core.cljc

(ns datascript.core
  (:refer-clojure :exclude [filter])
  (:require
    [#?(:cljs cljs.reader :clj clojure.edn) :as edn]
    [datascript.db :as db #?@(:cljs [:refer [FilteredDB]])]
    #?(:clj [datascript.pprint])
    [datascript.pull-api :as dp]
    [datascript.query :as dq]
    [datascript.impl.entity :as de])
  #?(:clj
    (:import
      [datascript.db FilteredDB]
      [datascript.impl.entity Entity]
      [java.util UUID])))


(def ^:const ^:no-doc tx0 db/tx0)


; Entities

(def ^{:arglists '([db eid])
       :doc "Retrieves an entity by its id from database. Entities are lazy map-like structures to navigate DataScript database content.

             For `eid` pass entity id or lookup attr:
             
                 (entity db 1)
                 (entity db [:unique-attr :value])
            
             If entity does not exist, `nil` is returned:

                 (entity db 100500) ; => nil

             Creating an entity by id is very cheap, almost no-op, as attr access is on-demand:

                 (entity db 1) ; => {:db/id 1}

             Entity attributes can be lazily accessed through key lookups:
             
                 (:attr (entity db 1)) ; => :value
                 (get (entity db 1) :attr) ; => :value

             Cardinality many attributes are returned sequences:

                 (:attrs (entity db 1)) ; => [:v1 :v2 :v3]

             Reference attributes are returned as another entities:

                 (:ref (entity db 1)) ; => {:db/id 2}
                 (:ns/ref (entity db 1)) ; => {:db/id 2}

             References can be walked backwards by prepending `_` to name part of an attribute:

                 (:_ref (entity db 2)) ; => [{:db/id 1}]
                 (:ns/_ref (entity db 2)) ; => [{:db/id 1}]
             
             Reverse reference lookup returns sequence of entities unless attribute is marked as `:db/isComponent`:

                 (:_component-ref (entity db 2)) ; => {:db/id 1}

             Entity gotchas:
               
             - Entities print as map, but are not exactly maps (they have compatible get interface though).
             - Entities are effectively immutable “views” into a particular version of a database.
             - Entities retain reference to the whole database.
             - You can’t change database through entities, only read.
             - Creating an entity by id is very cheap, almost no-op (attributes are looked up on demand).
             - Comparing entities just compares their ids. Be careful when comparing entities taken from differenct dbs or from different versions of the same db.
             - Accessed entity attributes are cached on entity itself (except backward references).
             - When printing, only cached attributes (the ones you have accessed before) are printed. See [[touch]]."}
  entity de/entity)


(def ^{:arglists '([db eid])
       :doc "Given lookup ref `[unique-attr value]`, returns numberic entity id.

             If entity does not exist, returns `nil`.

             For numeric `eid` returns `eid` itself (does not check for entity existence in that case)."}
  entid db/entid)


(defn entity-db
  "Returns a db that entity was created from."
  [^Entity entity]
  {:pre [(de/entity? entity)]}
  (.-db entity))


(def ^{:arglists '([e])
       :doc "Forces all entity attributes to be eagerly fetched and cached. Only usable for debug output.

             Usage:

             ```
             (entity db 1) ; => {:db/id 1}
             (touch (entity db 1)) ; => {:db/id 1, :dislikes [:pie], :likes [:pizza]}
             ```"}
  touch de/touch)


; Pull

(def ^{:arglists '([db selector eid])
       :doc "Fetches data from database using recursive declarative description. See [docs.datomic.com/on-prem/pull.html](https://docs.datomic.com/on-prem/pull.html).

             Unlike [[entity]], returns plain Clojure map (not lazy).

             Usage:

                 (pull db [:db/id, :name, :likes, {:friends [:db/id :name]}] 1)
                 ; => {:db/id   1,
                 ;     :name    \"Ivan\"
                 ;     :likes   [:pizza]
                 ;     :friends [{:db/id 2, :name \"Oleg\"}]}"}
  pull dp/pull)


(def ^{:arglists '([db selector eids])
       :doc "Same as [[pull]], but accepts sequence of ids and returns sequence of maps.

