#require "hbcairo"

PROCEDURE Main()

   LOCAL hSurface, hCairo, nI

   hSurface := cairo_pdf_surface_create( "lightning.pdf", 566.9, 793.7 )  // 200x280 mm in pt
   hCairo := cairo_create( hSurface )

   // 20 pages
   FOR nI := 1 TO 20
      cairo_set_source_rgb( hCairo, 0, 0, 0 )
      cairo_paint( hCairo )
      cairo_set_source_rgb( hCairo, 1, 0.7, 1 )
      DrawLightning( hCairo, 250, 50, 700, 3 )
      cairo_show_page( hCairo )
   NEXT
   cairo_destroy( hCairo )
   cairo_surface_destroy( hSurface )

   RETURN


PROCEDURE DrawLightning( hCairo, nX, nY, nLen, nW, nInit )

   LOCAL nI, nK, nW0, nX2

   cairo_move_to( hCairo, nX, nY )
   nW0 := nW
   IF nInit == NIL
      nInit := 0
   ENDIF
   nK := 0
   FOR nI := 1 TO nLen
      // AR(1) process
      nInit := nInit * 0.9 + ( hb_Random() - 0.5 )
      // ARIMA(1, 1, 0) process
      nK += nInit
      // ARIMA(1, 1, 0) + white noise
      nX2 := nX + nK + hb_Random()
      cairo_line_to( hCairo, nX2, nI + nY )
      nW -= 0.003
      IF nW < nW0 - 0.1
         cairo_set_line_width( hCairo, nW0 )
         cairo_stroke( hCairo )
         cairo_move_to( hCairo, nX2, nI + nY )
         nW0 := nW
      ENDIF
      // Branch
      IF Abs( nInit ) > 1.6
         cairo_set_line_width( hCairo, nW0 )
         cairo_stroke( hCairo )
         DrawLightning( hCairo, nX2, nI + nY, Int( ( nLen - nI ) / 2 ), nW / 2, nInit * 0.5 )
         nInit *= -0.3
         cairo_move_to( hCairo, nX2, nI + nY )
      ENDIF
   NEXT
   cairo_set_line_width( hCairo, nW0 )
   cairo_stroke( hCairo )

   RETURN