;; lib/relations/api.sx — relationship lifecycle + direct queries over lib/datalog/.
;;
;; A relations db is a live Datalog db holding rel(Src,Dst,Kind) facts. Phase 1
;; uses no rules — direct children/parents are plain queries on the rel
;; relation. Phase 2's engine.sx adds recursive reachability rules; build-db
;; will fold them in then.
;;
;; Two surfaces: db-threading core fns (relations-children-of db ...) and a
;; current-db convenience layer (relations/relate ...) for callers that load a
;; fact base once and query without passing the db around. This mirrors lib/acl.

(define relations-rules (list))

(define
  relations-build-db
  (fn (facts) (dl-program-data facts relations-rules)))

;; Pull one column (by keyword key) out of a list of substitution dicts.
(define
  relations-pluck
  (fn (substs key) (map (fn (s) (get s key)) substs)))

;; Direct children: every Dst with rel(node, Dst, kind).
(define
  relations-children-of
  (fn
    (db node kind)
    (relations-pluck
      (dl-query db (list (quote rel) node (quote Dst) kind))
      :Dst)))

;; Direct parents: every Src with rel(Src, node, kind).
(define
  relations-parents-of
  (fn
    (db node kind)
    (relations-pluck
      (dl-query db (list (quote rel) (quote Src) node kind))
      :Src)))

;; Directly related: neighbours in either direction under kind.
(define
  relations-related
  (fn
    (db node kind)
    (append
      (relations-children-of db node kind)
      (relations-parents-of db node kind))))

;; --- current-db convenience layer ---

(define relations-current-db nil)

(define
  relations/load!
  (fn
    (facts)
    (do
      (set! relations-current-db (relations-build-db facts))
      relations-current-db)))

(define
  relations-ensure-db!
  (fn
    ()
    (do
      (when
        (= relations-current-db nil)
        (set! relations-current-db (relations-build-db (list))))
      relations-current-db)))

;; Add a relationship to the current db (re-saturates).
(define
  relations/relate
  (fn
    (src dst kind)
    (dl-assert! (relations-ensure-db!) (relations-rel src dst kind))))

;; Remove a relationship from the current db (re-saturates).
(define
  relations/unrelate
  (fn
    (src dst kind)
    (dl-retract! (relations-ensure-db!) (relations-rel src dst kind))))

(define
  relations/children
  (fn (node kind) (relations-children-of (relations-ensure-db!) node kind)))

(define
  relations/parents
  (fn (node kind) (relations-parents-of (relations-ensure-db!) node kind)))

(define
  relations/related
  (fn (node kind) (relations-related (relations-ensure-db!) node kind)))