Scripts/parking.scm

;;;; A program to print out the current real-time status of the computer parking terminal equipped
;;;; parking garages in the city of Linköping using the municipal open data API.

(import medea)
(import http-client)
(import srfi-1)
(import (chicken io))
(import srfi-13)
(import srfi-14)
(import (chicken condition))

(define (curry fn a)
  (lambda (b)
    (fn a b)))

(define (curry2 fn a b)
  (lambda (c)
    (fn a b c)))

(define (perhaps fn a)
  (if a (fn a) a))

(define (upon pred? fn a)
  (if (pred? a)
      (fn a)
      a))

(define (member? a ln)
  (if (member a ln) #t #f))

(define (default b a)
  (if a a b))

;;; Return the value for a given key in an association list, or #f if it doesn't exist.
(define value
  (compose (curry perhaps cdr) assoc))

;;; Return the value for a given key in an association list, or a default value if it doesn't exist.
(define (value-with-default k v ln)
  (default v (value k ln)))

(define (has-key? k ln)
  (member? k (map car ln)))

(define (sort less-than? ln)
  (if (<= (length ln) 1)
      ln
      (append (sort less-than? (filter (complement (curry less-than? (car ln))) (cdr ln)))
              (list (car ln))
              (sort less-than? (filter (curry less-than? (car ln)) (cdr ln))))))

;;; Sort using fn to extract a value for less-than? to compare for each element in ln.
(define (sort-via less-than? fn ln)
  (sort (lambda (a b)
          (less-than? (fn a) (fn b))) ln))

(define (display-newline str)
  (display str)
  (newline))

(define (->string a)
  (cond ((string? a) a)
        ((symbol? a) (symbol->string a))
        ((number? a) (number->string a))
        ((char? a) (string a))))

;;; Given a list of either symbols or strings and an association list:
;;; look up every symbol from left to right in the association list and replace it with the value found
;;; then concatenate the resulting list of strings.
;;; Inspired by Python 3's dictionary formatting string syntax.
(define (format-assoc statement ln)
  (apply string-append
         (map (curry2 upon symbol? (compose ->string (curry (flip value) ln))) statement)))

;;; Get data from a uri and parse it as json.
(define (simple-json-request uri)
  (condition-case (with-input-from-request uri #f read-json)
    ((exn i/o net) (begin (display-newline "Connection failed.")
                          (exit 1)))))

;;; The API key is an alphanumeric string of length 32
;;; The Linköping municipal open data API key generator portal is located at http://opendata.linkoping.se
(define parking-key
  (string-delete char-set:whitespace (with-input-from-file "key-parking" read-line)))

;;; Get a list of parking lots with various metadata from the municipal servers.
;;; It should be noted that this API is both glitchy and bugged. Two bugs have been noted:
;;; the timestamp parameter, which is supposed to give you a list of lots changed since a given UNIX timestamp
;;; instead results in giving you every lot up until a timestamp of under one hour into the future, after which
;;; it returns nothing. This bug may be platform related; as of this writing the server system providing the API
;;; runs on Windows Server 2016 via IIS 10.0.
;;; The second bug involves returning only partial data upon request - no pattern to this bug has been observed.
(define (get-parking-lots)
  (value 'parkingareanewlist
         (simple-json-request (string-append "http://parkering.linkoping.se/Parkeringsdata/ParkeringsdataV1.svc/GetParkeringsYtaList/" parking-key "/0"))))

;;; Only lots tracked by parking lot computer terminals have the parkingspacesavailable key.
(define (computerized? lot)
  (has-key? 'parkingspacesavailable lot))

;;; The handicap lots are tracked as separate lots with duplicate names in the data set.
(define (only-has-handicap-spots? lot)
  (equal? '(4) (value 'parkingtypes lot)))

;;; Electric car lots have a separate lot type, and the downtown parking garages have spaces with charging stations
;;; but they are not implemented in the data set.
(define (electric-car-lot? lot)
  (member? 3 (value 'parkingtypes lot)))

(define (main)
  ;; We want members to be parsed as lowercase symbols, and arrays to be parsed as lists.
  ;; This is not a legal way to parse json, which requires case-sensitive members
  ;; but makes for neater scheme code.
  (json-parsers (append `((member . ,(lambda (k v)
                                       (cons (string->symbol (string-downcase k)) v)))
                          (array . ,identity)) (json-parsers)))
  (display-newline "Parking spaces available:")
  (map (compose display-newline (curry format-assoc '(name ": " parkingspacesavailable "/" parkingspaces)))
       (sort-via string-ci<? (curry value 'name)
                 (filter (conjoin computerized? (complement only-has-handicap-spots?)) (get-parking-lots)))))

(main)