using System;
using System.Collections.Generic;

namespace StardewModdingAPI.Framework.Utilities
{
    /// <summary>A memory cache with sliding expiry based on custom intervals, with no background processing.</summary>
    /// <typeparam name="TKey">The cache key type.</typeparam>
    /// <typeparam name="TValue">The cache value type.</typeparam>
    /// <remarks>This is optimized for small caches that are reset relatively rarely. Each cache entry is marked as hot (accessed since the interval started) or stale.
    /// When a new interval is started, stale entries are cleared and hot entries become stale.</remarks>
    internal class IntervalMemoryCache<TKey, TValue>
        where TKey : notnull
    {
        /*********
        ** Fields
        *********/
        /// <summary>The cached values that were accessed during the current interval.</summary>
        private Dictionary<TKey, TValue> HotCache = new();

        /// <summary>The cached values that will expire on the next interval.</summary>
        private Dictionary<TKey, TValue> StaleCache = new();


        /*********
        ** Public methods
        *********/
        /// <summary>Get a value from the cache, fetching it first if needed.</summary>
        /// <param name="cacheKey">The unique key for the cached value.</param>
        /// <param name="get">Get the latest data if it's not in the cache yet.</param>
        public TValue GetOrSet(TKey cacheKey, Func<TValue> get)
        {
            // from hot cache
            if (this.HotCache.TryGetValue(cacheKey, out TValue? value))
                return value;

            // from stale cache
            if (this.StaleCache.TryGetValue(cacheKey, out value))
            {
                this.HotCache[cacheKey] = value;
                return value;
            }

            // new value
            value = get();
            this.HotCache[cacheKey] = value;
            return value;
        }

        /// <summary>Start a new cache interval, removing any stale entries.</summary>
        public void StartNewInterval()
        {
            this.StaleCache.Clear();
            if (this.HotCache.Count is not 0)
                (this.StaleCache, this.HotCache) = (this.HotCache, this.StaleCache); // swap hot cache to stale
        }
    }
}