;; slice-on construct - iterate over analysis times with user function
;;
;; Usage:
;;   (construct slice-on :path "constructs/slice-on.sexp")
;;   (def segments (slice-on beats-data
;;                   :init 0
;;                   :reducer (fn [acc i start end]
;;                              {:source (nth (list video-a video-b) (mod acc 2))
;;                               :effects (list)
;;                               :acc (inc acc)})))
;;
;; The construct receives:
;;   - First positional arg as 'analysis' (the analysis data with :times)
;;   - :init as 'init' (initial accumulator value)
;;   - :reducer as 'reducer' (the reducer lambda)

(define-construct slice-on
  :params (
    (analysis :type any :desc "Analysis data with :times")
    (init :type any :default 0 :desc "Initial accumulator value")
    (reducer :type any :desc "Reducer function (fn [acc i start end] ...)")
  )
  ;; Get times from analysis data
  (let [times (get analysis :times)
        pairs (zip-pairs (cons 0 times))]
    ;; Use nth to get second element of reduce result (the segments list)
    (nth
      (reduce
        (fn [state pair]
          (let [acc (first state)
                segments (nth state 1)
                i (len segments)
                start (first pair)
                end (nth pair 1)
                ;; Call user's reducer function
                result (reducer acc i start end)
                ;; Extract new acc and build segment
                new-acc (get result :acc)
                segment (dict :source (get result :source)
                             :start start
                             :end end
                             :duration (- end start)
                             :effects (get result :effects))]
            (list new-acc (append segments segment))))
        (list init (list))
        pairs)
      1)))