Revert "Change M_PI to std::numbers::pi."

This reverts commit 0f515bbd97712bf0fb11012b898e314770d5255e.

Won't compile under gcc as deployed with Ubuntu 20.04.

1 files changed, 1 insertions, 2 deletions

diff --git a/src/window-function.cpp b/src/window-function.cpp
index 64ea14e..d52ae6d 100644
--- a/src/window-function.cpp
+++ b/src/window-function.cpp
@@ -7,7 +7,6 @@
 #include "window-function.hpp"
 
 #include <cassert>
-#include <numbers>
 
 WindowFunction::WindowFunction(std::size_t window_size) : window_size_(window_size)
 {
@@ -70,7 +69,7 @@ GeneralizedCosineWindowFunction::GeneralizedCosineWindowFunction(std::size_t win
         this->cached_factors_[n] = 0;
         for (unsigned int k = 0; k < a.size(); k++) {
             this->cached_factors_[n] +=
-                std::pow<double>(-1.0f, k) * a[k] * std::cos(2.0 * std::numbers::pi * k * (double)n / N);
+                std::pow<double>(-1.0f, k) * a[k] * std::cos(2.0 * (double)M_PI * k * (double)n / N);
         }
     }
 }