using System.Linq;

using System.Collections.Generic;

using System;

public class Program {

  public static (T? left, T? right) Fork<T>(IEnumerable<T?> source, 

                                            T? target, 

                                            IComparer<T>? comparer = default) {

    ArgumentNullException.ThrowIfNull(source);

​

    comparer ??= Comparer<T>.Default;

​

    ArgumentNullException.ThrowIfNull(comparer);

​

    T? left = default;

    T? secondLeft = default;

    T? right = default;

    T? secondRight = default;

​

    var hasLeft = false;

    var hasSecondLeft = false;

    var hasRight = false;

    var hasSecondRight = false;

​

    foreach (var item in source) {

      var compare = comparer.Compare(item, target);

​

      if (compare > 0) {

        if (!hasRight || comparer.Compare(item, right) <= 0) {

          (hasSecondRight, hasRight) = (hasRight, true);

          (secondRight, right) = (right, item);

        }

        else if (!hasSecondRight || comparer.Compare(item, secondRight) <= 0) {

          hasSecondRight = true;

          secondRight = item;

        }

      }

      else if (compare < 0) {

        if (!hasLeft || comparer.Compare(item, left) >= 0) {

          (hasSecondLeft, hasLeft) = (hasLeft, true);

          (secondLeft, left) = (left, item);

        }

        else if (!hasSecondLeft || comparer.Compare(item, secondLeft) <= 0) {

          hasSecondLeft = true;

          secondLeft = item;

        }

      }

    }

​

    return hasLeft && hasRight ? (left, right)

      : hasLeft ? (secondLeft, left)

      : (right, secondRight);

  }

  private static (string prevValue, string nextValue) FindClosestValues(double valueToFind,

                                                                        IEnumerable<(double time, string value, bool flag)> values) {

    var comparer = Comparer<(double time, string value, bool flag)>

      .Create((left, right) => left.time.CompareTo(right.time));

​

    var result = Fork(values.Where(item => item.flag), (valueToFind, "", true), comparer);

​

    return (result.left.Item2, result.right.Item2);

  } 

  public static void Main() {

    var listOfDoubleValues = new List<(double time, string value, bool flag)> {

      (1, "test1", true),

      (3, "test2", true),

      (5, "test3", true),

      (9, "test4", false),

      (78, "test5", true)

    };

    var testDataWithExpectedValues = new List<(double value, string expectedPrevValue, string expectedNextValue)> {

      (1, "test1", "test2"),

      (2, "test1", "test2"),

      (3, "test1", "test3"),

      (6, "test3", "test5"),

      (-50, "test1", "test2"),

      (77, "test3", "test5"),

      (800, "test3", "test5"),

    };

​

    var actual = testDataWithExpectedValues

      .Select(item => $"{item.value, 3} : {FindClosestValues(item.value, listOfDoubleValues)} expected ({item.expectedPrevValue}, {item.expectedNextValue})");

    Console.WriteLine(string.Join(Environment.NewLine, actual));   

  }

}