Discord-History-Tracker/app/Utils/Collections/LinqExtensions.cs

using System;
using System.Collections.Generic;

namespace DHT.Utils.Collections {
	public static class LinqExtensions {
		public static IEnumerable<TItem> DistinctByKeyStable<TItem, TKey>(this IEnumerable<TItem> collection, Func<TItem, TKey> getKeyFromItem) where TKey : IEquatable<TKey> {
			HashSet<TKey>? seenKeys = null;
			
			foreach (var item in collection) {
				seenKeys ??= new HashSet<TKey>();
				
				if (seenKeys.Add(getKeyFromItem(item))) {
					yield return item;
				}
			}
		}
	}
}