demo5/DatesLibrary/Calculations.cs

using System;
using System.Collections.Generic;

namespace DatesLibrary;

public class Calculations
{
    public static string[] AvailablePeriods(
        TimeSpan[] startTimes,
        int[] durations,
        TimeSpan beginWorkingTime,
        TimeSpan endWorkingTime,
        int consultationTime)
    {
        List<string> availablePeriods = new();
        TimeSpan workTime = beginWorkingTime;
        int i = 0;

        while (workTime < endWorkingTime)
        {
            TimeSpan nextBusyStart = (i < startTimes.Length) ? startTimes[i] : endWorkingTime;
            int availableMinutes = (int)(nextBusyStart - workTime).TotalMinutes;
            
            if (availableMinutes < consultationTime)
            {
                if (i < startTimes.Length)
                {
                    workTime = startTimes[i].Add(TimeSpan.FromMinutes(durations[i]));
                    i++;
                }
                else
                {
                    break;
                }
                continue;
            }

            while (availableMinutes >= consultationTime)
            {
                TimeSpan nextAvailableEnd = workTime.Add(TimeSpan.FromMinutes(consultationTime));

                if (nextAvailableEnd > endWorkingTime)
                    break;

                availablePeriods.Add($"{workTime:hh\\:mm}-{nextAvailableEnd:hh\\:mm}");

                workTime = nextAvailableEnd;
                availableMinutes -= consultationTime;
            }

            if (i < startTimes.Length)
            {
                workTime = startTimes[i].Add(TimeSpan.FromMinutes(durations[i]));
                i++;
            }
            else
            {
                break;
            }
        }

        return availablePeriods.ToArray();
    }
}