using System;
using System.Collections.Generic;

namespace Superlazy
{
    public static class SLLinqUtil
    {
        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");
            if (comparer == null) 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;
        }

        public static TSource MaxBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> selector)
        {
            return source.MaxBy(selector, null);
        }

        public static TSource MaxBy<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");
            if (comparer == null) comparer = Comparer<TKey>.Default;

            using var sourceIterator = source.GetEnumerator();
            if (!sourceIterator.MoveNext())
            {
                throw new InvalidOperationException("Sequence contains no elements");
            }

            var max = sourceIterator.Current;
            var maxKey = selector(max);
            while (sourceIterator.MoveNext())
            {
                var candidate = sourceIterator.Current;
                var candidateProjected = selector(candidate);
                if (comparer.Compare(candidateProjected, maxKey) > 0)
                {
                    max = candidate;
                    maxKey = candidateProjected;
                }
            }

            return max;
        }
    }
}