Function pearsonsChiSquareTest

internal/testutils/roundrobin/roundrobin.go:253–286

pearsonsChiSquareTest checks if the observed counts match the expected counts. Pearson's Chi-Squared Test Formula: χ² = ∑ (Oᵢ - Eᵢ)² / Eᵢ Where: - χ² is the chi-square statistic - Oᵢ is the observed count for category i - Eᵢ is the expected count for category i - The sum is over all categories (i

(t *testing.T, observedCounts, expectedCounts map[string]float64)

// See https://en.wikipedia.org/wiki/Pearson%27s_chi-squared_test for more
// details.
func pearsonsChiSquareTest(t *testing.T, observedCounts, expectedCounts map[string]float64) error {
	chiSquaredStat := 0.0
	for addr, want := range expectedCounts {
		got := observedCounts[addr]
		chiSquaredStat += (got - want) * (got - want) / want
	}
	degreesOfFreedom := len(expectedCounts) - 1
	const alpha = 1e-6
	chiSquareDist := distuv.ChiSquared{K: float64(degreesOfFreedom)}
	pValue := chiSquareDist.Survival(chiSquaredStat)
	t.Logf("Observed ratio: %v", observedCounts)
	t.Logf("Expected ratio: %v", expectedCounts)
	t.Logf("χ² statistic: %.4f", chiSquaredStat)
	t.Logf("Degrees of freedom: %d\n", degreesOfFreedom)
	t.Logf("p-value: %.10f\n", pValue)
	// Alpha (α) is the threshold we use to decide if the test "fails".
	// It controls how sensitive the chi-square test is.
	//
	// A smaller alpha means we require stronger evidence to say the load
	// balancing is wrong. A larger alpha makes the test more likely to fail,
	// even for small random fluctuations.
	//
	// For CI, we set alpha = 1e-6 to avoid flaky test failures.
	// That means:
	//  - There's only a 1-in-a-million chance the test fails due to random
	//    variation, assuming the load balancer is working correctly.
	//  - If the test *does* fail, it's very likely there's a real bug.
	//
	// TL;DR: smaller alpha = stricter test, fewer false alarms.
	if pValue > alpha {
		return nil
	}
	return fmt.Errorf("observed distribution significantly differs from expectations, observeredCounts: %v, expectedCounts: %v", observedCounts, expectedCounts)
}