calcualte if a Point is over a Polygon area

can somebody provide an formula for calculation  if a point ( record of 2 real) is over a polygon (defined by an array of ral values)

below is the code I using now, failure is a divison by 0
 

function IsPointInPolygon(p : FPoint; poly : TFPolygon) : Boolean;
var i,j : integer;
Begin
  result := false;
  j := High(poly);
  For i := Low(poly) to High(poly) do begin
  if (( ((poly[i].y <= p.y) and (p.y < poly[j].y)) or ((poly[j].y <= p.y) and (p.y < poly[i].y)) ) and
      (p.x < ((poly[j].x - poly[i].x) * (p.y - poly[i].y) / (poly[j].y - poly[i].y) + poly[i].x))) then result := not result;
  j := i
end;
end;

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