             Usage:

             ```
             (pull-many db [:db/id :name] [1 2])
             ; => [{:db/id 1, :name \"Ivan\"}
             ;     {:db/id 2, :name \"Oleg\"}]
             ```"}
  pull-many dp/pull-many)


; Query

(def
  ^{:arglists '([query & inputs])
    :doc "Executes a datalog query. See [docs.datomic.com/on-prem/query.html](https://docs.datomic.com/on-prem/query.html).

          Usage:
          
          ```
          (q '[:find ?value
               :where [_ :likes ?value]]
             db)
          ; => #{[\"fries\"] [\"candy\"] [\"pie\"] [\"pizza\"]}
          ```"}
  q dq/q)


; Creating DB

(def ^{:arglists '([] [schema])
       :doc "Creates an empty database with an optional schema.

             Usage:
             
             ```
             (empty-db) ; => #datascript/DB {:schema {}, :datoms []}
  
             (empty-db {:likes {:db/cardinality :db.cardinality/many}})
             ; => #datascript/DB {:schema {:likes {:db/cardinality :db.cardinality/many}}
             ;                    :datoms []}
             ```"}
  empty-db db/empty-db)


(def ^{:arglists '([x])
       :doc "Returns `true` if the given value is an immutable database, `false` otherwise."}
  db? db/db?)


(def ^{:arglists '([e a v] [e a v tx] [e a v tx added])
       :doc "Low-level fn to create raw datoms.

             Optionally with transaction id (number) and `added` flag (`true` for addition, `false` for retraction).

             See also [[init-db]]."}
  datom db/datom)


(def ^{:arglists '([x])
       :doc "Returns `true` if the given value is a datom, `false` otherwise."}
  datom? db/datom?)


(def ^{:arglists '([datoms] [datoms schema])
       :doc "Low-level fn for creating database quickly from a trusted sequence of datoms.

             Does no validation on inputs, so `datoms` must be well-formed and match schema.

             Used internally in db (de)serialization. See also [[datom]]."}
  init-db db/init-db)

; Schema

(def ^{:arglists '([db])
       :doc "Returns a schema of a database."}
  schema db/-schema)

; Filtered db

(defn is-filtered
  "Returns `true` if this database was filtered using [[filter]], `false` otherwise."
  [x]
  (instance? FilteredDB x))


(defn filter
  "Returns a view over database that has same interface but only includes datoms for which the `(pred db datom)` is true. Can be applied multiple times.
   
   Filtered DB gotchas:

   - All operations on filtered database are proxied to original DB, then filter pred is applied.
   - Not cached. You pay filter penalty every time.
   - Supports entities, pull, queries, index access.
   - Does not support [[with]] and [[db-with]]."
  [db pred]
  {:pre [(db/db? db)]}
  (if (is-filtered db)
    (let [^FilteredDB fdb db
          orig-pred (.-pred fdb)
          orig-db   (.-unfiltered-db fdb)]
      (FilteredDB. orig-db #(and (orig-pred %) (pred orig-db %)) (atom 0)))
    (FilteredDB. db #(pred db %) (atom 0))))


; Changing DB

(defn with
  "Same as [[transact!]], but applies to an immutable database value. Returns transaction report (see [[transact!]])."
  ([db tx-data] (with db tx-data nil))
  ([db tx-data tx-meta]
    {:pre [(db/db? db)]}
    (if (is-filtered db)
      (throw (ex-info "Filtered DB cannot be modified" {:error :transaction/filtered}))
      (db/transact-tx-data (db/map->TxReport
                             { :db-before db
                               :db-after  db
                               :tx-data   []
                               :tempids   {}
                               :tx-meta   tx-meta}) tx-data))))


(defn db-with
  "Applies transaction to an immutable db value, returning new immutable db value. Same as `(:db-after (with db tx-data))`."
  [db tx-data]
  {:pre [(db/db? db)]}
  (:db-after (with db tx-data)))


; Index lookups

(defn datoms
  "Index lookup. Returns a sequence of datoms (lazy iterator over actual DB index) which components (e, a, v) match passed arguments.

