1 files changed, 98 insertions, 0 deletions
diff --git a/Util/MemoryCache.cs b/Util/MemoryCache.cs
new file mode 100644
index 0000000..ba314cf
--- /dev/null
+++ b/Util/MemoryCache.cs
@@ -0,0 +1,98 @@
+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, (DateTime createTime, TValue value)> cache = new();
+
+    public TValue this[TKey key] {
+        get => GetValue(key);
+        set {
+            Prune();
+            cache[key] = (DateTime.Now, value);
+        }
+    }
+
+    public TValue GetValue(TKey key) {
+        bool success = cache.TryGetValue(key, out var result);
+        if(success) {
+            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] = (DateTime.Now, 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.createTime)
+            .Take(cache.Count() - (int) MaxItems + 1)
+            .Select(kv => kv.Key)
+            .ToArray();
+
+        foreach(var key in toRemove)
+            cache.Remove(key);
+    }
+}