;; lib/dream/flash.sx — Dream-on-SX flash messages.
;; A single-request cookie store: messages added during one request are read on
;; the NEXT request, then the cookie is cleared. Depends on types.sx + session.sx
;; (shared cookie helpers). A message is {:category c :message m}.

;; ── cookie codec ───────────────────────────────────────────────────
;; escape the field separators so categories/messages round-trip safely
(define
  dr/flash-esc
  (fn (s) (replace (replace (replace s "%" "%25") "|" "%7C") "~" "%7E")))
(define
  dr/flash-unesc
  (fn (s) (replace (replace (replace s "%7E" "~") "%7C" "|") "%25" "%")))

(define
  dr/flash-encode
  (fn
    (msgs)
    (join
      "~"
      (map
        (fn
          (m)
          (str
            (dr/flash-esc (get m :category))
            "|"
            (dr/flash-esc (get m :message))))
        msgs))))

(define
  dr/flash-decode
  (fn
    (s)
    (if
      (= s "")
      (list)
      (map
        (fn (part) (let ((i (index-of part "|"))) {:message (dr/flash-unesc (substr part (+ i 1))) :category (dr/flash-unesc (substr part 0 i))}))
        (split s "~")))))

;; ── mutable outbox cell ────────────────────────────────────────────
(define dr/flash-box (fn () (let ((items (list))) {:add (fn (x) (set! items (concat items (list x)))) :get (fn () items)})))

;; ── middleware ─────────────────────────────────────────────────────
(define dream-flash-cookie-name "dream.flash")

(define
  dream-flash
  (fn
    (next)
    (fn
      (req)
      (let
        ((incoming (dr/flash-decode (or (dream-cookie req dream-flash-cookie-name) "")))
          (box (dr/flash-box)))
        (let
          ((resp (next (assoc req :dream-flash {:box box :incoming incoming}))))
          (let
            ((out ((get box :get))))
            (cond
              ((not (empty? out))
                (dream-set-cookie
                  resp
                  dream-flash-cookie-name
                  (dr/flash-encode out)
                  {:path "/" :http-only true :same-site "Lax"}))
              ((not (empty? incoming))
                (dream-drop-cookie resp dream-flash-cookie-name))
              (else resp))))))))

;; ── handler-facing API ─────────────────────────────────────────────
(define
  dream-add-flash-message
  (fn
    (req category msg)
    (begin ((get (get (get req :dream-flash) :box) :add) {:message msg :category category}) req)))

(define
  dream-flash-messages
  (fn (req) (get (get req :dream-flash) :incoming)))
(define dream-flash-category (fn (m) (get m :category)))
(define dream-flash-message (fn (m) (get m :message)))

;; convenience: only messages of a given category
(define
  dream-flash-of
  (fn
    (req category)
    (filter
      (fn (m) (= (get m :category) category))
      (dream-flash-messages req))))