   Datoms are sorted in index sort order. Possible `index` values are: `:eavt`, `:aevt`, `:avet`.

   Usage:

       ; find all datoms for entity id == 1 (any attrs and values)
       ; sort by attribute, then value
       (datoms db :eavt 1)
       ; => (#datascript/Datom [1 :friends 2]
       ;     #datascript/Datom [1 :likes \"fries\"]
       ;     #datascript/Datom [1 :likes \"pizza\"]
       ;     #datascript/Datom [1 :name \"Ivan\"])
  
       ; find all datoms for entity id == 1 and attribute == :likes (any values)
       ; sorted by value
       (datoms db :eavt 1 :likes)
       ; => (#datascript/Datom [1 :likes \"fries\"]
       ;     #datascript/Datom [1 :likes \"pizza\"])
       
       ; find all datoms for entity id == 1, attribute == :likes and value == \"pizza\"
       (datoms db :eavt 1 :likes \"pizza\")
       ; => (#datascript/Datom [1 :likes \"pizza\"])
  
       ; find all datoms for attribute == :likes (any entity ids and values)
       ; sorted by entity id, then value
       (datoms db :aevt :likes)
       ; => (#datascript/Datom [1 :likes \"fries\"]
       ;     #datascript/Datom [1 :likes \"pizza\"]
       ;     #datascript/Datom [2 :likes \"candy\"]
       ;     #datascript/Datom [2 :likes \"pie\"]
       ;     #datascript/Datom [2 :likes \"pizza\"])
  
       ; find all datoms that have attribute == `:likes` and value == `\"pizza\"` (any entity id)
       ; `:likes` must be a unique attr, reference or marked as `:db/index true`
       (datoms db :avet :likes \"pizza\")
       ; => (#datascript/Datom [1 :likes \"pizza\"]
       ;     #datascript/Datom [2 :likes \"pizza\"])
  
       ; find all datoms sorted by entity id, then attribute, then value
       (datoms db :eavt) ; => (...)

   Useful patterns:

       ; get all values of :db.cardinality/many attribute
       (->> (datoms db :eavt eid attr) (map :v))
  
       ; lookup entity ids by attribute value
       (->> (datoms db :avet attr value) (map :e))
  
       ; find all entities with a specific attribute
       (->> (datoms db :aevt attr) (map :e))
  
       ; find “singleton” entity by its attr
       (->> (datoms db :aevt attr) first :e)
  
       ; find N entities with lowest attr value (e.g. 10 earliest posts)
       (->> (datoms db :avet attr) (take N))
  
       ; find N entities with highest attr value (e.g. 10 latest posts)
       (->> (datoms db :avet attr) (reverse) (take N))

   Gotchas:
   
   - Index lookup is usually more efficient than doing a query with a single clause.
   - Resulting iterator is calculated in constant time and small constant memory overhead.
   - Iterator supports efficient `first`, `next`, `reverse`, `seq` and is itself a sequence.
   - Will not return datoms that are not part of the index (e.g. attributes with no `:db/index` in schema when querying `:avet` index).
     - `:eavt` and `:aevt` contain all datoms.
     - `:avet` only contains datoms for references, `:db/unique` and `:db/index` attributes."
  ([db index]             {:pre [(db/db? db)]} (db/-datoms db index []))
  ([db index c1]          {:pre [(db/db? db)]} (db/-datoms db index [c1]))
  ([db index c1 c2]       {:pre [(db/db? db)]} (db/-datoms db index [c1 c2]))
  ([db index c1 c2 c3]    {:pre [(db/db? db)]} (db/-datoms db index [c1 c2 c3]))
  ([db index c1 c2 c3 c4] {:pre [(db/db? db)]} (db/-datoms db index [c1 c2 c3 c4])))


(defn seek-datoms
  "Similar to [[datoms]], but will return datoms starting from specified components and including rest of the database until the end of the index.

