シーケンスの比較を行う `IComparer<T>` 実装。

EnumerableComparer.cs

#nullable enable

using System;
using System.Collections.Generic;

namespace Commons {

    /// <summary>
    /// シーケンスの比較を行う <see cref="IComparer{T}"/> 実装。
    /// </summary>
    /// <typeparam name="T">比較対象となるシーケンスの要素型。</typeparam>
    public class EnumerableComparer<T> : IComparer<IEnumerable<T>?> {
        private readonly IComparer<T> _elementComparer;

        /// <summary>
        /// Initializes a new instance of the <see cref="EnumerableComparer{T}"/> class.
        /// </summary>
        /// <param name="elementComparer">要素の比較子。</param>
        /// <exception cref="ArgumentNullException"><paramref name="elementComparer"/> is <c>null</c>.</exception>
        public EnumerableComparer(IComparer<T> elementComparer) {
            _elementComparer = elementComparer ?? throw new ArgumentNullException(nameof(elementComparer));
        }

        /// <inheritdoc />
        public int Compare(IEnumerable<T>? x, IEnumerable<T>? y) {
            if (x == y) { return 0; }
            if (x is null) { return -1; }
            if (y is null) { return 1; }

            using IEnumerator<T> xIter = x.GetEnumerator();
            using IEnumerator<T> yIter = y.GetEnumerator();

            while (true) {
                switch ((xIter.MoveNext(), yIter.MoveNext())) {
                    case (false, false):
                        return 0;

                    case (false, true):
                        return -1;

                    case (true, false):
                        return 1;
                }

                switch (_elementComparer.Compare(xIter.Current, yIter.Current)) {
                    case < 0:
                        return -1;

                    case > 0:
                        return 1;
                }
            }
        }
    }
}

#nullable restore