using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;

namespace Vanara.PInvoke
{
	/// <summary>Gets a static field's name from its value and caches the list for faster lookups.</summary>
	public class StaticFieldValueHash
	{
		private static Dictionary<(Type, Type), IDictionary<int, string>> cache = new Dictionary<(Type, Type), IDictionary<int, string>>();

		/// <summary>Tries to get the name of a static field from it's value.</summary>
		/// <typeparam name="TType">The type of the type.</typeparam>
		/// <typeparam name="TFieldType">The type of the field type.</typeparam>
		/// <param name="value">The value for which to search.</param>
		/// <param name="fieldName">On success, the name of the field.</param>
		/// <returns><see langword="true"/> if the value was found, otherwise <see langword="false"/>.</returns>
		public static bool TryGetFieldName<TType, TFieldType>(TFieldType value, out string fieldName)
		{
			var tt = (typeof(TType), typeof(TFieldType));
			if (!cache.TryGetValue(tt, out var hash))
				cache.Add(tt, hash = typeof(TType).GetFields(BindingFlags.Public | BindingFlags.Static).Where(fi => fi.FieldType == typeof(TFieldType)).Distinct(FIValueComp<TFieldType>.Default).ToDictionary(fi => fi.GetValue(null).GetHashCode(), fi => fi.Name));
			return hash.TryGetValue(value.GetHashCode(), out fieldName);
		}

		private class FIValueComp<TFieldType> : IEqualityComparer<FieldInfo>
		{
			bool IEqualityComparer<FieldInfo>.Equals(FieldInfo x, FieldInfo y) => Comparer<TFieldType>.Default.Compare((TFieldType)x.GetValue(null), (TFieldType)y.GetValue(null)) == 0;

			int IEqualityComparer<FieldInfo>.GetHashCode(FieldInfo obj) => ((TFieldType)obj.GetValue(null)).GetHashCode();

			public static readonly FIValueComp<TFieldType> Default = new FIValueComp<TFieldType>();
		}
	}
}