servo: Merge #12327 - Profile stats (from malisas:profile-stats); r=jdm

<!-- Please describe your changes on the following line: -->
Pulled out profiler statistics calculation into its own function in time.rs, added tests for it, and simplified min and max calculation for pre-sorted data.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: b9af49f12093dfacdd3d8831efab4a58bee2e458

servo/components/profile/time.rs

@@ -327,6 +327,23 @@ impl Profiler {
         true
     }
 
+    /// Get tuple (mean, median, min, max) for profiler statistics.
+    pub fn get_statistics(data: &[f64]) -> (f64, f64, f64, f64) {
+        data.iter().fold(-f64::INFINITY, |a, &b| {
+            debug_assert!(a < b, "Data must be sorted");
+            b
+        });
+
+        let data_len = data.len();
+        debug_assert!(data_len > 0);
+        let (mean, median, min, max) =
+            (data.iter().sum::<f64>() / (data_len as f64),
+            data[data_len / 2],
+            data[0],
+            data[data_len - 1]);
+        (mean, median, min, max)
+    }
+
     fn print_buckets(&mut self) {
         match self.output {
             Some(OutputOptions::FileName(ref filename)) => {
@@ -349,11 +366,7 @@ impl Profiler {
                     });
                     let data_len = data.len();
                     if data_len > 0 {
-                        let (mean, median, min, max) =
-                            (data.iter().sum::<f64>() / (data_len as f64),
-                             data[data_len / 2],
-                             data.iter().fold(f64::INFINITY, |a, &b| a.min(b)),
-                             data.iter().fold(-f64::INFINITY, |a, &b| a.max(b)));
+                        let (mean, median, min, max) = Self::get_statistics(data);
                         write!(file, "{}\t{}\t{:15.4}\t{:15.4}\t{:15.4}\t{:15.4}\t{:15}\n",
                             category.format(&self.output), meta.format(&self.output),
                             mean, median, min, max, data_len).unwrap();
@@ -378,11 +391,7 @@ impl Profiler {
                     });
                     let data_len = data.len();
                     if data_len > 0 {
-                        let (mean, median, min, max) =
-                            (data.iter().sum::<f64>() / (data_len as f64),
-                             data[data_len / 2],
-                             data.iter().fold(f64::INFINITY, |a, &b| a.min(b)),
-                             data.iter().fold(-f64::INFINITY, |a, &b| a.max(b)));
+                        let (mean, median, min, max) = Self::get_statistics(data);
                         writeln!(&mut lock, "{:-35}{} {:15.4} {:15.4} {:15.4} {:15.4} {:15}",
                                  category.format(&self.output), meta.format(&self.output), mean, median, min, max,
                                  data_len).unwrap();

servo/tests/unit/profile/time.rs

@@ -15,3 +15,40 @@ fn time_profiler_smoke_test() {
     chan.send(ProfilerMsg::Exit(ipcchan));
     assert!(true, "Can tell the profiler thread to exit");
 }
+
+#[test]
+fn time_profiler_stats_test() {
+    let even_data = vec![1.234, 3.24567, 3.54578, 5.0, 5.324, 7.345,
+                         9.2345, 10.2342345, 13.2599, 15.0];
+    let (even_mean, even_median, even_min, even_max) = time::Profiler::get_statistics(&even_data);
+
+    assert_eq!(7.34230845, even_mean);
+    assert_eq!(7.345, even_median);
+    assert_eq!(1.234, even_min);
+    assert_eq!(15.0, even_max);
+
+    let odd_data = vec![1.234, 3.24567, 3.54578, 5.0, 5.324, 7.345,
+                        9.2345, 10.2342345, 13.2599];
+    let (odd_mean, odd_median, odd_min, odd_max) = time::Profiler::get_statistics(&odd_data);
+
+    assert_eq!(6.491453833333334, odd_mean);
+    assert_eq!(5.324, odd_median);
+    assert_eq!(1.234, odd_min);
+    assert_eq!(13.2599, odd_max);
+}
+
+#[cfg(debug_assertions)]
+#[test]
+#[should_panic]
+fn time_profiler_unsorted_stats_test() {
+    let unsorted_data = vec![5.0, 7.5, 1.0, 8.9];
+    time::Profiler::get_statistics(&unsorted_data);
+}
+
+#[cfg(debug_assertions)]
+#[test]
+#[should_panic]
+fn time_profiler_data_len_zero() {
+    let zero_data = vec![];
+    time::Profiler::get_statistics(&zero_data);
+}