using System;
using System.Collections.Generic;
using System.Linq;

namespace PivotExtensions
{
    public static class Ext
    {
        public static Dictionary<TKey1, Dictionary<TKey2, TValue>>
                        Pivot<TSource, TKey1, TKey2, TValue>
                        (
                            this IEnumerable<TSource> source,
                            Func<TSource, TKey1> key1Selector,
                            Func<TSource, TKey2> key2Selector,
                            Func<IEnumerable<TSource>, TValue> aggregate
                        )
        {
            return source.GroupBy(key1Selector)
                         .Select(
                            key1Group => new
                                {
                                    Key = key1Group.Key,
                                    Value = key1Group.GroupBy(key2Selector)
                                         .Select(
                                            key2Group => new
                                               {
                                                   K = key2Group.Key,
                                                   V = aggregate(key2Group)
                                               })
                                         .ToDictionary(e => e.K, o => o.V)
                                })
                         .ToDictionary(e => e.Key, o => o.Value);
        }
    }
}

var list = ExpenseDataSource.ExpensesDataSource();
var pivotList = list.Pivot(
                x =>
                    new
                    {
                        x.Date.Year,
                        x.Date.Month
                    },
                  x1 => x1.Department,
                  x2 => x2.Sum(x => x.Expenses));

var list2 = StudentsStatDataSource.CreateWeeklyReportDataSource();
var lst = list2.Pivot(
                x =>
                    new 
                    {
                        x.Id,
                        x.Name
                    },
              x1 => "Day " + x1.Date.Day,
              x2 => x2.First().IsPresent);