using System;

using System.Linq;

​

using static MoreLinq.Extensions.GroupAdjacentExtension;

using static MoreLinq.Extensions.ScanRightExtension;

using static MoreLinq.Extensions.WindowRightExtension;

using static MoreLinq.Extensions.ForEachExtension;

​

var items = new[] {

    new {

        Id = 1, 

        Quantity = 1,

        StartDate = DateTime.Parse("2021-03-10"),

        EndDate = DateTime.Parse("2021-03-21")

    },

    new {

        Id = 2,

        Quantity = 1,

        StartDate = DateTime.Parse("2021-03-10"),

        EndDate = DateTime.Parse("2021-03-21")

    },

    new {

        Id = 3, 

        Quantity = 2,

        StartDate = DateTime.Parse("2021-03-10"), 

        EndDate = DateTime.Parse("2021-03-23")

    },

    new {

        Id = 4, 

        Quantity = 1,

        StartDate = DateTime.Parse("2021-03-10"), 

        EndDate = DateTime.Parse("2021-03-25")

    }

};

​

items

    // aggregate identical end dates

    .GroupAdjacent(x => x.EndDate)

    .Select(xs => xs.Aggregate(

        new {

            Ids = Enumerable.Empty<int>(),

            Quantity = 0,

            StartDate = default(DateTime),

            EndDate = default(DateTime)

        },

        (acc, curr) => new {

            Ids = acc.Ids.Append(curr.Id),

            Quantity = acc.Quantity + curr.Quantity,

            curr.StartDate,

            curr.EndDate

        }))

    // cut-off overlapping start dates

    .WindowRight(2)

    .Select(win => win.Count == 1

        ? win[0]

        : new {

            win[1].Ids,

            win[1].Quantity,

            StartDate = win[0].EndDate.AddDays(1),

            win[1].EndDate

        })

    // accumulate IDs and quantities

    .ScanRight((curr, prev) => new {

        Ids = curr.Ids.Concat(prev.Ids),

        Quantity = curr.Quantity + prev.Quantity,

        curr.StartDate,

        curr.EndDate

    })

    // print

    .Select(x => $"{{ Ids: [{string.Join(',', x.Ids)}], Quantity: {x.Quantity}, StartDate: {x.StartDate.ToShortDateString()}, EndDate: {x.EndDate.ToShortDateString()} }}")

    .ForEach(x => Console.WriteLine(x));

​