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using System.Collections;
namespace HyperBooru.Util;
public class MemoryCache<TKey, TValue> : IEnumerable<KeyValuePair<TKey, TValue>> where TKey : struct {
/// <summary>Maximum number of items this cache may hold (unlimited if null)</summary>
public int? MaxItems { get; init; }
/// <summary>Maximum amount of time an item may stay in cache before it is removed (unlimited if null)</summary>
public TimeSpan? MaxAge { get; init; }
/// <summary>Function that will be called to populate the cache in the event of a cache-miss</summary>
public Func<TKey, TValue?>? DataSource { get; init; }
private Dictionary<TKey, CacheItem> cache = new();
private struct CacheItem {
public DateTime CreateTime { get; set; }
public DateTime LastAccessTime { get; set; }
public TValue Value { get; set; }
}
public TValue this[TKey key] {
get => GetValue(key);
set {
Prune();
cache[key] = new() {
CreateTime = DateTime.Now,
LastAccessTime = DateTime.Now,
Value = value
};
}
}
public TValue GetValue(TKey key) {
bool success = cache.TryGetValue(key, out var result);
if(success) {
result.LastAccessTime = DateTime.Now;
if(MaxAge is null)
return result.Value;
if(result.CreateTime > DateTime.Now - MaxAge)
return result.Value;
}
if(DataSource is null)
throw new KeyNotFoundException();
TValue? value = DataSource(key);
if(value is null)
throw new KeyNotFoundException();
Prune();
cache[key] = new() {
CreateTime = DateTime.Now,
LastAccessTime = DateTime.Now,
Value = value
};
return value;
}
public bool TryGetValue(TKey key, out TValue? value) {
try {
value = GetValue(key);
return true;
} catch(KeyNotFoundException) {
value = default;
return false;
}
}
public bool Remove(TKey key) => cache.Remove(key);
public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() {
DateTime? expiry = MaxAge is null ? null : DateTime.Now - MaxAge;
foreach(var kv in cache) {
// Don't return expired cache items
if(expiry is not null)
if(kv.Value.CreateTime < expiry)
continue;
yield return new KeyValuePair<TKey, TValue>(kv.Key, kv.Value.Value);
}
}
IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
private void Prune() {
DateTime? expiry = MaxAge is null ? null : DateTime.Now - MaxAge;
// If an expiry time for cache items was
// specified, remove expired cache items.
if(expiry is not null) {
foreach(var kv in cache) {
if(kv.Value.CreateTime < expiry)
cache.Remove(kv.Key);
}
}
// If this cache was created with a maximum size,
// remove elements until that size is reached.
if(MaxItems is null || cache.Count() < MaxItems)
return;
var toRemove = cache
.OrderBy(kv => kv.Value.LastAccessTime)
.Take(cache.Count() - (int) MaxItems + 1)
.Select(kv => kv.Key)
.ToArray();
foreach(var key in toRemove)
cache.Remove(key);
}
}
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