weights);
for item in &self.items {
index -= item.weight;
if index <= 0 {
return Some(item.item);
}
}

Some(self.items[self.n - 1].item)
}
fn add(&mut self, item: T, weight: isize) {
let weight_item = RandWeightItem { item, weight };

self.items.push(weight_item);
self.n += 1;
self.sum_of_weights += weight;
}

// all returns all items.
fn all(&self) -> HashMap<T, isize> {
let mut rt: HashMap<T, isize> = HashMap::new();
for w in &self.items {
rt.insert(w.item, w.weight);
}
rt
}

// remove_all removes all weighted items.
fn remove_all(&mut self) {
self.items.clear();
self.n = 0;
}

// reset resets the balancing algorithm.
fn reset(&mut self) {
}
}

#[cfg(test)]
mod tests {
use crate::{RandWeight, Weight};
use std::collections::HashMap;

#[test]
fn test_smooth_weight() {
let mut sw: RandWeight<&str> = RandWeight::new();

let mut results: HashMap<&str, usize> = HashMap::new();

for _ in 0..10000 {
let s = sw.next().unwrap();
// *results.get_mut(s).unwrap() += 1;
*results.entry(s).or_insert(0) += 1;
}

println!("{:?}", results);
// assert!(results["server1"] > 4000 && results["server1"] < 6000);
// assert!(results["server2"] > 1000 && results["server1"] < 3000);
// assert!(results["server3"] > 2000 && results["server1"] < 4000);
}
}
```