MeloongCore/Extensions/EnumerableExtensions.cs

using System.Collections.Concurrent;

namespace MeloongCore.Extensions;
public static class EnumerableExtensions {

    #region Distinct

    /// <summary>
    /// 通过比较器进行去重。
    /// 该方法的效率较低，建议仅在小型列表上使用，或换用 DistinctBy。
    /// </summary>
    public static IEnumerable<T> Distinct<T>(this IList<T> list, Func<T, T, bool> comparer) {
        for (int i = 0; i < list.Count; i++) {
            if (!list.Skip(i + 1).Any(item => comparer(list[i], item))) yield return list[i];
        }
    }
    /// <summary>
    /// 按对象的指定值去重。
    /// </summary>
    public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> source, Func<T, TKey> selector) {
        var seen = new HashSet<TKey>();
        foreach (var element in source) {
            if (seen.Add(selector(element))) yield return element;
        }
    }
    /// <summary>
    /// 按对象的指定值去重。
    /// </summary>
    public static ParallelQuery<T> DistinctBy<T, TKey>(this ParallelQuery<T> source, Func<T, TKey> selector) {
        var seen = new ConcurrentDictionary<TKey, bool>();
        return source.Where(element => seen.TryAdd(selector(element), true));
    }

    #endregion

    #region MinBy / MaxBy

    /// <summary>
    /// 选择所有最大值对应的对象。
    /// 若没有元素则返回空列表。
    /// </summary>
    public static List<T> MaxByAll<T, C>(this IEnumerable<T> source, Func<T, C> selector) where C : IComparable<C> {
        var results = new List<T>();
        using var enumerator = source.GetEnumerator();
        if (!enumerator.MoveNext()) return results;
        T maxItem = enumerator.Current;
        C maxValue = selector(maxItem);
        results.Add(maxItem);
        while (enumerator.MoveNext()) {
            T currentItem = enumerator.Current;
            C currentValue = selector(currentItem);
            int comparisonResult = currentValue.CompareTo(maxValue);
            if (comparisonResult > 0) {
                maxValue = currentValue;
                results.Clear();
                results.Add(currentItem);
            } else if (comparisonResult == 0) {
                results.Add(currentItem);
            }
        }
        return results;
    }
    /// <summary>
    /// 选择所有最小值对应的对象。
    /// 若没有元素则返回空列表。
    /// </summary>
    public static List<T> MinByAll<T, C>(this IEnumerable<T> source, Func<T, C> selector) where C : IComparable<C> {
        var results = new List<T>();
        using var enumerator = source.GetEnumerator();
        if (!enumerator.MoveNext()) return results;
        T minItem = enumerator.Current;
        C minValue = selector(minItem);
        results.Add(minItem);
        while (enumerator.MoveNext()) {
            T currentItem = enumerator.Current;
            C currentValue = selector(currentItem);
            int comparisonResult = currentValue.CompareTo(minValue);
            if (comparisonResult < 0) {
                minValue = currentValue;
                results.Clear();
                results.Add(currentItem);
            } else if (comparisonResult == 0) {
                results.Add(currentItem);
            }
        }
        return results;
    }

    /// <summary>
    /// 选择最大值对应的对象。
    /// 若没有元素则返回 null。
    /// </summary>
    public static T? MaxBy<T, C>(this IEnumerable<T> source, Func<T, C> selector) where C : IComparable<C> {
        using var enumerator = source.GetEnumerator();
        if (!enumerator.MoveNext()) return default;
        T maxItem = enumerator.Current;
        C maxValue = selector(maxItem);
        while (enumerator.MoveNext()) {
            C value = selector(enumerator.Current);
            if (value.CompareTo(maxValue) <= 0) continue;
            maxItem = enumerator.Current;
            maxValue = value;
        }
        return maxItem;
    }
    /// <summary>
    /// 选择最小值对应的对象。
    /// 若没有元素则返回 null。
    /// </summary>
    public static T? MinBy<T, C>(this IEnumerable<T> List, Func<T, C> Selector) where C : IComparable<C> {
        using var enumerator = List.GetEnumerator();
        if (!enumerator.MoveNext()) { return default; }
        T minItem = enumerator.Current;
        C minValue = Selector(minItem);
        while (enumerator.MoveNext()) {
            C value = Selector(enumerator.Current);
            if (value.CompareTo(minValue) >= 0) { continue; }
            minItem = enumerator.Current;
            minValue = value;
        }
        return minItem;
    }

    #endregion

    #region Join

    /// <summary>
    /// 用指定的分割符将集合连接为字符串。
    /// </summary>
    public static string Join(this IEnumerable list, char split) {
        var builder = new StringBuilder();
        bool isFirst = true;
        foreach (var element in list) {
            if (isFirst)
                isFirst = false;
            else
                builder.Append(split);
            if (element is not null) builder.Append(element.ToString());
        }
        return builder.ToString();
    }
    /// <summary>
    /// 用指定的分割符将集合连接为字符串。
    /// </summary>
    public static string Join(this IEnumerable list, string split) {
        if (split.IsSingle()) return list.Join(split[0]);
        var builder = new StringBuilder();
        bool isFirst = true;
        foreach (var element in list) {
            if (isFirst)
                isFirst = false;
            else
                builder.Append(split);
            if (element is not null) builder.Append(element.ToString());
        }
        return builder.ToString();
    }

    #endregion

    /// <summary>
    /// 判断集合是否有且仅有一个元素。
    /// 在输入 null 时返回 false。
    /// </summary>
    public static bool IsSingle<T>([AllowNull] this IEnumerable<T> source) {
        if (source is null) return false;
        if (source is IList<T> list) return list.Count == 1;
        using var enumerator = source.GetEnumerator();
        if (!enumerator.MoveNext()) return false;
        if (enumerator.MoveNext()) return false;
        return true;
    }

    /// <summary>
    /// 对集合的每个元素执行指定操作。
    /// </summary>
    public static IEnumerable<T> ForAll<T>(this IEnumerable<T> source, Action<T> action) {
        foreach (T item in source) action(item);
        return source;
    }

}