using System;

using System.Collections.Generic;

using System.Linq;

​

class IntPair(int x, int y)

{

    public int X = x;

    public int Y = y;

}

​

public static class MyExtensions

{

    public static IEnumerable<T> Scan<T>(this IEnumerable<T> collection,  Func<T, int> accessor, Func<T, bool> predicate, Func<T, int, T> result)

    {

        int total = 0;

        foreach (var element in collection)

        {

            if (predicate(element))

            {

                yield return result(element, total += accessor(element));

            }

            else

            {

                yield return element;

            }

        }

    }

}

​

public class Program

{

    public static void Main()

    {

        List<IntPair> values = [

            new(0, 1),

            new(1, 2),

            new(2, 3),

            new(3, 4),

        ];

        var result = values.Scan(elem => elem.Y, elem => elem.X != 3, (elem, tempResult) => { elem.Y = tempResult; return elem; });

        Console.WriteLine($"[\n\t{string.Join("\n\t", result.Select(elem => $"X: {elem.X} Y: {elem.Y}"))}\n]");

    }

}