cs2576

Module Module1
    Const K As Integer = 50

    Private Count As Integer

    Function RandK() As Integer
        Static r As New Random
        Count += 1
        Return r.Next(1, K + 1)
    End Function

    Private Function Rand2K1() As Integer
        Return RandK() * 2 - If(RandK() > K / 2, 1, 0)
    End Function

    Function Rand2Klist() As IEnumerable(Of Integer)
        Count = 0
        Dim lst = Enumerable.Range(1, 2 * K).ToList
        Dim result As New List(Of Integer)

        Dim j = Rand2K1() - 1

        For i = 0 To 2 * K - 1
            Debug.Assert(lst.Count = (2 * K - i))
            Debug.Assert(result.Count = i)
            Dim l = (j + RandK() + RandK() - 1) Mod (2 * K - i)
            result.Add(lst(l))
            lst.RemoveAt(l)
            j += RandK()
        Next
        Debug.Assert(lst.Count = 0)
        Return result
    End Function

    Private sum(0 To 2 * K - 1), pos(0 To 2 * K - 1) As Integer

    Sub CheckAndStats(ByVal lst As IEnumerable(Of Integer))
        Dim chkLst = lst.ToArray
        Array.Sort(chkLst)
        Debug.Assert(chkLst.Length = 2 * K)
        Debug.Assert(chkLst(0) = 1)
        Debug.Assert(chkLst(2 * K - 1) = 2 * K)

        chkLst = lst.ToArray
        For i = 0 To 2 * K - 1
            pos(chkLst(i) - 1) += i
            sum(i) += chkLst(i)
        Next
    End Sub

    Sub PrintStats()
        Console.WriteLine(" {0,4} {1,7} {2,7}", "no.", "Pos", "Avg")
        For i = 0 To 2 * K - 1
            Console.WriteLine(" {0,4} {1,7:f3} {2,7:f3}", i + 1, pos(i) / M - K, sum(i) / M - K)
        Next
    End Sub

    Const M As Integer = 10000

    Sub Main()
        Dim someLists(0 To M) As IEnumerable(Of Integer)
        For i = 0 To M
            someLists(i) = Rand2Klist.ToList
            Debug.Assert(Count = 6 * K + 2)
        Next
        For Each l In someLists
            CheckAndStats(l)
        Next
        PrintStats()
        If Debugger.IsAttached Then _
            Console.ReadLine()
    End Sub

End Module