Fix up the tests and remove some duplicate calculations for the t-test.

cpp/include/stats.hpp

@@ -65,8 +65,6 @@ struct Stats {
     auto end = std::chrono::high_resolution_clock::now();
     auto elapsed = std::chrono::duration<double>(end - start);
 
-    auto as_ms = std::chrono::duration_cast<std::chrono::nanoseconds>(elapsed);
-
     if(elapsed.count() > 0.0) {
       sample(1.0/elapsed.count());
     }

cpp/src/stats.cpp

@@ -44,14 +44,10 @@ TTest internal_t_test(auto& first, auto& second) {
   double den = (std::pow(var1 / n1, 2) / (n1 - 1)) + (std::pow(var2 / n2, 2) / (n2 - 1));
   double dof = num / den;
 
-  // welch calculation
-  double temp = first.sample_variance() / first.n + second.sample_variance() / second.n;
-  double welchs = (first.mean() - second.mean()) / std::sqrt(temp);
-
   double gamm = std::exp(
       std::lgamma(dof / 2.0) + std::lgamma(0.5)
         - std::lgamma(dof / 2.0 + 0.5));
-  double b = dof / ( welchs * welchs + dof);
+  double b = dof / (t_stat * t_stat + dof);
 
   auto func = [&](const double r) {
     return std::pow(r, dof / 2.0 - 1.0) / std::sqrt(1.0 - r);

cpp/tests/stats_tests.cpp

@@ -99,7 +99,7 @@ namespace stats_tests {
 
     std::string filename("samples.txt");
     std::fstream s{filename, s.binary | s.trunc | s.out};
-    double skew_factor = 0.8;
+    double skew_factor = uniform_real(0.01, 5.0);
 
     for(size_t i = 0; i < 10; i++) {
       double num = uniform_real(-10.0, 10.0);