gistfile1.scm

;; convert a hex rgb or argb colour string to a vector of 3 or 4 floats [0..1]

(define (hex2rgb str)
  (define (h2n h)
    (/ (string->number (string-append "#x" h) 16) 255.0))
  (cond ((= 9 (string-length str))
         (let ((a (substring str 1 3))
               (r (substring str 3 5))
               (g (substring str 5 7))
               (b (substring str 7 9)))
           (vector (h2n r) (h2n g) (h2n b) (h2n a))))
        ((= 7 (string-length str))
         (let ((r (substring str 1 3))
               (g (substring str 3 5))
               (b (substring str 5 7)))
           (vector (h2n r) (h2n g) (h2n b))))))

;; convert a vector of 3 or 4 floats [0..1] to a hex rgb or rgba colour string

(define (rgb2hex rgb)
  (define (f2h f)
    (let ((h (number->string (inexact->exact
                              (round (* 255.0 f))) 16)))
      (if (= 1 (string-length h))
          (string-append "0" h) h)))
  (let ((r (vector-ref rgb 0))
        (g (vector-ref rgb 1))
        (b (vector-ref rgb 2))
        (a (if (= 4 (vector-length rgb))
               (vector-ref rgb 3) #f)))
    (string-append "#" (f2h r) (f2h g) (f2h b) (if a (f2h a) ""))))

;; this could still be improved to check for a leading '#' and possibly 
;; 3 character colour strings (amongst other sanity checks. ..)