; persist/project — a projection folds a stream's events into a read model.
; A projection state is {:value v :seq s} where s is the last seq folded in,
; so a projection can resume incrementally from where it left off (replay only
; the tail). step : (value event) -> value. Determinism: step must be pure —
; time lives on the event (event-at), never a clock here.
; Requires: lib/persist/event.sx, lib/persist/log.sx.

; fold the tail (events with seq > prior's seq) onto a prior projection state
(define
  persist/project-resume
  (fn
    (b stream step prior)
    (let
      ((tail (persist/read-from b stream (+ 1 (get prior :seq)))))
      (reduce (fn (acc e) {:value (step (get acc :value) e) :seq (persist/event-seq e)}) prior tail))))

; project the whole stream from seed
(define
  persist/project
  (fn (b stream step seed) (persist/project-resume b stream step {:value seed :seq 0})))

(define persist/project-value (fn (p) (get p :value)))
(define persist/project-seq (fn (p) (get p :seq)))

; convenience: project and return just the value
(define
  persist/project-fold
  (fn
    (b stream step seed)
    (persist/project-value (persist/project b stream step seed))))