diff --git a/sm2.go b/sm2.go
index 05fd1cf..fba3e67 100644
--- a/sm2.go
+++ b/sm2.go
@@ -1,9 +1,22 @@
 package main
 
-func SM2Next(thisGrade int, consecutivePassesBeforeThis int, ef float64, lastInterval int) (int, float64, int) {
+import (
+	"time"
+)
+
+func Next(ts time.Time, history []int) time.Time {
+	n, ef, i := 0, 2.5, 1
+	for j := range history {
+		n, ef, i = sm2Next(history[j], n, ef, i)
+	}
+	return ts.Add(time.Duration(i) * time.Hour * 24)
+}
+
+// After all scheduled reviews are complete, SuperMemo asks the user to re-review any cards they marked with a grade less than 4 repeatedly until they give a grade ≥ 4.
+func sm2Next(thisGrade int, consecutivePassesBeforeThis int, efBefore float64, lastInterval int) (int, float64, int) {
 	q := thisGrade
 	n := consecutivePassesBeforeThis
-	ef = ef
+	ef := efBefore
 	i := lastInterval
 
 	if q >= 3 {
diff --git a/sm2_test.go b/sm2_test.go
new file mode 100644
index 0000000..b66f645
--- /dev/null
+++ b/sm2_test.go
@@ -0,0 +1,54 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+func TestSM2(t *testing.T) {
+	ts := time.Date(2000, 1, 1, 0, 0, 0, 0, time.Local)
+	cases := map[string]struct {
+		history []int
+		want    int
+	}{
+		"never before and no attempts": {
+			history: nil,
+			want:    1,
+		},
+		"0": {
+			history: []int{3, 3, 3, 0},
+			want:    1,
+		},
+		"1": {
+			history: []int{3, 3, 0, 3},
+			want:    1,
+		},
+		"2": {
+			history: []int{3, 0, 3, 3},
+			want:    6,
+		},
+		"3": {
+			history: []int{0, 3, 3, 3},
+			want:    8,
+		},
+		"4": {
+			history: []int{3, 3, 3, 3},
+			want:    27,
+		},
+		"5": {
+			history: []int{3, 3, 3, 3, 3},
+			want:    52,
+		},
+	}
+
+	for name, d := range cases {
+		c := d
+		t.Run(name, func(t *testing.T) {
+			got := Next(ts, c.history)
+			t.Logf("Next(\n\t%s, \n\t%+v) = \n\t%s", ts, c.history, got)
+			if got != ts.Add(time.Duration(c.want)*time.Hour*24) {
+				t.Error(c.want, got)
+			}
+		})
+	}
+}