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|
use std::time::Duration;
fn randomize(backoff_ms: usize) -> usize {
let new_value = (fastrand::usize(750..=1250) * backoff_ms) / 1000;
if new_value == 0 {
backoff_ms
} else {
new_value
}
}
/// A utility to calculate steps for exponential backoff similar to how it's done in `git`.
pub struct Exponential<Fn> {
multiplier: usize,
max_multiplier: usize,
exponent: usize,
transform: Fn,
}
impl Default for Exponential<fn(usize) -> usize> {
fn default() -> Self {
Exponential {
multiplier: 1,
max_multiplier: 1000,
exponent: 1,
transform: std::convert::identity,
}
}
}
impl Exponential<fn(usize) -> usize> {
/// Create a new exponential backoff iterator that backs off in randomized, ever increasing steps.
pub fn default_with_random() -> Self {
Exponential {
multiplier: 1,
max_multiplier: 1000,
exponent: 1,
transform: randomize,
}
}
}
impl<Transform> Exponential<Transform>
where
Transform: Fn(usize) -> usize,
{
/// Return an iterator that yields `Duration` instances to sleep on until `time` is depleted.
pub fn until_no_remaining(&mut self, time: Duration) -> impl Iterator<Item = Duration> + '_ {
let mut elapsed = Duration::default();
let mut stop_next_iteration = false;
self.take_while(move |d| {
if stop_next_iteration {
false
} else {
elapsed += *d;
if elapsed > time {
stop_next_iteration = true;
}
true
}
})
}
}
impl<Transform> Iterator for Exponential<Transform>
where
Transform: Fn(usize) -> usize,
{
type Item = Duration;
fn next(&mut self) -> Option<Self::Item> {
let wait = Duration::from_millis((self.transform)(self.multiplier) as u64);
self.multiplier += 2 * self.exponent + 1;
if self.multiplier > self.max_multiplier {
self.multiplier = self.max_multiplier;
} else {
self.exponent += 1;
}
Some(wait)
}
}
#[cfg(test)]
mod tests {
use std::convert::TryInto;
use super::*;
const EXPECTED_TILL_SECOND: &[usize] = &[
1usize, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529,
576, 625, 676, 729, 784, 841, 900, 961, 1000, 1000,
];
#[test]
fn random_exponential_produces_values_in_the_correct_range() {
let mut num_identities = 0;
for (actual, expected) in Exponential::default_with_random().zip(EXPECTED_TILL_SECOND) {
let actual: usize = actual.as_millis().try_into().unwrap();
if actual == *expected {
num_identities += 1;
}
assert!(
actual * 1000 >= (expected - 1) * 750,
"value too small: {actual} < {expected}"
);
assert!(
actual * 1000 <= (expected + 1) * 1250,
"value too big: {actual} > {expected}"
);
}
assert!(
num_identities < EXPECTED_TILL_SECOND.len(),
"too many untransformed values: {num_identities}"
);
}
#[test]
fn how_many_iterations_for_a_second_of_waittime() {
let max = Duration::from_millis(1000);
assert_eq!(Exponential::default().until_no_remaining(max).count(), 14);
assert_eq!(
Exponential::default()
.until_no_remaining(max)
.reduce(|acc, n| acc + n)
.unwrap(),
Duration::from_millis(1015),
"a little overshoot"
);
}
#[test]
fn output_with_default_settings() {
assert_eq!(
Exponential::default().take(33).collect::<Vec<_>>(),
EXPECTED_TILL_SECOND
.iter()
.map(|n| Duration::from_millis(*n as u64))
.collect::<Vec<_>>()
);
}
}
|
