; persist/backend — the injected storage protocol. Every facet (log, kv,
; snapshot) goes through a backend dict, never touching storage directly, so
; file/pg/ipfs-ref backends swap in unchanged. A backend is a dict of fns:
;   {:append :read :last-seq :truncate-through :streams
;    :kv-get :kv-put :kv-delete :kv-has? :kv-keys}
; The in-memory backend is the test default. State is three dicts held in a
; closure and mutated with set!: logs (stream -> event list), seqs (stream ->
; last assigned seq — a monotonic high-water mark that survives compaction so
; truncating the log prefix never lets a future append reuse a seq), kv. The
; stream catalog comes from seqs, so a fully-compacted stream still lists.

(define
  persist/mem-backend
  (fn
    ()
    (let ((logs {}) (seqs {}) (kv {})) {:truncate-through (fn (stream n) (let ((cur (get logs stream))) (set! logs (assoc logs stream (filter (fn (e) (> (persist/event-seq e) n)) (if cur cur (list))))))) :kv-keys (fn () (keys kv)) :read (fn (stream) (let ((cur (get logs stream))) (if cur cur (list)))) :kv-has? (fn (key) (has-key? kv key)) :last-seq (fn (stream) (let ((s (get seqs stream))) (if s s 0))) :streams (fn () (keys seqs)) :append (fn (stream event) (begin (let ((cur (get logs stream))) (set! logs (assoc logs stream (append (if cur cur (list)) event)))) (set! seqs (assoc seqs stream (persist/event-seq event))))) :kv-delete (fn (key) (set! kv (dissoc kv key))) :kv-put (fn (key val) (set! kv (assoc kv key val))) :kv-get (fn (key) (get kv key))})))

; protocol accessors — call a backend op by keyword
(define
  persist/backend-append
  (fn (b stream event) ((get b :append) stream event)))
(define persist/backend-read (fn (b stream) ((get b :read) stream)))
(define
  persist/backend-last-seq
  (fn (b stream) ((get b :last-seq) stream)))
(define persist/backend-streams (fn (b) ((get b :streams))))
(define
  persist/backend-truncate
  (fn (b stream n) ((get b :truncate-through) stream n)))
(define persist/backend-kv-get (fn (b key) ((get b :kv-get) key)))
(define persist/backend-kv-put (fn (b key val) ((get b :kv-put) key val)))
(define persist/backend-kv-delete (fn (b key) ((get b :kv-delete) key)))
(define persist/backend-kv-has? (fn (b key) ((get b :kv-has?) key)))
(define persist/backend-kv-keys (fn (b) ((get b :kv-keys))))
