from reportlab.lib import colors
import math

def getAngle(x1, y1, x2, y2):
    X = 1.0 * ( x2 - x1 )
    Y = 1.0 * ( y2 - y1 )
    if X==0:
        angle = 90 + (-1 * (Y<0)) * 180
    else:
        angle = math.atan ( Y / X ) / math.pi * 180
    if X<0:
        angle = angle + 180
    return angle

class arrow:
    def __init__(self, x1, y1, x2, y2, width):
        self.x1 = x1
        self.y1 = y1
        self.x2 = x2
        self.y2 = y2
        self.width = width
        self.arrowpoints = [
            ( 0, 2.0 * width ),
            ( 4.0 * width, 0 ),
            ( 0, -2.0 * width )
            ]
    def drawOn(self, canvas):
        canvas.saveState()
        canvas.setLineWidth(self.width)
        canvas.setStrokeColor(colors.black)
        canvas.setFillColor(colors.black)
        canvas.line(self.x1, self.y1, self.x2, self.y2)

        # draw the triangle for the arrowhead
        canvas.translate (self.x2, self.y2)
        angle = getAngle(self.x1, self.y1, self.x2, self.y2)
        canvas.rotate(angle)
        path = canvas.beginPath()
        (x,y) = self.arrowpoints[0]
        path.moveTo(x,y)
        for (x,y) in self.arrowpoints[1:]:
            path.lineTo(x,y)
        path.close()
        canvas.drawPath(path, fill=1)

        canvas.restoreState()