Public Class Vector2D
    Public x As Double
    Public y As Double
    Public Sub New()
        x = 0.0
        y = 0.0
    End Sub

    Public Sub New(ByVal x1 As Double, ByVal y1 As Double)
        x = x1
        y = y1
    End Sub

    Public ReadOnly Property IsZero() As Boolean
        Get
            Return x * x + y * y < Double.MinValue
        End Get
    End Property
    Public Sub Zero()
        x = 0.0
        y = 0.0
    End Sub

    '返回向量的長度
    Public ReadOnly Property Length() As Double
        Get
            Return Math.Sqrt(x * x + y * y)
        End Get
    End Property

    '返回向量的長度的平方
    Public ReadOnly Property LengthSQ() As Double
        Get
            Return x * x + y * y
        End Get
    End Property

    Public Sub Normalize()
        Dim vector_length As Double = Length

        If vector_length > Double.Epsilon Then
            x /= vector_length
            y /= vector_length
        End If
    End Sub

    Public Function Dot(ByVal v2 As Vector2D) As Double
        Return x * v2.x + y * v2.y
    End Function

    Enum enumSign
        clockwise = 1   '順時針
        anticlockwise = -1  '逆時針
    End Enum

    '返回向量的方向
    Public Function Sign(ByVal v2 As Vector2D) As enumSign
        If (y * v2.x > x * v2.y) Then
            Return enumSign.anticlockwise
        Else
            Return enumSign.clockwise
        End If
    End Function

    '返回該向量的正交向量
    Public Function Perp() As Vector2D
        Return New Vector2D(-y, x)
    End Function

    '調整向量，使它的長度不超過max
    Public Sub Truncate(ByVal max As Double)
        If Length > max Then
            Normalize()
            'TODO:需要重寫操作符 *=
            '*this *= max
            Me.MutiplyEqual(max)
        End If
    End Sub

    Public Sub Reflect(ByVal norm As Vector2D)
        'TODO:需要重寫操作符 +=
        '*this += 2.0 * this->Dot(norm) * norm.GetReverse();
        Me.PlusEqual(norm.GetReverse().Mutiply(2.0 * Dot(norm)))
    End Sub

    Public Function GetReverse() As Vector2D
        Return New Vector2D(-x, -y)
    End Function

    Public Function Distance(ByVal v2 As Vector2D) As Double
        Dim ySeparation As Double = v2.y - y
        Dim xSeparation As Double = v2.x - x

        Return Math.Sqrt(ySeparation * ySeparation + xSeparation * xSeparation)
    End Function

    Public Function DistanceSq(ByVal v2 As Vector2D) As Double
        Dim ySeparation As Double = v2.y - y
        Dim xSeparation As Double = v2.x - x

        Return ySeparation * ySeparation + xSeparation * xSeparation
    End Function

    '''''''''''''''''''
    '模仿操作符重載程序
    '''''''''''''''''''

    '+=
    Public Sub PlusEqual(ByVal v As Vector2D)
        x += v.x
        y += v.y
    End Sub

    '-=
    Public Sub MinusEqual(ByVal v As Vector2D)
        x -= v.x
        y -= v.y
    End Sub

    '*=
    Public Sub MutiplyEqual(ByVal rhs As Double)
        x *= rhs
        y *= rhs
    End Sub

    '/=
    Public Sub DividedEqual(ByVal rhs As Double)
        x /= rhs
        y /= rhs
    End Sub

    '==
    Public Function IsEqual(ByVal v As Vector2D) As Boolean
        Return DoubleEqual(x, v.x) And DoubleEqual(y, v.y) ' System.Double.Equals(x, v.x) And System.Double.Equals(y, v.y)
    End Function

    '!=
    Public Function NotEqual(ByVal v As Vector2D) As Boolean
        Return x <> v.x And y <> v.y

    End Function

    Public Shared Function DoubleEqual(ByVal d1 As Double, ByVal d2 As Double)
        Return Math.Abs(d1 - d2) < 0.000000000001
    End Function

    '+
    Public Function Plus(ByVal v As Vector2D) As Vector2D
        Return New Vector2D(x + v.x, y + v.y)
    End Function

    '-
    Public Function Minus(ByVal v As Vector2D) As Vector2D
        Return New Vector2D(x - v.x, y - v.y)
    End Function

