;;; -*- lexical-binding: t -*-
;;
;; this file:
;;   https://dataswamp.org/~incal/emacs-init/color-incal.el

(defun color-p (c)
  (and (integerp c)
       (<= 0  c)
       (< c 256) ))

(defun blue-p (r g b)
  (unless (and (color-p r)
               (color-p g)
               (color-p b) )
    (error "Bogus data") )
  (let*((min-adv 64)
        (rg      (ceiling (/ (+ r g) 2.0)))
        (diff    (- b rg))
        (rg-diff (- r g)) )
    (and (<= min-adv diff)
         (< -64 rg-diff)
         (< rg-diff 32) )))

(defun all-blue ()
  (cl-member-if
   (lambda (c)
     (blue-p
      (/ (nth 0 #1=(color-values c)) #2=256)
      (/ (nth 1 #1#)                 #2#)
      (/ (nth 2 #1#)                 #2#))
   (defined-colors))))

;; (all-blue)

;; (blue-p   0   0  64) ; t
;; (blue-p   1   0  64) ; nil
;; (blue-p 191 191 255) ; t
;; (blue-p 191 192 255) ; nil

(provide 'color-incal)