;;; Battery monitoring daemon script.

(import shell)

(import srfi-1)

;; Constant values.

;; Battery level control interval, in seconds.

(define check-interval 5) 

;; Path to the virtual file containing the battery percentage number.

(define capacity-path "/sys/class/power_supply/BAT1/capacity")

;; Association list of threshold levels and formatting functions.

(define thresholds
  `((20 . ,(lambda (battery-level)
	     (string-append "Battery level low: " (number->string battery-level) "%.")))
    (5 . ,(lambda (battery-level)
	    (string-append "Battery level critical: " (number->string battery-level) "%.")))))

;; Utility functions.

;; Get the battery level, return it as a number.

(define (get-battery-level)
  (with-input-from-file capacity-path read))

;; Method to display a notification dialog.

(define (user-notify message)
  (run ("notify-send '" ,message "'")))

;; Core daemon loop.

(define (monitor-battery previous-level)
  (let ((current-level (get-battery-level)))
    (let ((crossed-thresholds
	   (filter (lambda (n)
		     (and (>= n current-level)
			  (< n previous-level)))
		   (map car thresholds))))
      (if (not (null? crossed-thresholds))
	  (user-notify ((cdr (assoc (last crossed-thresholds) thresholds)) current-level))))
    (sleep check-interval)
    (monitor-battery current-level)))

;; Entry point of the program.

(monitor-battery 100)