Background Prefetching for Kavita+ (#2707)

API/Helpers/RateLimiter.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+
+namespace API.Helpers;
+
+public class RateLimiter(int maxRequests, TimeSpan duration, bool refillBetween = true)
+{
+    private readonly Dictionary<string, (int Tokens, DateTime LastRefill)> _tokenBuckets = new();
+    private readonly object _lock = new();
+
+    public bool TryAcquire(string key)
+    {
+        lock (_lock)
+        {
+            if (!_tokenBuckets.TryGetValue(key, out var bucket))
+            {
+                bucket = (Tokens: maxRequests, LastRefill: DateTime.UtcNow);
+                _tokenBuckets[key] = bucket;
+            }
+
+            RefillTokens(key);
+
+            lock (_lock)
+            {
+
+                if (_tokenBuckets[key].Tokens > 0)
+                {
+                    _tokenBuckets[key] = (Tokens: _tokenBuckets[key].Tokens - 1, LastRefill: _tokenBuckets[key].LastRefill);
+                    return true;
+                }
+            }
+
+            return false;
+        }
+    }
+
+    private void RefillTokens(string key)
+    {
+        lock (_lock)
+        {
+            var now = DateTime.UtcNow;
+            var timeSinceLastRefill = now - _tokenBuckets[key].LastRefill;
+            var tokensToAdd = (int) (timeSinceLastRefill.TotalSeconds / duration.TotalSeconds);
+
+            // Refill the bucket if the elapsed time is greater than or equal to the duration
+            if (timeSinceLastRefill >= duration)
+            {
+                _tokenBuckets[key] = (Tokens: maxRequests, LastRefill: now);
+                Console.WriteLine($"Tokens Refilled to Max: {maxRequests}");
+            }
+            else if (tokensToAdd > 0 && refillBetween)
+            {
+                _tokenBuckets[key] = (Tokens: Math.Min(maxRequests, _tokenBuckets[key].Tokens + tokensToAdd), LastRefill: now);
+                Console.WriteLine($"Tokens Refilled: {_tokenBuckets[key].Tokens}");
+            }
+        }
+    }
+}
+