fix underflow in fuzz code, leading to large intervals

https://forums.ankiweb.net/t/buried-cards-in-ankimobile-beta-20081-3/14753

rslib/src/scheduler/states/mod.rs

@@ -156,7 +156,7 @@ fn constrained_fuzz_bounds(interval: f32, minimum: u32, maximum: u32) -> (u32, u
     if upper == lower && upper != 1 {
         upper = lower + 1;
     };
-    (lower, upper.min(maximum))
+    (lower, upper.min(maximum).max(lower))
 }
 
 fn fuzz_bounds(interval: f32) -> (u32, u32) {

rslib/src/scheduler/states/review.rs

@@ -253,4 +253,25 @@ mod test {
         assert!(leech_threshold_met(2, 1));
         assert!(leech_threshold_met(3, 1));
     }
+
+    #[test]
+    fn low_multiplier_fuzz() {
+        let mut ctx = StateContext::defaults_for_testing();
+        // our calculations should work correctly with a low ease or non-default multiplier
+        let state = ReviewState {
+            scheduled_days: 1,
+            elapsed_days: 1,
+            ease_factor: 1.3,
+            lapses: 0,
+            leeched: false,
+        };
+        ctx.fuzz_factor = Some(0.0);
+        assert_eq!(state.passing_review_intervals(&ctx), (2, 3, 4));
+
+        // this is a silly multiplier, but it shouldn't underflow
+        ctx.interval_multiplier = 0.1;
+        assert_eq!(state.passing_review_intervals(&ctx), (2, 3, 4));
+        ctx.fuzz_factor = Some(0.99);
+        assert_eq!(state.passing_review_intervals(&ctx), (2, 3, 4));
+    }
 }