   If no datom matches passed arguments exactly, iterator will start from first datom that could be considered “greater” in index order.

   Usage:

       (seek-datoms db :eavt 1)
       ; => (#datascript/Datom [1 :friends 2]
       ;     #datascript/Datom [1 :likes \"fries\"]
       ;     #datascript/Datom [1 :likes \"pizza\"]
       ;     #datascript/Datom [1 :name \"Ivan\"]
       ;     #datascript/Datom [2 :likes \"candy\"]
       ;     #datascript/Datom [2 :likes \"pie\"]
       ;     #datascript/Datom [2 :likes \"pizza\"])

       (seek-datoms db :eavt 1 :name)
       ; => (#datascript/Datom [1 :name \"Ivan\"]
       ;     #datascript/Datom [2 :likes \"candy\"]
       ;     #datascript/Datom [2 :likes \"pie\"]
       ;     #datascript/Datom [2 :likes \"pizza\"])
  
       (seek-datoms db :eavt 2) 
       ; => (#datascript/Datom [2 :likes \"candy\"]
       ;     #datascript/Datom [2 :likes \"pie\"]
       ;     #datascript/Datom [2 :likes \"pizza\"])
  
       ; no datom [2 :likes \"fish\"], so starts with one immediately following such in index
       (seek-datoms db :eavt 2 :likes \"fish\")
       ; => (#datascript/Datom [2 :likes \"pie\"]
       ;     #datascript/Datom [2 :likes \"pizza\"])"
  ([db index]             {:pre [(db/db? db)]} (db/-seek-datoms db index []))
  ([db index c1]          {:pre [(db/db? db)]} (db/-seek-datoms db index [c1]))
  ([db index c1 c2]       {:pre [(db/db? db)]} (db/-seek-datoms db index [c1 c2]))
  ([db index c1 c2 c3]    {:pre [(db/db? db)]} (db/-seek-datoms db index [c1 c2 c3]))
  ([db index c1 c2 c3 c4] {:pre [(db/db? db)]} (db/-seek-datoms db index [c1 c2 c3 c4])))


(defn rseek-datoms
  "Same as [[seek-datoms]], but goes backwards until the beginning of the index."
  ([db index]             {:pre [(db/db? db)]} (db/-rseek-datoms db index []))
  ([db index c1]          {:pre [(db/db? db)]} (db/-rseek-datoms db index [c1]))
  ([db index c1 c2]       {:pre [(db/db? db)]} (db/-rseek-datoms db index [c1 c2]))
  ([db index c1 c2 c3]    {:pre [(db/db? db)]} (db/-rseek-datoms db index [c1 c2 c3]))
  ([db index c1 c2 c3 c4] {:pre [(db/db? db)]} (db/-rseek-datoms db index [c1 c2 c3 c4])))


(defn index-range
  "Returns part of `:avet` index between `[_ attr start]` and `[_ attr end]` in AVET sort order.
  
   Same properties as [[datoms]].
   
   `attr` must be a reference, unique attribute or marked as `:db/index true`.
   
   Usage:

       (index-range db :likes \"a\" \"zzzzzzzzz\")
       ; => (#datascript/Datom [2 :likes \"candy\"]
       ;     #datascript/Datom [1 :likes \"fries\"]
       ;     #datascript/Datom [2 :likes \"pie\"]
       ;     #datascript/Datom [1 :likes \"pizza\"]
       ;     #datascript/Datom [2 :likes \"pizza\"])
        
       (index-range db :likes \"egg\" \"pineapple\")
       ; => (#datascript/Datom [1 :likes \"fries\"]
       ;     #datascript/Datom [2 :likes \"pie\"])
           
   Useful patterns:
     
       ; find all entities with age in a specific range (inclusive)
       (->> (index-range db :age 18 60) (map :e))"
  [db attr start end]
  {:pre [(db/db? db)]}
  (db/-index-range db attr start end))


