(ns datascript-diff
  (:require [datascript.core :as d]
            [datascript.btset :as btset]))
            

(defn- keys-eq? [a b]
  (or (identical? (.-keys a) (.-keys b))
      (let [achunk (btset/iter-chunk a)
            bchunk (btset/iter-chunk b)]
        (and (== (count achunk) (count bchunk))
             (every? #(= (nth achunk %)
                         (nth bchunk %))
                     (range (count achunk)))))))

(defn datom-exists? [db datom]
  (let [e (.-e datom)
        a (.-a datom)
        v (.-v datom)]
    (not-empty (d/datoms db :eavt e a v))))

(defn add-retract [action datom]
  [action (.-e datom) (.-a datom) (.-v datom)])

(defn datoms-diff
  "Given two datascript dbs, return the :db/add :db/retract statements to bring them in sync"
  ([a-db b-db] (datoms-diff a-db b-db (d/datoms a-db :eavt) (d/datoms b-db :eavt) []))
  ([a-db b-db a b accum]
   (if (and (nil? a) (nil? b))
     accum
     (if (and (some? a) (some? b) (keys-eq? a b))
       (recur a-db b-db (btset/iter-chunked-next a) (btset/iter-chunked-next b) accum)
       (let [achunk (btset/iter-chunk a)
             bchunk (btset/iter-chunk b)
             diff (map (fn [x]
                         (let [a-datom (nth achunk x)
                               b-datom (nth bchunk x)
                               same? (= a-datom b-datom)]
                           (when-not same?
                             (cond-> []
                                     ;; If a exists, but b doesn't, then we need to add a
                                     (and a-datom (nil? b-datom) (not (datom-exists? b-db a-datom)))
                                     (conj (add-retract :db/add a-datom))

                                     ;; if b exists, but a doesn't, then we need to add b
                                     (and b-datom (nil? a-datom) (not (datom-exists? a-db b-datom)))
                                     (conj (add-retract :db/add b-datom))

                                     ;; if a and b exist, but a isn't in b-db, then issue a retraction
                                     (and a-datom b-datom (not (datom-exists? b-db a-datom)))
                                     (conj (add-retract :db/retract a-datom))

                                     ;; if a and b exist, but b isn't in a-db, then issue an addition
                                     (and a-datom b-datom (not (datom-exists? a-db b-datom)))
                                     (conj (add-retract :db/add b-datom))))))
                       (range 25))
             filtered (filter not-empty diff)
             merged (mapcat into filtered)]
         (recur a-db b-db (btset/iter-chunked-next a) (btset/iter-chunked-next b) (into accum merged)))))))


(defn simple-test []
  (let [sample-db (-> (d/empty-db {:first-name {:db/index true}
                                   :last-name {:db/index true}
                                   :phone {:db/index true}
                                   :email {:db/index true}})
                      (d/db-with [[:db/add 1 :first-name "Sean"]
                                  [:db/add 1 :last-name "Tempesta"]
                                  [:db/add 1 :phone "555-5555"]]))
        changes [[:db/add 1 :email "sean.tempesta@gmail.com"]
                 [:db/retract 1 :last-name "Tempesta"]
                 [:db/retract 1 :phone "555-5555"]
                 [:db/add 1 :phone "555-1234"]]
        changed-db (d/db-with sample-db changes)]
    (= changes (datoms-diff sample-db changed-db))))