;; beat-alternate construct
;; Alternates between sources on each beat
;;
;; Usage in recipe:
;;   (construct beat-alternate :path "constructs/beat-alternate.sexp")
;;   (def segments (beat-alternate beats-data (list video-a video-b)))

(define-construct beat-alternate
  :params (
    (analysis :type any :desc "Analysis data with :times")
    (sources :type any :desc "List of source nodes to alternate between")
  )
  ;; Body: map over time pairs, return segment descriptors
  (let [times (get analysis :times)
        pairs (zip-pairs (cons 0 times))
        n-sources (len sources)]
    (map-indexed
      (fn [i pair]
        (dict :source (nth sources (mod i n-sources))
              :start (first pair)
              :end (nth pair 1)
              :effects (list)))
      pairs)))