Feat/get retrievability (#24)

fsrs.go

@@ -19,3 +19,13 @@ func (f *FSRS) Repeat(card Card, now time.Time) RecordLog {
 func (f *FSRS) Next(card Card, now time.Time, grade Rating) SchedulingInfo {
 	return f.scheduler(card, now).Review(grade)
 }
+
+func (f *FSRS) GetRetrievability(card Card, now time.Time) float64 {
+	if card.State == New {
+		return 0
+	} else {
+		elapsedDays := now.Sub(card.LastReview).Hours() / 24
+		retrievability := f.Parameters.forgettingCurve(elapsedDays, card.Stability)
+		return retrievability
+	}
+}

fsrs_test.go

@@ -148,3 +148,20 @@ func TestLongTermScheduler(t *testing.T) {
 		t.Errorf("excepted:%v, got:%v", wantDHistory, dHistory)
 	}
 }
+
+func TestGetRetrievability(t *testing.T) {
+	retrievabilityList := []float64{}
+	fsrs := NewFSRS(DefaultParam())
+	card := NewCard()
+	now := time.Date(2022, 11, 29, 12, 30, 0, 0, time.UTC)
+	retrievabilityList = append(retrievabilityList, roundFloat(fsrs.GetRetrievability(card, now), 4))
+	for i := 0; i < 3; i++ {
+		card = fsrs.Next(card, now, Good).Card
+		now = card.Due
+		retrievabilityList = append(retrievabilityList, roundFloat(fsrs.GetRetrievability(card, now), 4))
+	}
+	wantRetrievabilityList := []float64{0, 0.9997, 0.9036, 0.9017}
+	if !reflect.DeepEqual(retrievabilityList, wantRetrievabilityList) {
+		t.Errorf("excepted:%v, got:%v", wantRetrievabilityList, retrievabilityList)
+	}
+}