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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,99 @@ | ||
| use std::{time::Instant, mem::swap}; | ||
| use modality_ingest_client::types::TimelineId; | ||
|
|
||
| /// A Least Recently Used (LRU) cache of timeline information | ||
| pub struct TimelineLru { | ||
| // TODO(AJM): At the moment, the contents of the LRU cache are unsorted. | ||
| // | ||
| // Sorting them by time would be BAD, because there would be lots of churn. | ||
| // However, sorting them by `user_id` might be good, because then we could | ||
| // use a binary search for the fast path of query, but then we'd need to | ||
| // still do a linear search to determine the oldest item. | ||
| // | ||
| // For now, a linear search of a relatively small number (64-128) of items | ||
| // is likely to be more than fast enough. If it later turns out that users | ||
| // need a much larger LRU cache size (e.g. 1024+), we might consider making this | ||
| // optimization at that time. | ||
| data: Vec<LruItem>, | ||
| } | ||
|
|
||
| impl TimelineLru { | ||
| /// Create a timeline LRU cache with a maximum size of `cap`. | ||
| pub fn with_capacity(cap: usize) -> Self { | ||
| Self { | ||
| data: Vec::with_capacity(cap), | ||
| } | ||
| } | ||
|
|
||
| /// Check to see if the given `user_id` is known by the Lru | ||
| pub fn query(&mut self, user_id: u64) -> Result<TimelineId, LruToken> { | ||
| let full = self.data.len() >= self.data.capacity(); | ||
|
|
||
| if !full { | ||
| // If we're not full, we don't need to keep track of the oldest item. | ||
| // Just find it or don't. | ||
| match self.data.iter_mut().find(|li| li.user_id == user_id) { | ||
| Some(li) => { | ||
| li.last_use = Instant::now(); | ||
| Ok(li.timeline_id) | ||
| }, | ||
| None => Err(LruToken { val: LruError::DoesntExistNotFull }), | ||
| } | ||
| } else { | ||
| // Traverse the list, remembering the oldest slot we've seen. We'll either: | ||
| // | ||
| // * Find the item, bailing out early | ||
| // * Reach the end, in which case we've already found the oldest item | ||
| let mut oldest_time = Instant::now(); | ||
| let mut oldest_slot = 0; | ||
| for (idx, li) in self.data.iter_mut().enumerate() { | ||
| if li.user_id == user_id { | ||
| li.last_use = Instant::now(); | ||
| return Ok(li.timeline_id); | ||
| } | ||
|
|
||
| if oldest_time >= li.last_use { | ||
| oldest_time = li.last_use; | ||
| oldest_slot = idx; | ||
| } | ||
| } | ||
|
|
||
| // We reached the end, it doesn't exist. | ||
| Err(LruToken { val: LruError::DoesntExistReplace(oldest_slot) }) | ||
| } | ||
| } | ||
|
|
||
| pub fn insert(&mut self, user_id: u64, timeline_id: TimelineId, token: LruToken) { | ||
| let mut item = LruItem { | ||
| user_id, | ||
| timeline_id, | ||
| last_use: Instant::now(), | ||
| }; | ||
|
|
||
| match token.val { | ||
| LruError::DoesntExistNotFull => self.data.push(item), | ||
| LruError::DoesntExistReplace(idx) => { | ||
| if let Some(old) = self.data.get_mut(idx) { | ||
| swap(old, &mut item) | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| struct LruItem { | ||
| user_id: u64, | ||
| timeline_id: TimelineId, | ||
| last_use: Instant, | ||
| } | ||
|
|
||
| pub struct LruToken { | ||
| // Note: This is a struct with a private field to ensure that | ||
| // `query` must be called before `insert`. | ||
| val: LruError | ||
| } | ||
|
|
||
| enum LruError { | ||
| DoesntExistNotFull, | ||
| DoesntExistReplace(usize), | ||
| } |