Scontrini-Daemon/scontrini.rkt

#lang racket

(require racket/tcp)
(require json)

(define serial-port-device "/dev/ttyUSB0")
(define port 4444)
(define hostname "127.0.0.1")

; Return a list of pizzas from the local file
(define (get-pizzas-list)
	(let ((file (open-input-file "pizza_list.json")))
		(read-json file)))

; Send the pizza list as a JSON, prepending it with 4 bytes that contain the list length
(define (send-pizzas-list output)
	(let ((pizzas-list (get-pizzas-list)))
		(write-bytes (integer->integer-bytes (bytes-length (jsexpr->bytes pizzas-list)) 4 #f) output)
		(write-json pizzas-list output)))

; Print the to-print string on the printer
; TODO add synchronization to avoid race conditions
(define (printer-print to-print)
	(let ((file (open-output-file serial-port-device #:exists 'append)))
		(display (string-replace to-print "\n" "\n\n") file) ; always add an extra newline because the printer is a little messed up
		(close-output-port file)))

; Format the pizzas array from the JSON in a human-readable way
(define (format-pizza pizzas-list)
	(let ((str ""))
		(for-each (lambda (pizza-hashmap)
			(set! str (string-append str (format "~a : ~s\n" (hash-ref pizza-hashmap 'quantity) (hash-ref pizza-hashmap 'name)))))
			pizzas-list)
		str))

; Parse the commands given in the JSON
(define (parse-json message output)
  	(let ((parsed-json (with-input-from-string message (lambda () (read-json))))) ; parse the JSON
		(case (hash-ref parsed-json 'command)
			(("print") (printer-print (hash-ref parsed-json 'text)))
			(("pizza") (printer-print (format-pizza (hash-ref parsed-json 'pizzas))))
			(("get-pizzas") (send-pizzas-list output))
			(else (displayln "Unknown command")))))

; Parse the JSON and execute the command specified in it
;
; Each message starts with the length of the JSON object, put into the first 4 bytes
(define (execute-commands input output)
  	; read the first 4 bytes, little-endian, unsigned
	(let ((message-length (integer-bytes->integer (read-bytes 4 input) #f)))
		(parse-json (bytes->string/utf-8 (read-bytes message-length input)) output)
		(close-input-port input)
		(close-output-port output)))

; Wait on a port and spawn new threads on each connection
(define (wait-for-connection)
  	(let ((listener (tcp-listen port 4 #t hostname)))
	  	(let infinite-loop ()
			(let-values (((input output) (tcp-accept listener)))
				(thread (lambda () (execute-commands input output)))
				(infinite-loop)))))

(wait-for-connection)