replace history-index with count

README.md

@@ -130,7 +130,8 @@ Here's a taste of how your queries could look like:
 ```
 
 ## History
-Since the database is immutable, all previous values are accessing by reading
+
+Since the database is immutable, all previous values are accessed by reading
 from the respective `history index`.
 The root data structure of a xitdb database is a ArrayList, called 'history'.
 Each transaction adds a new entry into this array, which points to the latest value 
@@ -143,6 +144,18 @@ of the database (usually a map).
 (xdb/deref-at db 1)  ;; the second value
 ```
 
+You can get the latest history index from the `count` of the database:
+
+```clojure
+(def history-index (dec (count db)))
+```
+
+After making further transactions, you can revert back to it simply like this:
+
+```clojure
+(reset! db (xdb/deref-at db history-index))
+```
+
 It is also possible to create a transaction which returns the previous and current 
 values of the database, by setting the `*return-history?*` binding to `true`.
 

src/xitdb/db.clj

@@ -122,10 +122,7 @@
   [^Database db]
   (ReadArrayList. (-> db .rootCursor)))
 
-(defn history-index
-  "Returns the current size of the transaction history array."
-  [xdb]
-  (.count (read-history (-> xdb .tldbro .get))))
+(def ^:deprecated history-index count)
 
 (defn deref-at
   "Returns the version of the data at the specified index."
@@ -145,6 +142,10 @@
   (deref [this]
     (deref-at this -1))
 
+  clojure.lang.Counted
+  (count [this]
+    (.count (read-history (.get tldbro))))
+
   clojure.lang.IAtom
 
   (reset [this new-value]

test/xitdb/history_test.clj

@@ -1,7 +1,7 @@
 (ns xitdb.history-test
   "Tests for database history and versioning features:
    - deref-at: access historical versions
-   - history-index: get current transaction count"
+   - count: get current transaction count"
   (:require
     [clojure.test :refer :all]
     [xitdb.db :as xdb]
@@ -55,17 +55,17 @@
       (is (= [1 2] (tu/materialize (xdb/deref-at db 1))))
       (is (= [1 2 3] (tu/materialize (xdb/deref-at db 2)))))))
 
-(deftest history-index-test
-  (testing "history-index returns the current transaction count"
+(deftest count-test
+  (testing "count returns the current transaction count"
     (with-open [db (xdb/xit-db :memory)]
       (reset! db {:a 1})
-      (is (= 1 (xdb/history-index db)))
+      (is (= 1 (count db)))
 
       (swap! db assoc :b 2)
-      (is (= 2 (xdb/history-index db)))
+      (is (= 2 (count db)))
 
       (swap! db assoc :c 3)
-      (is (= 3 (xdb/history-index db))))))
+      (is (= 3 (count db))))))
 
 (deftest deref-at-with-nil-values-test
   (testing "deref-at works correctly with nil values in history"