;; Conn

(defn conn?
  "Returns `true` if this is a connection to a DataScript db, `false` otherwise."
  [conn]
  (and #?(:clj  (instance? clojure.lang.IDeref conn)
          :cljs (satisfies? cljs.core/IDeref conn))
    (db/db? @conn)))


(defn conn-from-db
  "Creates a mutable reference to a given immutable database. See [[create-conn]]."
  [db]
  (atom db :meta { :listeners (atom {}) }))


(defn conn-from-datoms
  "Creates an empty DB and a mutable reference to it. See [[create-conn]]."
  ([datoms]        (conn-from-db (init-db datoms)))
  ([datoms schema] (conn-from-db (init-db datoms schema))))


(defn create-conn
  "Creates a mutable reference (a “connection”) to an empty immutable database.

   Connections are lightweight in-memory structures (~atoms) with direct support of transaction listeners ([[listen!]], [[unlisten!]]) and other handy DataScript APIs ([[transact!]], [[reset-conn!]], [[db]]).

   To access underlying immutable DB value, deref: `@conn`."
  ([]       (conn-from-db (empty-db)))
  ([schema] (conn-from-db (empty-db schema))))


(defn ^:no-doc -transact! [conn tx-data tx-meta]
  {:pre [(conn? conn)]}
  (let [report (atom nil)]
    (swap! conn (fn [db]
                  (let [r (with db tx-data tx-meta)]
                    (reset! report r)
                    (:db-after r))))
    @report))


(defn transact!
  "Applies transaction the underlying database value and atomically updates connection reference to point to the result of that transaction, new db value.
  
   Returns transaction report, a map:

       { :db-before ...       ; db value before transaction
         :db-after  ...       ; db value after transaction
         :tx-data   [...]     ; plain datoms that were added/retracted from db-before
         :tempids   {...}     ; map of tempid from tx-data => assigned entid in db-after
         :tx-meta   tx-meta } ; the exact value you passed as `tx-meta`

  Note! `conn` will be updated in-place and is not returned from [[transact!]].
  
  Usage:

      ; add a single datom to an existing entity (1)
      (transact! conn [[:db/add 1 :name \"Ivan\"]])
  
      ; retract a single datom
      (transact! conn [[:db/retract 1 :name \"Ivan\"]])
  
      ; retract single entity attribute
      (transact! conn [[:db.fn/retractAttribute 1 :name]])
  
      ; ... or equivalently (since Datomic changed its API to support this):
      (transact! conn [[:db/retract 1 :name]])
      
      ; retract all entity attributes (effectively deletes entity)
      (transact! conn [[:db.fn/retractEntity 1]])
  
      ; create a new entity (`-1`, as any other negative value, is a tempid
      ; that will be replaced with DataScript to a next unused eid)
      (transact! conn [[:db/add -1 :name \"Ivan\"]])
  
      ; check assigned id (here `*1` is a result returned from previous `transact!` call)
      (def report *1)
      (:tempids report) ; => {-1 296}
  
      ; check actual datoms inserted
      (:tx-data report) ; => [#datascript/Datom [296 :name \"Ivan\"]]
  
      ; tempid can also be a string
      (transact! conn [[:db/add \"ivan\" :name \"Ivan\"]])
      (:tempids *1) ; => {\"ivan\" 297}
  
      ; reference another entity (must exist)
      (transact! conn [[:db/add -1 :friend 296]])
  
      ; create an entity and set multiple attributes (in a single transaction
      ; equal tempids will be replaced with the same unused yet entid)
      (transact! conn [[:db/add -1 :name \"Ivan\"]
                       [:db/add -1 :likes \"fries\"]
                       [:db/add -1 :likes \"pizza\"]
                       [:db/add -1 :friend 296]])
  
      ; create an entity and set multiple attributes (alternative map form)
      (transact! conn [{:db/id  -1
                        :name   \"Ivan\"
                        :likes  [\"fries\" \"pizza\"]
                        :friend 296}])
      
