iqdb_cache/

config.rs

//! Cache configuration.

use core::time::Duration;

/// Default cache capacity: the number of distinct recent searches whose results
/// are kept resident when no explicit capacity is given.
pub(crate) const DEFAULT_CAPACITY: usize = 1024;

/// Which entry an eviction discards when the cache is full.
///
/// All four policies keep the cache within its capacity; they differ only in
/// *which* entry they sacrifice to make room. The default is [`Lru`](Self::Lru),
/// the best general-purpose choice for search workloads.
///
/// # Examples
///
/// ```
/// use iqdb_cache::EvictionPolicy;
///
/// assert_eq!(EvictionPolicy::default(), EvictionPolicy::Lru);
/// ```
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[non_exhaustive]
pub enum EvictionPolicy {
    /// **Least Recently Used.** Discards the entry untouched for the longest.
    /// The default: it adapts to shifting query hot-sets and is the strongest
    /// general default for search.
    #[default]
    Lru,
    /// **Least Frequently Used.** Discards the entry with the fewest lifetime
    /// hits, breaking ties by least-recently-used. Favors stable, skewed
    /// workloads where a few queries dominate.
    Lfu,
    /// **First In, First Out.** Discards the oldest *inserted* entry regardless
    /// of access. The cheapest policy; a good fit when reuse is uniform.
    Fifo,
    /// **Adaptive Replacement Cache.** Balances recency and frequency
    /// automatically using ghost lists, tuning itself between LRU- and
    /// LFU-like behavior as the workload shifts.
    Arc,
}

/// Tuning for a [`CachedIndex`](crate::CachedIndex) — the Tier-2 configured path.
///
/// Build one with [`CacheConfig::new`] and the chaining setters, then hand it to
/// [`CachedIndex::with_config`](crate::CachedIndex::with_config). Every setting
/// has a sensible default, so `CacheConfig::new()` alone is a valid config.
///
/// | Setting | Default | Meaning |
/// |---|---|---|
/// | [`capacity`](CacheConfig::capacity) | `1024` | Max distinct searches cached; `0` disables caching. |
/// | [`ttl`](CacheConfig::ttl) | none | Optional per-entry time-to-live; expired results are recomputed. |
/// | [`policy`](CacheConfig::policy) | `Lru` | Which entry to evict when full. |
///
/// # Examples
///
/// ```
/// use std::time::Duration;
///
/// use iqdb_cache::{CacheConfig, CachedIndex, EvictionPolicy};
///
/// let config = CacheConfig::new()
///     .capacity(4096)
///     .ttl(Duration::from_secs(30))
///     .policy(EvictionPolicy::Lfu);
///
/// let cached = CachedIndex::with_config(iqdb_cache::doc_stub::stub_index(), config);
/// assert_eq!(cached.capacity(), 4096);
/// assert_eq!(cached.ttl(), Some(Duration::from_secs(30)));
/// assert_eq!(cached.policy(), EvictionPolicy::Lfu);
/// ```
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct CacheConfig {
    /// Maximum number of distinct cached searches.
    pub(crate) capacity: usize,
    /// Optional per-entry time-to-live.
    pub(crate) ttl: Option<Duration>,
    /// Eviction policy applied when the cache is full.
    pub(crate) policy: EvictionPolicy,
}

impl CacheConfig {
    /// A configuration with the default capacity (1024), no TTL, and the LRU
    /// policy.
    ///
    /// # Examples
    ///
    /// ```
    /// use iqdb_cache::CacheConfig;
    ///
    /// let config = CacheConfig::new();
    /// // Equivalent to `CachedIndex::new(..)`'s defaults.
    /// # let _ = config;
    /// ```
    #[must_use]
    pub fn new() -> Self {
        Self {
            capacity: DEFAULT_CAPACITY,
            ttl: None,
            policy: EvictionPolicy::Lru,
        }
    }

    /// Sets the eviction policy applied when the cache is full.
    ///
    /// # Examples
    ///
    /// ```
    /// use iqdb_cache::{CacheConfig, EvictionPolicy};
    ///
    /// let config = CacheConfig::new().policy(EvictionPolicy::Arc);
    /// # let _ = config;
    /// ```
    #[must_use]
    pub fn policy(mut self, policy: EvictionPolicy) -> Self {
        self.policy = policy;
        self
    }

    /// Sets the maximum number of distinct cached searches.
    ///
    /// A `capacity` of `0` disables caching: searches pass straight through.
    ///
    /// # Examples
    ///
    /// ```
    /// use iqdb_cache::CacheConfig;
    ///
    /// let config = CacheConfig::new().capacity(256);
    /// # let _ = config;
    /// ```
    #[must_use]
    pub fn capacity(mut self, capacity: usize) -> Self {
        self.capacity = capacity;
        self
    }

    /// Sets a per-entry time-to-live: a cached result older than `ttl` is
    /// treated as a miss and recomputed.
    ///
    /// TTL bounds staleness from changes the wrapper cannot observe (for
    /// example, the wrapped index mutated through another handle). Mutations
    /// *through* the wrapper already invalidate exactly, independent of TTL.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::time::Duration;
    ///
    /// use iqdb_cache::CacheConfig;
    ///
    /// let config = CacheConfig::new().ttl(Duration::from_secs(60));
    /// # let _ = config;
    /// ```
    #[must_use]
    pub fn ttl(mut self, ttl: Duration) -> Self {
        self.ttl = Some(ttl);
        self
    }

    /// Clears any previously set TTL, so cached results never expire on time
    /// (only on mutation).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::time::Duration;
    ///
    /// use iqdb_cache::CacheConfig;
    ///
    /// let config = CacheConfig::new().ttl(Duration::from_secs(60)).no_ttl();
    /// # let _ = config;
    /// ```
    #[must_use]
    pub fn no_ttl(mut self) -> Self {
        self.ttl = None;
        self
    }
}

impl Default for CacheConfig {
    fn default() -> Self {
        Self::new()
    }
}