Public Class ObjectiveFunction

    Inherits LibOptimization.Optimization.absObjectiveFunction

    Private fxb As Func(Of Double(), Double)

    Private fxg As Func(Of Double(), Double())

    Private epsilon As Double

    Private n As Integer

    Public Sub New(func1 As Func(Of Double(), Double), deriv1 As Func(Of Double(), Double()), eps As Double, nvars As Integer)

        fxb = func1
        fxg = deriv1
        epsilon = eps
        n = nvars

    End Sub

    Public Overrides Function F(ByVal a As List(Of Double)) As Double

        Return fxb.Invoke(a.ToArray())

    End Function

    Public Overrides Function Gradient(x As List(Of Double), Optional h As Double = 0.00000001) As List(Of Double)

        If fxg IsNot Nothing Then
            Return fxg.Invoke(x.ToArray()).ToList()
        Else
            Return FunctionGradient(x.ToArray()).ToList()
        End If

    End Function

    Public Overrides Function Hessian(x As List(Of Double), Optional h As Double = 0.00000001) As List(Of List(Of Double))

        Return FunctionHessian(x.ToArray())

    End Function

    Public Overrides Function NumberOfVariable() As Integer

        Return n

    End Function

    Private Function FunctionGradient(ByVal x() As Double) As Double()

        Dim f1, f2 As Double
        Dim g(x.Length - 1), x1(x.Length - 1), x2(x.Length - 1) As Double
        Dim j, k As Integer

        For j = 0 To x.Length - 1
            For k = 0 To x.Length - 1
                x1(k) = x(k)
                x2(k) = x(k)
            Next
            If x(j) <> 0.0# Then
                x1(j) = x(j) * (1.0# + epsilon)
                x2(j) = x(j) * (1.0# - epsilon)
            Else
                x1(j) = x(j) + epsilon
                x2(j) = x(j) - epsilon
            End If
            f1 = fxb.Invoke(x1)
            f2 = fxb.Invoke(x2)
            g(j) = (f2 - f1) / (x2(j) - x1(j))
        Next

        Return g

    End Function

    Private Function FunctionHessian(ByVal x() As Double) As List(Of List(Of Double))

        Dim n = x.Length - 1

        Dim f2() As Double
        Dim f3() As Double
        Dim x2(n), x3(n) As Double
        Dim k As Integer
        Dim h As New List(Of List(Of Double))

        For i = 0 To n
            For j = 0 To n
                If i <> j Then
                    x2(j) = x(j)
                    x3(j) = x(j)
                Else
                    x2(j) = x(j) * (1 + epsilon)
                    x3(j) = x(j) * (1 - epsilon)
                End If
            Next
            f2 = FunctionGradient(x2)
            f3 = FunctionGradient(x3)
            Dim l As New List(Of Double)
            For k = 0 To n
                l.Add((f2(k) - f3(k)) / (x2(i) - x3(i)))
            Next
            h.Add(l)
        Next

        Return h

    End Function

End Class