Utils.cs

using System;
using System.Collections.Generic;

namespace Hashcode2022
{
    public static class Utils
    {
        public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector)
        {
            return source.MinBy(selector, null);
        }

        public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
            Func<TSource, TKey> selector, IComparer<TKey> comparer)
        {
            if (source == null) throw new ArgumentNullException("source");
            if (selector == null) throw new ArgumentNullException("selector");
            comparer ??= Comparer<TKey>.Default;

            using (var sourceIterator = source.GetEnumerator())
            {
                if (!sourceIterator.MoveNext())
                {
                    throw new InvalidOperationException("Sequence contains no elements");
                }
                var min = sourceIterator.Current;
                var minKey = selector(min);
                while (sourceIterator.MoveNext())
                {
                    var candidate = sourceIterator.Current;
                    var candidateProjected = selector(candidate);
                    if (comparer.Compare(candidateProjected, minKey) < 0)
                    {
                        min = candidate;
                        minKey = candidateProjected;
                    }
                }
                return min;
            }
        }
    }
}