    '*
    Public Function Mutiply(ByVal rhs As Double) As Vector2D
        Dim v As New Vector2D(x, y)
        v.MutiplyEqual(rhs)
        Return v
    End Function

    '/
    Public Function Divided(ByVal rhs As Double) As Vector2D
        Dim v As New Vector2D(x, y)
        v.DividedEqual(rhs)
        Return v
    End Function
    '''''''''''''''''''
    '模仿操作符重載程序結束
    '''''''''''''''''''

    '''''''''''''''''''
    '實用程序
    ''''''''''''''''''
    Public Shared Function Vec2DNormalize(ByVal v As Vector2D) As Vector2D
        Dim vec As Vector2D = v

        Dim vector_length As Double = vec.Length()

        If vector_length > Double.Epsilon Then

            vec.x /= vector_length
            vec.y /= vector_length
        End If
        Return vec
    End Function

    Public Shared Function Vec2DDistance(ByVal v1 As Vector2D, ByVal v2 As Vector2D) As Double
        Dim ySeparation As Double = v2.y - v1.y
        Dim xSeparation As Double = v2.x - v1.x

        Return Math.Sqrt(ySeparation * ySeparation + xSeparation * xSeparation)
    End Function

    Public Shared Function Vec2DDistanceSQ(ByVal v1 As Vector2D, ByVal v2 As Vector2D) As Double
        Dim ySeparation As Double = v2.y - v1.y
        Dim xSeparation As Double = v2.x - v1.x

        Return ySeparation * ySeparation + xSeparation * xSeparation
    End Function

    Public Shared Function Vec2DLength(ByVal v As Vector2D) As Double
        Return Math.Sqrt(v.x * v.x + v.y * v.y)
    End Function

    Public Shared Function Vec2DLengthSQ(ByVal v As Vector2D) As Double
        Return v.x * v.x + v.y * v.y
    End Function

    Public Shared Function POINTStoVector(ByVal p As PointF) As Vector2D
        Return New Vector2D(p.X, p.Y)
    End Function

    Public Shared Function POINTtoVector(ByVal p As Point) As Vector2D
        Return New Vector2D(p.X, p.Y)
    End Function

    Public Shared Function VectorToPOINTS(ByVal v As Vector2D) As PointF
        Return New PointF(v.x, v.y)
    End Function

    Public Shared Function VectorToPOINT(ByVal v As Vector2D) As Point
        Return New Point(v.x, v.y)
    End Function

    Public Shared Sub WrapAround(ByVal pos As Vector2D, ByVal MaxX As Integer, ByVal MaxY As Integer)
        If pos.x > MaxX Then pos.x = 0.0

        If pos.x < 0 Then pos.x = MaxX

        If pos.y < 0 Then pos.y = MaxY

        If pos.y > MaxY Then pos.y = 0.0
    End Sub

    '測試點p是否在top_left與bot_rgt所在的區域
    Public Shared Function NotInsideRegion(ByVal p As Vector2D, ByVal top_left As Vector2D, ByVal bot_rgt As Vector2D) As Boolean
        Return (p.x < top_left.x) Or (p.x > bot_rgt.x) Or (p.y < top_left.y) Or (p.y > bot_rgt.y)
    End Function

    Public Shared Function InsideRegion(ByVal p As Vector2D, ByVal top_left As Vector2D, ByVal bot_rgt As Vector2D) As Boolean
        Return Not ((p.x < top_left.x) Or (p.x > bot_rgt.x) Or (p.y < top_left.y) Or (p.y > bot_rgt.y))
    End Function

    Public Shared Function InsideRegion(ByVal p As Vector2D, ByVal left As Integer, ByVal top As Integer, ByVal right As Integer, ByVal bottom As Integer) As Boolean
        Return Not ((p.x < left) Or (p.x > right) Or (p.y < top) Or (p.y > bottom))
    End Function

    Public Shared Function isSecondInFOVOfFirst(ByVal posFirst As Vector2D, ByVal facingFirst As Vector2D, ByVal posSecond As Vector2D, ByVal fov As Double) As Boolean
        Dim toTarget As Vector2D = Vec2DNormalize(posSecond.Minus(posFirst))
        Return facingFirst.Dot(toTarget) >= Math.Cos(fov / 2.0)
    End Function

    Public Function Copy() As Vector2D
        Return New Vector2D(x, y)
    End Function
End Class