feat: added sm-2 implementation

This is fairly naive and probably has many bugs, but it wil be deployed
for now and trialled, with improvements to come.

src/methods/sm-2.rhai

@@ -0,0 +1,65 @@
+// An implementation of the SuperMemo v2 algorithm.
+//
+// Algorithm SM-2, (C) Copyright SuperMemo World, 1991. (https://www.supermemo.com)
+
+const RESPONSES = [ "0", "1", "2", "3", "4", "5" ];
+
+fn get_weight(data, difficult) {
+    // Cards that are not yet ready to review will be excluded (unless they're difficult)
+    if difficult {
+        return 2.0;
+    } else if data.next_review <= get_seconds_since_epoch() {
+        // Once a card is ready to review, it should be reviewed
+        return 1.0;
+    } else {
+        return 0.0;
+    }
+}
+
+fn adjust_card(res, data, difficult) {
+    let quality = parse_int(res);
+
+    // Any cards the user is failing should be repeated zealously until they get them consistently right
+    if quality < 4 {
+        difficult = true;
+    } else {
+        difficult = false;
+    }
+
+    // Taken from https://stackoverflow.com/questions/49047159/spaced-repetition-algorithm-from-supermemo-sm-2#49047160
+    data.easiness = max(1.3, data.easiness + 0.1 - (5.0 - quality) * (0.08 + (5.0 - quality) * 0.02));
+
+    if (quality < 3) {
+        data.repetitions = 0;
+    } else {
+        data.repetitions += 1;
+    }
+
+    if (data.repetitions <= 1) {
+        data.interval = 1;
+    } else if (data.repetitions == 2) {
+        data.interval = 6;
+    } else {
+        data.interval = round(data.interval * data.easiness);
+    }
+
+    let now = get_seconds_since_epoch();
+    let seconds_in_day = 60 * 60 * 24;
+    data.next_review = now + seconds_in_day * data.interval;
+
+    return [data, difficult];
+}
+fn get_default_metadata() {
+    return #{
+        repetitions: 0,
+        easiness: 2.5,
+        interval: 1,
+        // First review can be immediate
+        next_review: get_seconds_since_epoch(),
+    };
+}
+
+// Utility functions
+fn max(x, y) {
+    if x > y {  return x; } else { return y; }
+}