From dd51ba396babac675846f14d9159e32c1c864cb8 Mon Sep 17 00:00:00 2001
From: Raulshc <33253777+Raulshc@users.noreply.github.com>
Date: Sun, 20 May 2018 17:21:50 +0200
Subject: Common: Implement discrete Hilbert transform

---
 common/alcomplex.c | 35 +++++++++++++++++++++++++++++++++++
 common/alcomplex.h |  9 +++++++++
 2 files changed, 44 insertions(+)

diff --git a/common/alcomplex.c b/common/alcomplex.c
index d68277e3..8a0786e9 100644
--- a/common/alcomplex.c
+++ b/common/alcomplex.c
@@ -59,3 +59,38 @@ void complex_fft(ALcomplex *FFTBuffer, ALsizei FFTSize, ALdouble Sign)
         }
     }
 }
+
+/*Discrete Hilbert Transform (analytic signal form)*/
+void hilbert(ALsizei size, ALcomplex *InOutBuffer )
+{
+    ALsizei k;
+    const ALdouble inverse_size = 1.0/(ALfloat)size;
+
+    for ( k = 0; k < size;k++ ) 
+          InOutBuffer[k].Imag = 0.0;
+
+    complex_fft( InOutBuffer, size, 1.0 );
+
+    for( k = 0; k < size; k++ )
+    {
+        if( k == 0 || k == size/2 )
+        {
+            InOutBuffer[k].Real *= inverse_size;
+            InOutBuffer[k].Imag *= inverse_size;
+        }
+
+        else if ( k >=1 && k < size/2 )
+        {
+            InOutBuffer[k].Real *= 2.0*inverse_size;
+            InOutBuffer[k].Imag *= 2.0*inverse_size;
+        }
+
+        else
+        {
+            InOutBuffer[k].Real   = 0.0;
+            InOutBuffer[k].Imag   = 0.0;
+        }
+    }
+
+    complex_fft( InOutBuffer, size,-1.0 );
+}
diff --git a/common/alcomplex.h b/common/alcomplex.h
index cf4683fa..cfd164b6 100644
--- a/common/alcomplex.h
+++ b/common/alcomplex.h
@@ -55,6 +55,15 @@ inline ALcomplex complex_mult(ALcomplex a, ALcomplex b)
  */
 void complex_fft(ALcomplex *FFTBuffer, ALsizei FFTSize, ALdouble Sign);
 
+/**
+ *Calculate the complex helical sequence (or discrete-time analytical signal)
+ *of the given input using the discrete Hilbert transform (In-place algorithm).
+ *Fills InOutBuffer[0...size-1] with the discrete-time analytical signal stored 
+ *in InOutBuffer[0...size-1]. InOutBuffer is an array of complex numbers, 
+ *size MUST BE power of two.
+ */
+void hilbert(ALsizei size, ALcomplex *InOutBuffer );
+
 #ifdef __cplusplus
 } // extern "C"
 #endif
-- 
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