Added "Touch" method for sliding expiration support.

Author: alex-jitbit

FastCache/FastCache.cs

@@ -274,6 +274,19 @@ public TValue GetOrAdd<TArg>(TKey key, Func<TKey, TArg, TValue> valueFactory, Ti
 		public TValue GetOrAdd(TKey key, TValue value, TimeSpan ttl)
 			=> GetOrAddCore(key, static (_, v) => v, value, (long)ttl.TotalMilliseconds);
 
+		/// <summary>
+		/// Resets the TTL for an existing (non-expired) item, essentially implementing "sliding expiration"
+		/// </summary>
+		/// <param name="key">The key to touch</param>
+		/// <param name="ttl">The new TTL to set</param>
+		public void Touch(TKey key, TimeSpan ttl)
+		{
+			if (_dict.TryGetValue(key, out var ttlValue))
+			{
+				ttlValue.ResetExpiration(ttl);
+			}
+		}
+
 		/// <summary>
 		/// Tries to remove item with the specified key
 		/// </summary>
@@ -368,6 +381,11 @@ public TtlValue(TValue value, TimeSpan ttl)
 			[MethodImpl(MethodImplOptions.AggressiveInlining)]
 			public bool IsExpired(long currTime) => currTime > TickCountWhenToKill;
 
+			public void ResetExpiration(TimeSpan ttl)
+			{
+				TickCountWhenToKill = Environment.TickCount64 + (long)ttl.TotalMilliseconds;
+			}
+
 			/// <summary>
 			/// Updates the value and TTL only if the item is expired, using a factory function
 			/// </summary>

UnitTests/UnitTests.cs

@@ -244,5 +244,19 @@ public async Task TestTtlExtended()
 			Assert.IsTrue(_cache.TryGet(42, out int result2)); //still not evicted
 			Assert.AreEqual(42, result2);
 		}
+
+		[TestMethod]
+		public async Task TestTouch()
+		{
+			using var _cache = new FastCache<int, int>();
+			_cache.AddOrUpdate(42, 42, TimeSpan.FromMilliseconds(200));
+
+			await Task.Delay(150);
+			_cache.Touch(42, TimeSpan.FromMilliseconds(200)); //reset TTL
+
+			await Task.Delay(150); //300ms total, would have expired without Touch
+			Assert.IsTrue(_cache.TryGet(42, out int v));
+			Assert.AreEqual(42, v);
+		}
 	}
 }