update-in-idx.clj

(defn- ensure-min-vec-size [v size]
  "Ensure v has at least size elements. Fill with `nil` if necessary."
  (let [d (- size (count v))]
    (if (> d 0) (into v (vec (repeat d nil))) v)))

(defn- find-or-create
  "Build up nested path with possibly-numeric indices as vectors and other keys
   as maps"
  [m [k1 k2 & path]]
  (let [ds (cond-> (get-in m [k1] (if (number? k2) (vec (repeat k2 nil)) {}))
             ;; if k2 is a number we need to ensure the vector is large enough
             (number? k2) (ensure-min-vec-size k2)
             ;; recurse if path is non nil
             path (find-or-create (into [k2] path)))]
    (update-in m [k1] (constantly ds))))

(defn- update-in-idx
  "Like clojure.core/update-in except creates vectors insated of hash-maps when
   key is a number"
  [m path v]
  (update-in (find-or-create m path) path (constantly v)))