;; content-on-sx — locate a block in the tree (ancestor section path).
;;
;; The read-side companion to doc-find-deep (which returns the block) and the
;; move/reparent ops (which relocate it): content/block-path returns the list of
;; ancestor section ids, root-first, leading to a block id — i.e. where the
;; block sits in the tree. A top-level block has an empty path; a block one
;; section deep has a one-element path; a missing id returns nil (distinct from
;; the empty-list path of a present top-level block). content/block-depth is the
;; path length (0 = top level, -1 = absent). Useful for breadcrumbs and for
;; scoping an edit to a block's enclosing section. Pure traversal; descends into
;; any block carrying a children list, like the rest of the tree helpers.
;;
;; Requires (loaded by harness): block.sx, doc.sx.

(define
  bp-in-blocks
  (fn
    (blocks id trail)
    (if
      (= (len blocks) 0)
      nil
      (let
        ((b (first blocks)))
        (if
          (= (blk-id b) id)
          trail
          (let
            ((ch (st-iv-get b "children")))
            (let
              ((found (if (list? ch) (bp-in-blocks ch id (append trail (list (blk-id b)))) nil)))
              (if (= found nil) (bp-in-blocks (rest blocks) id trail) found))))))))

;; ancestor section ids (root-first) for `id`, or nil if the block is absent.
(define
  content/block-path
  (fn (doc id) (bp-in-blocks (doc-blocks doc) id (list))))

;; depth of `id`: 0 at top level, n nested n sections deep, -1 if absent.
(define
  content/block-depth
  (fn
    (doc id)
    (let
      ((p (content/block-path doc id)))
      (if (= p nil) -1 (len p)))))