Option Strict On
Option Explicit On
Public Class Form1

    Private Function ValidateStreamed(ByVal streamedStr As String, ByRef streamed As Integer) As Boolean

        Dim isOk As Boolean
        isOk = Integer.TryParse(streamedStr, streamed)
        
        ' validate inputs - process can be the same for each text box
        ' handle streamed
        If isOk Then
            ' check range
            If streamed < 0 OrElse streamed > 21600 Then
                ' bad value - indicate problem
                isOk = False
                MsgBox("Please enter a number of songs streamed from 0 to 21,600")
            End If
        Else
            MsgBox("Please enter a whole number of songs streamed")
        End If
        Return isOk

    End Function


    Private Function ValidateSaved(ByVal savedStr As String, ByRef saved As Integer) As Boolean

        Dim isOk As Boolean
        isOk = Integer.TryParse(savedStr, saved)

        ' validate inputs - process can be the same for each text box
        ' handle saved
        If isOk Then
            ' check range
            If saved < 0 OrElse saved > 20000 Then
                ' bad value - indicate problem
                isOk = False
                MsgBox("Please enter a number of songs saved from 0 to 20,000")
            End If
        Else
            MsgBox("Please enter a whole number of songs saved")
        End If
        Return isOk

    End Function

    Private Function CalcPremiumCost(ByVal streamed As Integer, ByVal saved As Integer) As Double
        Const freeStreamed As Integer = 1000
        Const freeSaved As Integer = 100
        Dim fee As Double
        Dim streamedCost As Double
        Dim savedCost As Double
        Dim final As Double
        fee = 50
        If streamed <= freeStreamed Then
            streamedCost = 0
        Else
            streamedCost = (streamed - freeStreamed) * 0.05
        End If
        If saved <= freeSaved Then
            savedCost = 0
        Else
            savedCost = (saved - freeSaved) * 0.5
        End If
        final = fee + streamedCost + savedCost
        Return final
    End Function

    Private Sub BtnPremium_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnPremium.Click

        Dim stream As Integer
        Dim save As Integer
        Dim isOkStreamed As Boolean
        Dim isOkSaved As Boolean
        Dim cost As Double
        isOkStreamed = ValidateStreamed(TxtStreamed.Text, stream)
        isOkSaved = ValidateSaved(TxtDownloaded.Text, save)
        If isOkStreamed AndAlso isOkSaved Then
            cost = CalcPremiumCost(stream, save)
            TxtPremium.Text = cost.ToString("c2")
        End If
    End Sub
End Class