AdventOfCode/AdventOfCode.Extensions/EnumerableExtensions.cs

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

namespace AdventOfCode.Extensions;

public static class EnumerableExtensions
{
    public static IEnumerable<TValue[]> Combinations<TValue>(this IEnumerable<TValue> values, int count)
    {
        var pool = values.ToArray();
        var poolLength = pool.Length;
        if (count > poolLength)
            yield break;
        var indices = Range(0, count).ToArray();
        yield return GetCombination(indices, pool);
        while (true)
        {
            var validIndex = false;
            var currentIndex = 0;
            for(var i = count - 1; i >= 0; i--)
            {
                currentIndex = i;
                if (indices[i] != i + poolLength - count)
                {
                    validIndex = true;
                    break;
                }
            }
            if(!validIndex)
                yield break;
            
            indices[currentIndex] += 1;
            foreach (var j in Range(currentIndex + 1, count))
            {
                Index ix = j - 1;
                indices[j] = indices[ix] + 1;
            }

            yield return GetCombination(indices, pool);
        }

        static TValue[] GetCombination(int[] innerIndices, TValue[] innerPool) => 
            innerIndices.Select(i => innerPool[i]).ToArray();

        static IEnumerable<int> Range(int start, int end)
        {
            for (var i = start; i < end; i++)
                yield return i;
        }
    }
}