Движение точки по контуру воображаемого треугольника - QBasic

CLS
SCREEN 11
 
x1 = 100
y1 = 200
x2 = 300
y2 = 400
x3 = 500
y3 = 100
 
x = x1
y = y1
dx = 1
dy = 1
 
k1 = (y2 - y1) / (x2 - x1)
k2 = (y3 - y2) / (x3 - x2)
k3 = (y1 - y3) / (x1 - x3)
 
PRINT "Press any key"
 
DO
   LINE (x1, y1)-(x2, y2)
   LINE (x2, y2)-(x3, y3)
   LINE (x3, y3)-(x1, y1)
  
   IF x < x2 AND s = 0 THEN
      x = x + dx
      y = y1 + k1 * (x - x1)
      CIRCLE (x, y), 3
 
      FOR i = 1 TO 10000: NEXT
 
      CIRCLE (x, y), 3, 0
   ELSEIF x < x3 AND s = 0 THEN
      x = x + dx
      y = y2 + k2 * (x - x2)
      CIRCLE (x, y), 3
 
      FOR i = 1 TO 10000: NEXT
 
      CIRCLE (x, y), 3, 0
      IF x3 - x < 1 THEN s = 1
   ELSEIF x > x1 THEN
      x = x - dx
      y = y3 + k3 * (x - x3)
      CIRCLE (x, y), 3
 
      FOR i = 1 TO 10000: NEXT
 
      CIRCLE (x, y), 3, 0
      IF x - x1 < 2 THEN s = 0
   END IF
 
LOOP UNTIL INKEY$ <> ""
END