map-inversions.clj

(defn invert-one-to-one
  "returns a one-to-one mapping"
  [m]
  (persistent!
   (reduce (fn [m [k v]] (assoc! m v k)) (transient {}) m)))

(defn invert-many-to-one
  "returns a one-to-many mapping"
  ([m] (invert-many-to-one #{} m))
  ([to m]
   (persistent!
    (reduce (fn [m [k v]]
              (assoc! m v (conj (get m v to) k)))
            (transient {}) m))))

(defn invert-one-to-many
  "returns a many-to-one mapping"
  [m]
  (persistent!
   (reduce (fn [m [k vs]] (reduce (fn [m v] (assoc! m v k)) m vs))
           (transient {}) m)))

(defn invert-many-to-many
  "returns a many-to-many mapping"
  ([m] (invert-many-to-many #{} m))
  ([to m]
   (persistent!
    (reduce (fn [m [k vs]]
              (reduce (fn [m v]
                        (assoc! m v (conj (get m v to) k))) m vs))
            (transient {}) m))))