using System.Collections.Concurrent;

namespace molilian.core
{
    public static class SpecialBusinessRateLimiter
    {
        private static readonly ConcurrentDictionary<string, SlidingWindowState> States = new();

        public static bool TryAcquire(string bucketKey, int limit, TimeSpan window)
        {
            if (string.IsNullOrWhiteSpace(bucketKey)) return true;
            if (limit <= 0 || window <= TimeSpan.Zero) return true;

            var state = States.GetOrAdd(bucketKey, _ => new SlidingWindowState());
            var nowTicks = DateTime.UtcNow.Ticks;
            var minTicks = nowTicks - window.Ticks;

            lock (state.SyncRoot)
            {
                while (state.Timestamps.Count > 0 && state.Timestamps.Peek() < minTicks)
                {
                    state.Timestamps.Dequeue();
                }

                if (state.Timestamps.Count >= limit)
                {
                    return false;
                }

                state.Timestamps.Enqueue(nowTicks);
                return true;
            }
        }

        /// <summary>
        /// 归还 acquiredSinceTicks 之后占用的一个名额（请求未真正发生时使用）。
        /// 超过窗口时间的占用已自然释放，不再处理，避免误删其他请求的名额。
        /// </summary>
        public static void Release(string bucketKey, TimeSpan window, long acquiredSinceTicks)
        {
            if (string.IsNullOrWhiteSpace(bucketKey)) return;
            if (!States.TryGetValue(bucketKey, out var state)) return;
            if (DateTime.UtcNow.Ticks - acquiredSinceTicks >= window.Ticks) return;

            lock (state.SyncRoot)
            {
                var timestamps = state.Timestamps.ToArray();
                int index = Array.FindLastIndex(timestamps, t => t >= acquiredSinceTicks);
                if (index < 0) return;

                state.Timestamps.Clear();
                for (int i = 0; i < timestamps.Length; i++)
                {
                    if (i != index) state.Timestamps.Enqueue(timestamps[i]);
                }
            }
        }

        private sealed class SlidingWindowState
        {
            public object SyncRoot { get; } = new();
            public Queue<long> Timestamps { get; } = new();
        }
    }
}