Spaced repetitioin interval should reset to 0 if review session has b…

…een interrupted

functions/services/review-card.test.ts

@@ -37,3 +37,22 @@ test("hit yes all the time", () => {
     3600 * 24 * 1000 * 15.625,
   ]);
 });
+
+test("forgetting resets interval", () => {
+  const date = DateTime.fromSQL("2021-05-20 10:00:00");
+
+  const { interval: newInterval1 } = reviewCard(date, 0.4, "correct");
+  expect(newInterval1).toBe(1);
+
+  const { interval: newInterval2 } = reviewCard(date, 1, "wrong");
+  expect(newInterval2).toBe(0);
+
+  const { interval: newInterval3 } = reviewCard(date, 0, "wrong");
+  expect(newInterval3).toBe(0);
+
+  const { interval: newInterval4 } = reviewCard(date, 0, "correct");
+  expect(newInterval4).toBe(0.4);
+
+  const { interval: newInterval5 } = reviewCard(date, 0.4, "correct");
+  expect(newInterval5).toBe(1);
+});

functions/services/review-card.ts

@@ -17,9 +17,12 @@ export const reviewCard = (
   const easeFactor = 2.5;
 
   if (reviewOutcome === "correct") {
+    if (interval === 0) {
+      interval = 0.16;
+    }
     interval *= easeFactor;
   } else if (reviewOutcome === "wrong") {
-    interval = 0.4;
+    interval = 0;
   }
 
   const nextReviewDate = now.plus({ day: interval });