using System;

using System.Linq;

using System.Collections.Generic;

using System.Text.RegularExpressions;

​

public class Program

{

​

    public static void Main()

    {

        var things = GetThings();

        //The original S/O answer

        var stackFlowAnswer = things

                    .Select(p => new { ThingId = p.ID, Age = DateTime.Now.Year - p.YearBuilt })

                    .GroupBy(p => p.Age > 65 ? 5 : (int)((p.Age - 16) / 10))

                    .Select(g => new { 

                        AgeGroup = g.Key == 5 ? "> 65" : string.Format("{0} - {1}", 

                        g.Key * 10 + 16, 

                        g.Key * 10 + 25), 

                        Count = g.Count() });

        //my attempt to increase the numbner of bucets and start from zero.

        var thingsByAge = things

                    .Select(p => new { ThingId = p.ID, Age = DateTime.Now.Year - p.YearBuilt })

                    .GroupBy(p => p.Age > 40 ? 8 : (int)((p.Age + 1) / 5))

                    .Select(g => new { 

                        AgeGroup = g.Key == 8 ? "> 40" : string.Format("{0} - {1}",  //"g.Key == 7" puts that group at index 7 (the last one)

                        g.Key,

                        g.Key * 5 + 4,

                        g.Key * 5 + 9,

                        g.Key * 5 + 14,

                        g.Key * 5 + 19,

                        g.Key * 5 + 24,

                        g.Key * 5 + 29,

                        g.Key * 5 + 34,

                        g.Key * 5 + 39), 

                        Count = g.Count() }

                    );

        var json = Newtonsoft.Json.JsonConvert.SerializeObject(thingsByAge, Newtonsoft.Json.Formatting.Indented);

        //var json = Newtonsoft.Json.JsonConvert.SerializeObject(stackFlowAnswer, Newtonsoft.Json.Formatting.Indented);

        Console.WriteLine(json);

    }

    public static List<Thing> GetThings()

    {

         var things = new List<Thing>();

            things.AddRange(MakeThings(2, 2019)); things.AddRange(MakeThings(12, 2018)); things.AddRange(MakeThings(2, 2017)); things.AddRange(MakeThings(0, 2016)); things.AddRange(MakeThings(0, 2016));

            things.AddRange(MakeThings(7, 2015)); things.AddRange(MakeThings(8, 2014)); things.AddRange(MakeThings(15, 2013)); things.AddRange(MakeThings(5, 2012)); things.AddRange(MakeThings(5, 2011));

            things.AddRange(MakeThings(0, 2010)); things.AddRange(MakeThings(0, 2009)); things.AddRange(MakeThings(0, 2008)); things.AddRange(MakeThings(0, 2007)); things.AddRange(MakeThings(0, 2006));

            things.AddRange(MakeThings(9, 2005)); things.AddRange(MakeThings(4, 2004)); things.AddRange(MakeThings(5, 2003)); things.AddRange(MakeThings(5, 2002)); things.AddRange(MakeThings(5, 2001));

            things.AddRange(MakeThings(1, 2000)); things.AddRange(MakeThings(5, 1999)); things.AddRange(MakeThings(5, 1998)); things.AddRange(MakeThings(5, 1997)); things.AddRange(MakeThings(5, 1996));

            things.AddRange(MakeThings(1, 1995));

            things.AddRange(MakeThings(1, 1990));

            things.AddRange(MakeThings(1, 1985));

            things.AddRange(MakeThings(1, 1980));

            things.AddRange(MakeThings(1, 1975));

            things.AddRange(MakeThings(1, 1970));

            things.AddRange(MakeThings(1, 1965));

            things.AddRange(MakeThings(1, 1960));

            things.AddRange(MakeThings(1, 1955));

            things.AddRange(MakeThings(1, 1950));

            things.AddRange(MakeThings(1, 1945));

        return things;

    }

    public static List<Thing> MakeThings(int count, int year)

    {

        var ret = new List<Thing>();

        for(var i = 0; i < count; i++)

        {

            ret.Add(new Thing(year));

        }

        return ret;

    }

    public class Thing

    {

        public Thing(int yearBuilt)

        {

            this.ID = Guid.NewGuid().ToString();

            this.YearBuilt = yearBuilt;

        }

        public string ID { get; set; }

        public int YearBuilt { get; set; }

    }

}