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mathutils.c: add aubio_zero_crossing_rate at Amaury's request

--- a/src/mathutils.c

+++ b/src/mathutils.c

@@ -440,6 +440,23 @@

       return loudness;

 }

+smpl_t aubio_zero_crossing_rate(fvec_t * input) {

+  uint_t i=0,j;

+  uint_t zcr = 0;

+  for ( j = 1; j < input->length; j++ ) {

+    // previous was negative

+    if( input->data[i][j-1] <= 0. ) {

+      if ( input->data[i][j] > 0. ) {

+        zcr += 1;

+      }

+    //previous was positive

+    } else if ( input->data[i][j] <= 0. ){

+      zcr += 1;

+    }

+  }

+  return zcr/(smpl_t)input->length;

+}

+

 void aubio_autocorr(fvec_t * input, fvec_t * output){

         uint_t i = 0, j = 0, length = input->length;

         smpl_t * data = input->data[0];

--- a/src/mathutils.h

+++ b/src/mathutils.h

@@ -204,6 +204,10 @@

  */

 void aubio_autocorr(fvec_t * input, fvec_t * output);

 /**

+ * zero-crossing rate (number of zero cross per sample)

+ */

+smpl_t aubio_zero_crossing_rate(fvec_t * input);

+/**

  * clean up cached memory at the end of program

  *

  * use this function at the end of programs to purge all