Option Explicit

Sub CircleTrimmer

  ' Local variable declarations
  Dim curve, circle, ccx, ccx_t(1), interval

  ' Select closed curve to split
  curve = Rhino.GetObject("Select closed curve to split", 4)
  If IsNull(curve) Then Exit Sub
  If Not Rhino.IsCurveClosed(curve) Then Exit Sub

  ' Select circle to split with    
  circle = Rhino.GetObject("Select circle to split with", 4)
  If IsNull(circle) Then Exit Sub
  If Not Rhino.IsCircle(circle) Then Exit Sub

  ' Intersect the two curves
  ccx = Rhino.CurveCurveIntersection(curve, circle)
  If IsNull(ccx) Then
    Rhino.Print "Curve and circle do not intersect"
    Exit Sub
  End If

  ' Make sure there are only two intersection events
  If UBound(ccx) <> 1 Then
    Rhino.Print "Unable to split curve"
    Exit Sub
  End If

  ' Get two intersection parameters on the curve
  ccx_t(0) = ccx(0,5)
  ccx_t(1) = ccx(1,5)

  ' If the input curve is closed and the interval is decreasing,
  ' then the portion of the curve across the start and end of the
  ' curve is returned.
  If ccx_t(0) < ccx_t(1) Then
    interval = Array(ccx_t(1), ccx_t(0))
  Else
    interval = Array(ccx_t(0), ccx_t(1))
  End If

  ' Trim the curve
  Rhino.TrimCurve curve, interval

End Sub