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use crate::time::wheel::Stack;
use std::fmt;
/// Wheel for a single level in the timer. This wheel contains 64 slots.
pub(crate) struct Level<T> {
level: usize,
/// Bit field tracking which slots currently contain entries.
///
/// Using a bit field to track slots that contain entries allows avoiding a
/// scan to find entries. This field is updated when entries are added or
/// removed from a slot.
///
/// The least-significant bit represents slot zero.
occupied: u64,
/// Slots
slot: [T; LEVEL_MULT],
}
/// Indicates when a slot must be processed next.
#[derive(Debug)]
pub(crate) struct Expiration {
/// The level containing the slot.
pub(crate) level: usize,
/// The slot index.
pub(crate) slot: usize,
/// The instant at which the slot needs to be processed.
pub(crate) deadline: u64,
}
/// Level multiplier.
///
/// Being a power of 2 is very important.
const LEVEL_MULT: usize = 64;
impl<T: Stack> Level<T> {
pub(crate) fn new(level: usize) -> Level<T> {
// Rust's derived implementations for arrays require that the value
// contained by the array be `Copy`. So, here we have to manually
// initialize every single slot.
macro_rules! s {
() => {
T::default()
};
}
Level {
level,
occupied: 0,
slot: [
// It does not look like the necessary traits are
// derived for [T; 64].
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],
}
}
/// Finds the slot that needs to be processed next and returns the slot and
/// `Instant` at which this slot must be processed.
pub(crate) fn next_expiration(&self, now: u64) -> Option<Expiration> {
// Use the `occupied` bit field to get the index of the next slot that
// needs to be processed.
let slot = match self.next_occupied_slot(now) {
Some(slot) => slot,
None => return None,
};
// From the slot index, calculate the `Instant` at which it needs to be
// processed. This value *must* be in the future with respect to `now`.
let level_range = level_range(self.level);
let slot_range = slot_range(self.level);
// TODO: This can probably be simplified w/ power of 2 math
let level_start = now - (now % level_range);
let deadline = level_start + slot as u64 * slot_range;
debug_assert!(
deadline >= now,
"deadline={}; now={}; level={}; slot={}; occupied={:b}",
deadline,
now,
self.level,
slot,
self.occupied
);
Some(Expiration {
level: self.level,
slot,
deadline,
})
}
fn next_occupied_slot(&self, now: u64) -> Option<usize> {
if self.occupied == 0 {
return None;
}
// Get the slot for now using Maths
let now_slot = (now / slot_range(self.level)) as usize;
let occupied = self.occupied.rotate_right(now_slot as u32);
let zeros = occupied.trailing_zeros() as usize;
let slot = (zeros + now_slot) % 64;
Some(slot)
}
pub(crate) fn add_entry(&mut self, when: u64, item: T::Owned, store: &mut T::Store) {
let slot = slot_for(when, self.level);
self.slot[slot].push(item, store);
self.occupied |= occupied_bit(slot);
}
pub(crate) fn remove_entry(&mut self, when: u64, item: &T::Borrowed, store: &mut T::Store) {
let slot = slot_for(when, self.level);
self.slot[slot].remove(item, store);
if self.slot[slot].is_empty() {
// The bit is currently set
debug_assert!(self.occupied & occupied_bit(slot) != 0);
// Unset the bit
self.occupied ^= occupied_bit(slot);
}
}
pub(crate) fn pop_entry_slot(&mut self, slot: usize, store: &mut T::Store) -> Option<T::Owned> {
let ret = self.slot[slot].pop(store);
if ret.is_some() && self.slot[slot].is_empty() {
// The bit is currently set
debug_assert!(self.occupied & occupied_bit(slot) != 0);
self.occupied ^= occupied_bit(slot);
}
ret
}
}
impl<T> fmt::Debug for Level<T> {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("Level")
.field("occupied", &self.occupied)
.finish()
}
}
fn occupied_bit(slot: usize) -> u64 {
1 << slot
}
fn slot_range(level: usize) -> u64 {
LEVEL_MULT.pow(level as u32) as u64
}
fn level_range(level: usize) -> u64 {
LEVEL_MULT as u64 * slot_range(level)
}
/// Convert a duration (milliseconds) and a level to a slot position
fn slot_for(duration: u64, level: usize) -> usize {
((duration >> (level * 6)) % LEVEL_MULT as u64) as usize
}
#[cfg(all(test, not(loom)))]
mod test {
use super::*;
#[test]
fn test_slot_for() {
for pos in 0..64 {
assert_eq!(pos as usize, slot_for(pos, 0));
}
for level in 1..5 {
for pos in level..64 {
let a = pos * 64_usize.pow(level as u32);
assert_eq!(pos as usize, slot_for(a as u64, level));
}
}
}
}