      ; update an entity (alternative map form). Can’t retract attributes in
      ; map form. For cardinality many attrs, value (fish in this example)
      ; will be added to the list of existing values
      (transact! conn [{:db/id  296
                        :name   \"Oleg\"
                        :likes  [\"fish\"]}])

      ; ref attributes can be specified as nested map, that will create netsed entity as well
      (transact! conn [{:db/id  -1
                        :name   \"Oleg\"
                        :friend {:db/id -2
                                 :name \"Sergey\"}])
                                 
      ; reverse attribute name can be used if you want created entity to become
      ; a value in another entity reference
      (transact! conn [{:db/id  -1
                        :name   \"Oleg\"
                        :_friend 296}])
      ; equivalent to
      (transact! conn [{:db/id  -1, :name   \"Oleg\"}
                       {:db/id 296, :friend -1}])
      ; equivalent to
      (transact! conn [[:db/add  -1 :name   \"Oleg\"]
                       {:db/add 296 :friend -1]])"
  ([conn tx-data] (transact! conn tx-data nil))
  ([conn tx-data tx-meta]
    {:pre [(conn? conn)]}
    (let [report (-transact! conn tx-data tx-meta)]
      (doseq [[_ callback] (some-> (:listeners (meta conn)) (deref))]
        (callback report))
      report)))


(defn reset-conn!
  "Forces underlying `conn` value to become `db`. Will generate a tx-report that will remove everything from old value and insert everything from the new one."
  ([conn db] (reset-conn! conn db nil))
  ([conn db tx-meta]
    (let [report (db/map->TxReport
                  { :db-before @conn
                    :db-after  db
                    :tx-data   (concat
                                 (map #(assoc % :added false) (datoms @conn :eavt))
                                 (datoms db :eavt))
                    :tx-meta   tx-meta})]
      (reset! conn db)
      (doseq [[_ callback] (some-> (:listeners (meta conn)) (deref))]
        (callback report))
      db)))


(defn- atom? [a]
  #?(:cljs (instance? Atom a)
     :clj  (instance? clojure.lang.IAtom a)))


(defn listen!
  "Listen for changes on the given connection. Whenever a transaction is applied to the database via [[transact!]], the callback is called
   with the transaction report. `key` is any opaque unique value.
   
   Idempotent. Calling [[listen!]] with the same key twice will override old callback with the new value.
   
   Returns the key under which this listener is registered. See also [[unlisten!]]."
  ([conn callback] (listen! conn (rand) callback))
  ([conn key callback]
     {:pre [(conn? conn) (atom? (:listeners (meta conn)))]}
     (swap! (:listeners (meta conn)) assoc key callback)
     key))


(defn unlisten!
  "Removes registered listener from connection. See also [[listen!]]."
  [conn key]
  {:pre [(conn? conn) (atom? (:listeners (meta conn)))]}
  (swap! (:listeners (meta conn)) dissoc key))


; Data Readers

(def ^{:doc "Data readers for EDN readers. In CLJS they’re registered automatically. In CLJ, if `data_readers.clj` do not work, you can always do

             ```
             (clojure.edn/read-string {:readers data-readers} \"...\")
             ```"}
  data-readers {'datascript/Datom db/datom-from-reader
                'datascript/DB    db/db-from-reader})

#?(:cljs
   (doseq [[tag cb] data-readers] (edn/register-tag-parser! tag cb)))


;; Datomic compatibility layer

(def ^:private last-tempid (atom -1000000))


(defn tempid
  "Allocates and returns an unique temporary id (a negative integer). Ignores `part`. Returns `x` if it is specified.
  
   Exists for Datomic API compatibility. Prefer using negative integers directly if possible."
  ([part]
    (if (= part :db.part/tx)
      :db/current-tx
      (swap! last-tempid dec)))
  ([part x]
    (if (= part :db.part/tx)
      :db/current-tx
      x)))


(defn resolve-tempid
  "Does a lookup in tempids map, returning an entity id that tempid was resolved to.
   
