diff --git a/src/main/frontend/extensions/fsrs.cljs b/src/main/frontend/extensions/fsrs.cljs
new file mode 100644
index 000000000..04eb46706
--- /dev/null
+++ b/src/main/frontend/extensions/fsrs.cljs
@@ -0,0 +1,71 @@
+(ns frontend.extensions.fsrs
+  (:require))
+
+(def ratings
+  {:again 1
+   :hard 2
+   :good 3
+   :easy 4})
+
+(def w [1, 1, 5, -0.5, -0.5, 0.2, 1.4, -0.12, 0.8, 2, -0.2, 0.2, 1])
+
+(def enable-fuzz false)
+(def requestRetention 0.9)
+(def maximumInterval 36500)
+;; const intervalModifier = Math.log(requestRetention) / Math.log(0.9);
+(def interval-modifier (/ (js/Math.log requestRetention) (js/Math.log 0.9)))
+
+(defn- constrain-difficulty
+  [difficulty]
+  {:pre [(number? difficulty)]}
+  (js/Math.min (js/Math.max (.toFixed difficulty 2) 1) 10))
+
+
+(defn- init-difficulty
+  [rating]
+  (parse-double (.toFixed (constrain-difficulty (+ (nth w 2) (* (nth w 3) (- (get ratings rating) 3)))) 2)))
+
+(defn- init-stability
+  [rating]
+  (parse-double (.toFixed (js/Math.max (+ (nth w 0) (* (nth w 1) (- (get ratings rating) 1))) 0.1) 2)))
+
+(defn- apply-fuzz
+  [ivl]
+  ivl)
+
+(defn- next-interval
+  [stability]
+  (let [new-interval (apply-fuzz (* stability interval-modifier))]
+    (js/Math.min maximumInterval (js/Math.max 1 (js/Math.round new-interval)))))
+
+(defn- mean-reversion
+  [init current]
+  (+ (* (nth w 5) init) (* (- 1 (nth w 5)) current)))
+
+(defn- next-difficulty
+  [d rating]
+  (let [next-d (+ d (* (nth w 4) (- (get ratings rating) 3)))]
+    (constrain-difficulty (mean-reversion (nth w 2) next-d))))
+
+(defn- next-recall-stability
+  [d s r]
+  (-> (* s (+ 1 (* (js/Math.exp (nth w 6)) (- 11 d) (js/Math.pow s (nth w 7)) (- (js/Math.exp (* (- 1 r) (nth w 8))) 1))))
+      (.toFixed 2)
+      parse-double))
+
+(defn- next-forget-stability
+  [d s r]
+  (-> (* (nth w 9) (js/Math.pow d (nth w 10)) (js/Math.pow s (nth w 11)) (js/Math.exp (* (- 1 r) (nth w 12))))
+      (.toFixed 2)
+      parse-double))
+
+(defn- init-states
+  []
+  {:again {:d (init-difficulty :again)
+           :s (init-stability :again)}
+   :hard {:d (init-difficulty :hard)
+          :s (init-stability :hard)}
+   :good {:d (init-difficulty :good)
+          :s (init-stability :good)}
+   :easy {:d (init-difficulty :easy)
+          :s (init-stability :easy)}})