1 files changed, 76 insertions, 0 deletions
diff --git a/common/alcomplex.cpp b/common/alcomplex.cpp
new file mode 100644
index 00000000..de2d1be9
--- /dev/null
+++ b/common/alcomplex.cpp
@@ -0,0 +1,76 @@
+
+#include "config.h"
+
+#include "alcomplex.h"
+
+#include <cmath>
+
+namespace {
+
+constexpr double Pi{3.141592653589793238462643383279502884};
+
+} // namespace
+
+void complex_fft(std::complex<double> *FFTBuffer, int FFTSize, double Sign)
+{
+    /* Bit-reversal permutation applied to a sequence of FFTSize items */
+    for(int i{1};i < FFTSize-1;i++)
+    {
+        int j{0};
+        for(int mask{1};mask < FFTSize;mask <<= 1)
+        {
+            if((i&mask) != 0)
+                j++;
+            j <<= 1;
+        }
+        j >>= 1;
+
+        if(i < j)
+            std::swap(FFTBuffer[i], FFTBuffer[j]);
+    }
+
+    /* Iterative form of DanielsonLanczos lemma */
+    int step{2};
+    for(int i{1};i < FFTSize;i<<=1, step<<=1)
+    {
+        int step2{step >> 1};
+        double arg{Pi / step2};
+
+        std::complex<double> w{std::cos(arg), std::sin(arg)*Sign};
+        std::complex<double> u{1.0, 0.0};
+        for(int j{0};j < step2;j++)
+        {
+            for(int k{j};k < FFTSize;k+=step)
+            {
+                std::complex<double> temp{FFTBuffer[k+step2] * u};
+                FFTBuffer[k+step2] = FFTBuffer[k] - temp;
+                FFTBuffer[k] += temp;
+            }
+
+            u *= w;
+        }
+    }
+}
+
+void complex_hilbert(std::complex<double> *Buffer, int size)
+{
+    const double inverse_size = 1.0/(double)size;
+
+    for(int i{0};i < size;i++)
+        Buffer[i].imag(0.0);
+
+    complex_fft(Buffer, size, 1.0);
+
+    int todo{size>>1};
+    int i{0};
+
+    Buffer[i++] *= inverse_size;
+    while(i < todo)
+        Buffer[i++] *= 2.0*inverse_size;
+    Buffer[i++] *= inverse_size;
+
+    for(;i < size;i++)
+        Buffer[i] = std::complex<double>{};
+
+    complex_fft(Buffer, size, -1.0);
+}