[benchmark] Fix index computation for quantiles

Turns out that both the old code in `DriverUtils` that computed median, as well as newer quartiles in `PerformanceTestSamples` had off-by-1 error.

It trully is the 3rd of the 2 hard things in computer science!

Author: palimondo

Diff for: benchmark/scripts/compare_perf_tests.py

@@ -141,20 +141,24 @@ def max(self):
         """Maximum sampled value."""
         return self.samples[-1].runtime
 
+    def _quantile_index(self, q, i):
+        """Return index of the element nearest to the i-th q-quantile."""
+        return int(round((self.count - 1) / float(q) * float(i)))
+
     @property
     def median(self):
         """Median sampled value."""
-        return self.samples[self.count / 2].runtime
+        return self.samples[self._quantile_index(2, 1)].runtime
 
     @property
     def q1(self):
         """First Quartile (25th Percentile)."""
-        return self.samples[self.count / 4].runtime
+        return self.samples[self._quantile_index(4, 1)].runtime
 
     @property
     def q3(self):
         """Third Quartile (75th Percentile)."""
-        return self.samples[(self.count / 2) + (self.count / 4)].runtime
+        return self.samples[self._quantile_index(4, 3)].runtime
 
     @property
     def iqr(self):

Diff for: benchmark/scripts/test_compare_perf_tests.py

@@ -78,10 +78,10 @@ def test_computes_five_number_summary(self):
             self.samples, (1000, 1000, 1100, 1100, 1100))
         self.samples.add(Sample(3, 1, 1050))
         self.assertEqualFiveNumberSummary(
-            self.samples, (1000, 1000, 1050, 1050, 1100))
+            self.samples, (1000, 1050, 1050, 1100, 1100))
         self.samples.add(Sample(4, 1, 1025))
         self.assertEqualFiveNumberSummary(
-            self.samples, (1000, 1025, 1050, 1100, 1100))
+            self.samples, (1000, 1025, 1050, 1050, 1100))
         self.samples.add(Sample(5, 1, 1075))
         self.assertEqualFiveNumberSummary(
             self.samples, (1000, 1025, 1050, 1075, 1100))
@@ -156,11 +156,12 @@ def test_excludes_outliers_zero_IQR(self):
         self.samples.add(Sample(0, 2, 23))
         self.samples.add(Sample(1, 2, 18))
         self.samples.add(Sample(2, 2, 18))
+        self.samples.add(Sample(3, 2, 18))
         self.assertEquals(self.samples.iqr, 0)
 
         self.samples.exclude_outliers()
 
-        self.assertEquals(self.samples.count, 2)
+        self.assertEquals(self.samples.count, 3)
         self.assertEqualStats(
             (self.samples.min, self.samples.max), (18, 18))
 

Diff for: benchmark/utils/DriverUtils.swift

@@ -454,7 +454,7 @@ func runBench(_ test: BenchmarkInfo, _ c: TestConfig) -> BenchResults? {
                       min: samples.first!, max: samples.last!,
                       mean: UInt64(stats.mean),
                       sd: UInt64(stats.standardDeviation),
-                      median: samples[samples.count / 2],
+                      median: samples[(samples.count - 1) / 2],
                       maxRSS: UInt64(sampler.measureMemoryUsage()))
 }