   Exists for Datomic API compatibility. Prefer using map lookup directly if possible."
  [_db tempids tempid]
  (get tempids tempid))


(defn db
  "Returns the underlying immutable database value from a connection.
   
   Exists for Datomic API compatibility. Prefer using `@conn` directly if possible."
  [conn]
  {:pre [(conn? conn)]}
  @conn)


(defn transact
  "Same as [[transact!]], but returns an immediately realized future.
  
   Exists for Datomic API compatibility. Prefer using [[transact!]] if possible."
  ([conn tx-data] (transact conn tx-data nil))
  ([conn tx-data tx-meta]
    {:pre [(conn? conn)]}
    (let [res (transact! conn tx-data tx-meta)]
      #?(:cljs
         (reify
           IDeref
           (-deref [_] res)
           IDerefWithTimeout
           (-deref-with-timeout [_ _ _] res)
           IPending
           (-realized? [_] true))
         :clj
         (reify
           clojure.lang.IDeref
           (deref [_] res)
           clojure.lang.IBlockingDeref
           (deref [_ _ _] res)
           clojure.lang.IPending
           (isRealized [_] true))))))


;; ersatz future without proper blocking
#?(:cljs
   (defn- future-call [f]
     (let [res      (atom nil)
           realized (atom false)]
       (js/setTimeout #(do (reset! res (f)) (reset! realized true)) 0)
       (reify
         IDeref
         (-deref [_] @res)
         IDerefWithTimeout
         (-deref-with-timeout [_ _ timeout-val] (if @realized @res timeout-val))
         IPending
         (-realized? [_] @realized)))))


(defn transact-async
  "In CLJ, calls [[transact!]] on a future thread pool, returning immediately.
  
   In CLJS, just calls [[transact!]] and returns a realized future."
  ([conn tx-data] (transact-async conn tx-data nil))
  ([conn tx-data tx-meta]
    {:pre [(conn? conn)]}
    (future-call #(transact! conn tx-data tx-meta))))


(defn- rand-bits [pow]
  (rand-int (bit-shift-left 1 pow)))


#?(:cljs
  (defn- to-hex-string [n l]
    (let [s (.toString n 16)
          c (count s)]
      (cond
        (> c l) (subs s 0 l)
        (< c l) (str (apply str (repeat (- l c) "0")) s)
        :else   s))))


(defn squuid
  "Generates a UUID that grow with time. Such UUIDs will always go to the end  of the index and that will minimize insertions in the middle.
  
   Consist of 64 bits of current UNIX timestamp (in seconds) and 64 random bits (2^64 different unique values per second)."
  ([]
    (squuid #?(:clj  (System/currentTimeMillis)
               :cljs (.getTime (js/Date.)))))
  ([msec]
  #?(:clj
      (let [uuid     (UUID/randomUUID)
            time     (int (/ msec 1000))
            high     (.getMostSignificantBits uuid)
            low      (.getLeastSignificantBits uuid)
            new-high (bit-or (bit-and high 0x00000000FFFFFFFF)
                             (bit-shift-left time 32)) ]
        (UUID. new-high low))
     :cljs
       (uuid
         (str
               (-> (int (/ msec 1000))
                   (to-hex-string 8))
           "-" (-> (rand-bits 16) (to-hex-string 4))
           "-" (-> (rand-bits 16) (bit-and 0x0FFF) (bit-or 0x4000) (to-hex-string 4))
           "-" (-> (rand-bits 16) (bit-and 0x3FFF) (bit-or 0x8000) (to-hex-string 4))
           "-" (-> (rand-bits 16) (to-hex-string 4))
               (-> (rand-bits 16) (to-hex-string 4))
               (-> (rand-bits 16) (to-hex-string 4)))))))

(defn squuid-time-millis
  "Returns time that was used in [[squuid]] call, in milliseconds, rounded to the closest second."
  [uuid]
  #?(:clj (-> (.getMostSignificantBits ^UUID uuid)
              (bit-shift-right 32)
              (* 1000))
     :cljs (-> (subs (str uuid) 0 8)
               (js/parseInt 16)
               (* 1000))))