(define (head l) (car l))
(define (tail l) (cdr l))

(define (ifsnow a b c d)
(let ((p (+ (* a a) (* b b) (* c c) (* d d) 3))
      (s60 0.866025403)
      (c60 0.5))
  (ifs-iterate 0 (list
    (list (/ (* a a) p)
          (vector a 0 0)
          (vector 0 a 0))
    (list (/ (* b b) p)
          (vector b 0 0)
          (vector 0 b a))
    (list (/ (* c c) p)
          (vector (*      c60  c) (*      s60  c) 0)
          (vector (* (- 0 s60) c) (*      c60  c) a))
    (list (/ (* d d) p)
          (vector (* (- 0 c60) d) (*      s60  d) 0)
          (vector (* (- 0 s60) d) (* (- 0 c60) c) a))
    (list (/ 1 p)
          (vector   1   0  0)
          (vector   0 -1 0))
    (list (/ 1 p)
          (vector -1  0  0)
          (vector   0   1  0))
    (list (/ 1 p)
          (vector      c60  s60 0)
          (vector (- 0 s60) c60 0))
  ))
))

(define (ifs-iterate n t) (cond
  ((= n 0) (begin
    (pdata-set "p" n (vector 0 0 1))
    (ifs-iterate (+ n 1) t))
  )
  ((< n (pdata-size))
    (let ((z (pdata-get "p" (- n 1))))
    (let ((tr (pickprob t (flxrnd))))
    (let ((x (car tr)) (y (cadr tr))) (begin
      (pdata-set "p" n (vector (vdot x z) (vdot y z) 1))
      (ifs-iterate (+ n 1) t)
  )))))
))

(define (pickprob l r)
  (if (<= r (head (head l)))
    (tail (head l))
    (pickprob (tail l) (- r (head (head l))))
))

(backfacecull 0)
(clear-colour (vector 0.2 0.5 0.6))
(clear)
(hint-points)
;(hint-unlit)
;(hint-anti-alias)
;(hint-wire)
;(line-width 1)
(wire-colour (vector 1 1 1))

(define count 10000)

(define l0 0.5)
(define l1 0.5)
(define l2 0.33)
(define l3 0.33)

(define pi 3.1415926)
(define t 0)

(colour (vector 1 1 1))

(define obj (build-particles count))

(define (ifsnow-init n)
  (if (< n (pdata-size)) (begin
    (pdata-set "s" n (vector 0.01 0.01 0.01))
;    (pdata-set "c" n (vector 1 1 1)) ;; this line fails an assert
    (ifsnow-init (+ n 1))
  ))
)

(grab obj)
(ifsnow-init 0)
(ungrab)

(define (render)
  (grab obj)
    (ifsnow l0 l1 l2 l3)
  (ungrab)
  (set! t (+ t 0.040))
  (set! l0 (+ (* 0.3 (cos (* t 0.5 pi))) 0.5))
  (set! l1 (- 1 l0))
  (set! l2 (+ (* -0.25 (cos (/ (* t 3 2 pi) 217))) 0.4))
  (set! l3 (+ (* -0.25 (cos (/ (* t 4 2 pi) 217))) 0.4))
)

(every-frame (render))

;(start-framedump "out" "png")