已知条件求圆心角

在一整圆上有两个任意点，已知起点坐标(X1,Y1),终点坐标（X2,Y2),圆心坐标（XC,YC),半径（R)，起点终点间的直线距离（L)，起点和终点将圆打断为两段弧长不相等的圆弧，已知弧长（C1,C2)

求如果起点到终点是顺时针方向的话，起点到终点间圆弧的圆心角是大于180度还是小于180度
求如果起点到终点是逆时针方向的话，起点到终点间圆弧的圆心角是大于180度还是小于180度

求一高效的算法，谢谢

[解决办法]

VB code

 
  
 Const PI = 3.1415927 
  
 Private Function Angle(x1 As Single, y1 As Single, x2 As Single, y2 As Single, xc As Single, yc As Single, bisClockwise As Boolean) 
  
   Dim x3 As Single, y3 As Single 
   Dim x4 As Single, y4 As Single 
    
   '//transform your coordinate 
   x3 = x1 - xc: y3 = y1 - yc: x4 = x2 - xc: y4 = y2 - yc 
  
   '//From the first point to the 2nd poinit, use cross product to check the angle 
   Dim z As Single, bisLessThanPi As Boolean 
    
   If bisClockwise Then 
     z = x3 * y4 - y3 * x4 
   Else 
      z = y3 * x4 - x3 * y4 
   End If 
    
   If z > 0 Then 
     bisLessThanPi = True 
   Else 
     bisLessThanPi = False 
   End If 
    
   '//Get the angle between 
   Dim cos As Double 
   cos = (x3 * x4 + y3 * y4) / (x3 ^ 2 + y3 ^ 2) 
    
   '//check for 0 or 180 degrees 
   If Round(Abs(cos), 3)  < 0.001 Then 
     Angle = PI / 2 
   ElseIf Round(Abs(Abs(cos) - 1), 3)  < 0.001 And cos  < 0 Then 
     Angle = PI 
   ElseIf Round(Abs(Abs(cos) - 1), 3)  < 0.001 And cos > 0 Then 
     Angle = 0 
   Else 
     Angle = Arccos(cos) 
   End If 
    
   If bisLessThanPi = False Then 
     Angle = 2 * PI - Angle 
   End If 
    
   '//Convert to degree 
   Angle = Angle * 180 / PI 
  
 End Function 
  
 Private Function Arccos(x As Double) As Double 
   Arccos = PI / 2 - Atn(x / Sqr(-x * x + 1)) 
 End Function 
  
 

VB code

Option ExplicitPrivate Sub Rotate(X As Single, Y As Single, ByVal sinT As Single, ByVal cosT As Single)    Dim dX As Single    Dim dY As Single    dX = X    dY = Y    X = dX * cosT + dY * sinT    Y = -dX * sinT + dY * cosTEnd SubPrivate Sub Command1_Click()    Dim X1 As Single    Dim X2 As Single    Dim Y1 As Single    Dim Y2 As Single    Dim XC As Single    Dim YC As Single    Dim R As Single    Dim sinT As Single    Dim cosT As Single        Me.Cls    Me.Line (-100, 0)-(100, 0)    Me.Line (0, -100)-(0, 100)        X1 = 61.132    Y1 = -44.549    X2 = 54.107    Y2 = -52.519    XC = 81.698    YC = -69.759        '平移'    X2 = X2 - X1    Y2 = Y2 - Y1    XC = XC - X1    YC = YC - Y1    X1 = 0    Y1 = 0    R = Sqr(XC ^ 2 + YC ^ 2)        '旋转'    sinT = YC / R    cosT = XC / R    Rotate X2, Y2, sinT, cosT    Rotate XC, YC, sinT, cosT        Me.Circle (X1, Y1), 1, vbBlue    Me.Circle (X2, Y2), 1, vbGreen    Me.Circle (XC, YC), 1, vbRed    Me.Circle (XC, YC), R        '判断'    Me.CurrentX = 20    Me.CurrentY = 20    If Y2 > 0 Then Me.Print "顺时针 小弧"    If Y2 < 0 Then Me.Print "顺时针 大弧"End SubPrivate Sub Form_Load()    Me.AutoRedraw = True    Me.Scale (-100, 100)-(100, -100)End Sub