◊(Local Yarn Code "Diff")

;; Provides common helper functions used throughout the project

(provide maybe-meta     ; Select from (current-metas) or default value ("") if not available
         maybe-attr     ; Return an attribute’s value or a default ("") if not available
         series-noun    ; Retrieve noun-singular from current 'series meta, or ""
         series-title   ; Retrieve title of series in current 'series meta, or ""
         series-pagenode
         attr-present?  ; Test if an attribute is present
         make-tag-predicate
         tx-strs
         ymd->english
         ymd->dateformat
         default-authorname
         default-title
         articles-path

  `(root ,@(include-in-pagetree articles-path ".poly.pm")))

(define (series-pagetree)
  `(root ,@(include-in-pagetree series-path ".poly.pm")))

;; ~~~ Convenience functions for tagged x-expressions ~~~

(define (attr-present? name attrs)
  (for/or ([attr-pair (in-list attrs)])
          (equal? name (car attr-pair))))

(define (maybe-attr name attrs [missing ""])
  (define result (assoc name attrs))
  (cond
    [(pair? result) (cadr result)]
    [else missing]))

;; Returns a function will test if a txexpr's tag matches the given symbol.

  (define test-attrs '([name "Hazel"] [rank "Chief"]))

  (parameterize ([current-metas test-metas])
    (check-equal? (maybe-meta 'name) "Fiver") ; present meta
    (check-equal? (maybe-meta 'age) "")       ; missing meta
    (check-equal? (maybe-meta 'age 2) 2))      ; alternate default value
  
  (check-equal? (attr-present? 'name test-attrs) #t)
  (check-equal? (attr-present? 'dingus test-attrs) #f)
  (check-equal? (maybe-attr 'rank test-attrs) "Chief")
  (check-equal? (maybe-attr 'dingus test-attrs) "")
  (check-equal? (maybe-attr 'dingus test-attrs "zippy") "zippy"))

;; Return the first N words out of a list of txexprs. This function will unpack the strings out of
;; the elements of one txexpr at a time until it finds the requested number of words. It aims to be
;; both reliable and fast for any size of list you pass it, and smart about the punctuation it

(define (build-note-id txpr)
  (string-append (maybe-attr 'date (get-attrs txpr))
                 "_"
                 (uri-encode (maybe-attr 'author (get-attrs txpr) default-authorname))))

;; Extract the last disposition (if any), and the ID of the disposing note, out of a list of notes
(define (notes->last-disposition-values txprs)
  (define (contains-disposition? tx) (attr-present? 'disposition (get-attrs tx)))
  (define (contains-disposition? tx) (attrs-have-key? tx 'disposition))
  (define disp-notes (filter contains-disposition? txprs))
  (cond [(not (empty? disp-notes))
         (define latest-disposition-note (last disp-notes))
         (values (attr-ref latest-disposition-note 'disposition)
                 (build-note-id latest-disposition-note))]
        [else (values "" "")]))