PKHeX/PKHeX.Core/Util/RandUtil.cs

using System;
using System.Threading;

namespace PKHeX.Core
{
    public static partial class Util
    {
        // Multithread safe rand, ha
        public static Random Rand => _local.Value;
        private static int randomSeed = Environment.TickCount;

        private static readonly ThreadLocal<Random> _local = new(() =>
        {
            // threads should never really step on each other when starting, but we'll play it safe.
            var seed = Interlocked.Increment(ref randomSeed);
            var mix = (0x41C64E6D * seed) + 0x00006073; // why not
            return new Random(mix);
        });

        public static uint Rand32() => Rand32(Rand);
        public static uint Rand32(Random rnd) => (uint)rnd.Next(1 << 30) << 2 | (uint)rnd.Next(1 << 2);

        /// <summary>
        /// Shuffles the order of items within a collection of items.
        /// </summary>
        /// <typeparam name="T">Item type</typeparam>
        /// <param name="items">Item collection</param>
        public static void Shuffle<T>(Span<T> items) => Shuffle(items, 0, items.Length, Rand);

        /// <summary>
        /// Shuffles the order of items within a collection of items.
        /// </summary>
        /// <typeparam name="T">Item type</typeparam>
        /// <param name="items">Item collection</param>
        /// <param name="start">Starting position</param>
        /// <param name="end">Ending position</param>
        /// <param name="rnd">RNG object to use</param>
        public static void Shuffle<T>(Span<T> items, int start, int end, Random rnd)
        {
            for (int i = start; i < end; i++)
            {
                int index = i + rnd.Next(end - i);
                (items[index], items[i]) = (items[i], items[index]);
            }
        }
    }
}