Media: tests for the averaged MP4 frame rate (#1442)

crates/media/src/probe.rs

@@ -306,3 +306,48 @@ fn averaged_rate(timescale: u32, durations: impl Iterator<Item = u32>) -> Option
         .map(|(r, _)| r)
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Durations (in 1/`timescale` s) of `n` frames at `rate` whose times are rounded to the timescale.
+    fn rounded(rate: FrameRate, timescale: i64, n: i64) -> Vec<u32> {
+        let at = |k: i64| (2 * k * timescale * rate.den + rate.num) / (2 * rate.num);
+        (0..n).map(|k| (at(k + 1) - at(k)) as u32).collect()
+    }
+
+    #[test]
+    fn millisecond_rounded_frame_times_average_to_their_rate() {
+        assert_eq!(rounded(FrameRate::FPS_30, 1000, 6), [33, 34, 33, 33, 34, 33]);
+        assert_eq!(averaged_rate(1000, rounded(FrameRate::FPS_30, 1000, 120).into_iter()), Some(FrameRate::FPS_30));
+        assert_eq!(averaged_rate(1000, rounded(FrameRate::FPS_60, 1000, 240).into_iter()), Some(FrameRate::FPS_60));
+        // NTSC rates stay NTSC, not the round rate next to them.
+        for r in [FrameRate::FPS_23_976, FrameRate::FPS_29_97, FrameRate::FPS_59_94] {
+            assert_eq!(averaged_rate(1000, rounded(r, 1000, 240).into_iter()), Some(r), "{r:?}");
+        }
+        // A short clip: its few frames average to the rate as well.
+        assert_eq!(averaged_rate(1000, [33, 34, 33].into_iter()), Some(FrameRate::FPS_30));
+    }
+
+    #[test]
+    fn exact_and_variable_rates_keep_the_readers_estimate() {
+        // A median that is a standard rate already reads right.
+        assert_eq!(averaged_rate(30000, rounded(FrameRate::FPS_29_97, 30000, 240).into_iter()), None);
+        assert_eq!(averaged_rate(90000, [3000; 10].into_iter()), None);
+        assert_eq!(averaged_rate(600, [20, 20, 19, 21, 20].into_iter()), None);
+        // Durations averaging to no standard rate (22.2 fps).
+        assert_eq!(averaged_rate(1000, [40, 45, 50].repeat(20).into_iter()), None);
+        // A truly 30.3 fps clip is not pulled to 30 (1 % off).
+        assert_eq!(averaged_rate(1000, [33; 30].into_iter()), None);
+    }
+
+    #[test]
+    fn degenerate_durations_have_no_rate() {
+        assert_eq!(averaged_rate(1000, std::iter::empty()), None);
+        assert_eq!(averaged_rate(1000, [0, 0, 0].into_iter()), None);
+        assert_eq!(averaged_rate(0, [33, 34, 33].into_iter()), None);
+        assert_eq!(averaged_rate(u32::MAX, [u32::MAX, u32::MAX - 1, 1, 0].into_iter()), None);
+        assert_eq!(averaged_rate(u32::MAX, [u32::MAX; 300].into_iter()), None);
+        assert_eq!(video_rate(b"not a movie"), None);
+    }
+}