using System;

using System.Net.Http;

using System.Collections.Generic;

using System.Linq;

​

const short ResetValueAfterReproduction = 6;

const short NewFishReproductionDays = 8;

​

​

// Get data

var response = await new HttpClient().GetStringAsync("https://tompafireadventofcode.free.beeceptor.com/day6");

var reproductionValues = response.Split(',').Select(short.Parse);

​

// Step 1

​

var timePeriod = TimeSpan.FromDays(80);

​

var laternFishes1 = new List<short>(reproductionValues);

var count1 = RunReproductionPeriod(timePeriod, laternFishes1);

​

Console.WriteLine($"Step 1: {count1}");

​

// Step 2

​

var timePeriod2 = TimeSpan.FromDays(256);

​

var laternFishes2 = new List<short>(reproductionValues);

var count2 = RunReproductionPeriod(timePeriod2, laternFishes2);

​

Console.WriteLine($"Step 2: {count2}");

​

​

long RunReproductionPeriod(TimeSpan timePeriod, List<short> reproductionValues)

{

    var countByDaysToReproduction = reproductionValues.GroupBy(v => v)

        .ToDictionary(v => v.Key, v => v.LongCount());

​

    for (var day = 1; day <= timePeriod.Days; day++)

    {

        var changes = CalculateChanges(countByDaysToReproduction, day).ToList();

​

        ApplyChanges(countByDaysToReproduction, changes);

    }

​

    var count = countByDaysToReproduction.Values.Sum(v => v);

​

    return count;

}

​

IEnumerable<(short day, Func<long, long> change)> CalculateChanges(Dictionary<short, long> countByDaysToReproduction, int day)

{

    foreach (var entry in countByDaysToReproduction.OrderBy(x => x.Key))

    {

        var daysUntilReproduction = entry.Key;

​

        if (daysUntilReproduction == 0)

        {

            var moveToKey = ResetValueAfterReproduction;

​

            yield return (0, v => v - entry.Value);

            yield return (moveToKey, v => v + entry.Value);

​

            var fishesToAdd = entry.Value;

            yield return (NewFishReproductionDays, v => v + fishesToAdd);

        }

        else

        {

            var moveToKey = (short)(daysUntilReproduction - 1);

​

            yield return (daysUntilReproduction, v => v - entry.Value);

            yield return (moveToKey, v => v + entry.Value);

        }

    }

}

​

void ApplyChanges(Dictionary<short, long> countByDaysToReproduction, IEnumerable<(short day, Func<long, long> change)> changes)

{

    foreach (var change in changes)

    {

        if (!countByDaysToReproduction.ContainsKey(change.day))

        {

            countByDaysToReproduction.Add(change.day, 0);

        }

​

        countByDaysToReproduction[change.day] = change.change(countByDaysToReproduction[change.day]);

    }

}