PKHeX/PKHeX.Core/Legality/Encounters/Verifiers/GenerationTraversal.cs

using System;

namespace PKHeX.Core
{
    public static class GenerationTraversal
    {
        /// <summary>
        /// Gets the generation numbers in descending order for iterating over.
        /// </summary>
        public static int[] GetVisitedGenerationOrder(PKM pkm, int origin)
        {
            if (pkm.Format < 3)
                return GetVisitedGenerationOrderGB(pkm, pkm.Format);
            if (pkm.VC)
                return GetVisitedGenerationOrderVC(pkm, origin);
            return GetVisitedGenerationOrder(pkm.Format, origin);
        }

        private static int[] GetVisitedGenerationOrderVC(PKM pkm, int origin)
        {
            // VC case: check transfer games in reverse order (8, 7..) then past games.
            int[] xfer = GetVisitedGenerationOrder(pkm.Format, 7);
            int[] past = GetVisitedGenerationOrderGB(pkm, origin);
            int end = xfer.Length;
            Array.Resize(ref xfer, xfer.Length + past.Length);
            past.CopyTo(xfer, end);
            return xfer;
        }

        private static readonly int[] G2 = { 2 };
        private static readonly int[] G12 = { 1, 2 };
        private static readonly int[] G21 = { 2, 1 };

        private static int[] GetVisitedGenerationOrderGB(PKM pkm, int originalGeneration)
        {
            if (originalGeneration == 2)
                return pkm.Korean ? G2 : G21;
            return G12; // RBY
        }

        private static int[] GetVisitedGenerationOrder(int start, int end)
        {
            if (end < 0)
                return Array.Empty<int>();
            if (start <= end)
                return new[] { start };
            var order = new int[start - end + 1];
            for (int i = 0; i < order.Length; i++)
                order[i] = start - i;
            return order;
        }
    }
}