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src/mathutils.h: add fvec_quadratic_peak_mag to find the magnitude of interpolated peaks

--- a/src/mathutils.c

+++ b/src/mathutils.c

@@ -1,5 +1,5 @@

 /*

-  Copyright (C) 2003-2013 Paul Brossier <[email protected]>

+  Copyright (C) 2003-2014 Paul Brossier <[email protected]>

   This file is part of aubio.

@@ -427,6 +427,17 @@

   s1 = x->data[pos];

   s2 = x->data[x2];

   return pos + 0.5 * (s0 - s2 ) / (s0 - 2.* s1 + s2);

+}

+

+smpl_t fvec_quadratic_peak_mag (fvec_t *x, smpl_t pos) {

+  smpl_t x0, x1, x2;

+  uint_t index = (uint_t)(pos - .5) + 1;

+  if (pos >= x->length || pos < 0.) return 0.;

+  if ((smpl_t)index == pos) return x->data[index];

+  x0 = x->data[index - 1];

+  x1 = x->data[index];

+  x2 = x->data[index + 1];

+  return x1 - .25 * (x0 - x2) * (pos - index);

 }

 uint_t fvec_peakpick(fvec_t * onset, uint_t pos) {

--- a/src/mathutils.h

+++ b/src/mathutils.h

@@ -1,5 +1,5 @@

 /*

-  Copyright (C) 2003-2013 Paul Brossier <[email protected]>

+  Copyright (C) 2003-2014 Paul Brossier <[email protected]>

   This file is part of aubio.

@@ -233,6 +233,19 @@

 */

 smpl_t fvec_quadratic_peak_pos (fvec_t * x, uint_t p);

+

+/** finds magnitude of peak by quadratic interpolation

+

+  See [Quadratic Interpolation of Spectral

+  Peaks](https://ccrma.stanford.edu/~jos/sasp/Quadratic_Peak_Interpolation.html),

+  by Julius O. Smith III

+

+  \param x vector to get the magnitude of the interpolated peak position from

+  \param p index of the peak in vector `x`

+  \return magnitude of interpolated peak

+

+*/

+smpl_t fvec_quadratic_peak_mag (fvec_t * x, smpl_t p);

 /** Quadratic interpolation using Lagrange polynomial.