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(define-module (calp html util)
:use-module (hnh util))
(define-public (date-link date)
((@ (datetime) date->string) date "~Y-~m-~d"))
;; Generate an html id for an event.
;; TODO? same event placed multiple times, when spanning multiple cells
(define-public html-id
(let ((id (make-object-property)))
(lambda (ev)
(or (id ev)
(set/r! (id ev) (symbol->string (gensym "__html_id_")))))))
;; Returns a color with good contrast to the given background color.
;; https://stackoverflow.com/questions/1855884/determine-font-color-based-on-background-color/1855903#1855903
(define-public (calculate-fg-color c)
;; TODO what errors can actually appear here?
(catch #t
(lambda ()
(define (str->num c n) (string->number (substring/shared c n (+ n 2)) 16))
;; (format (current-error-port) "COLOR = ~s~%" c)
(let ((r (str->num c 1))
(g (str->num c 3))
(b (str->num c 5)))
(if (< 1/2 (/ (+ (* 0.299 r)
(* 0.587 g)
(* 0.114 b))
#xFF))
"#000000" "#FFFFFF")))
(lambda args
(format (current-error-port) "Error calculating foreground color?~%~s~%" args)
"#FF0000"
)))
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