text: /*
  ** Copyright (c) 2002-2016, Erik de Castro Lopo <erikd@mega-nerd.com>
  ** All rights reserved.
  **
  ** This code is released under 2-clause BSD license. Please see the
  ** file at : https://github.com/libsndfile/libsamplerate/blob/master/COPYING
  */
  
  #ifdef HAVE_CONFIG_H
  #include "config.h"
  #endif
  
  #include "util.h"
  
  #if (HAVE_FFTW3)
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <math.h>
  
  #include <fftw3.h>
  
  #define	MAX_SPEC_LEN	(1<<18)
  #define	MAX_PEAKS		10
  
  static void log_mag_spectrum (double *input, int len, double *magnitude) ;
  static void smooth_mag_spectrum (double *magnitude, int len) ;
  static double find_snr (const double *magnitude, int len, int expected_peaks) ;
  
  typedef struct
  {	double	peak ;
  	int		index ;
  } PEAK_DATA ;
  
  double
  calculate_snr (float *data, int len, int expected_peaks)
  {	static double magnitude [MAX_SPEC_LEN] ;
  	static double datacopy [MAX_SPEC_LEN] ;
  
  	double snr = 200.0 ;
  	int k ;
  
  	if (len > MAX_SPEC_LEN)
  	{	printf ("%s : line %d : data length too large.\n", __FILE__, __LINE__) ;
  		exit (1) ;
  		} ;
  
  	for (k = 0 ; k < len ; k++)
  		datacopy [k] = data [k] ;
  
  	/* Pad the data just a little to speed up the FFT. */
  	while ((len & 0x1F) && len < MAX_SPEC_LEN)
  	{	datacopy [len] = 0.0 ;
  		len ++ ;
  		} ;
  
  	log_mag_spectrum (datacopy, len, magnitude) ;
  	smooth_mag_spectrum (magnitude, len / 2) ;
  
  	snr = find_snr (magnitude, len, expected_peaks) ;
  
  	return snr ;
  } /* calculate_snr */
  
  /*==============================================================================
  ** There is a slight problem with trying to measure SNR with the method used
  ** here; the side lobes of the windowed FFT can look like a noise/aliasing peak.
  ** The solution is to smooth the magnitude spectrum by wiping out troughs
  ** between adjacent peaks as done here.
  ** This removes side lobe peaks without affecting noise/aliasing peaks.
  */
  
  static void linear_smooth (double *mag, PEAK_DATA *larger, PEAK_DATA *smaller) ;
  
  static void
  smooth_mag_spectrum (double *mag, int len)
  {	PEAK_DATA peaks [2] ;
  
  	int k ;
  
  	memset (peaks, 0, sizeof (peaks)) ;
  
  	/* Find first peak. */
  	for (k = 1 ; k < len - 1 ; k++)
  	{	if (mag [k - 1] < mag [k] && mag [k] >= mag [k + 1])
  		{	peaks [0].peak = mag [k] ;
  			peaks [0].index = k ;
  			break ;
  			} ;
  		} ;
  
  	/* Find subsequent peaks ans smooth between peaks. */
  	for (k = peaks [0].index + 1 ; k < len - 1 ; k++)
  	{	if (mag [k - 1] < mag [k] && mag [k] >= mag [k + 1])
  		{	peaks [1].peak = mag [k] ;
  			peaks [1].index = k ;
  
  			if (peaks [1].peak > peaks [0].peak)
  				linear_smooth (mag, &peaks [1], &peaks [0]) ;
  			else
  				linear_smooth (mag, &peaks [0], &peaks [1]) ;
  			peaks [0] = peaks [1] ;
  			} ;
  		} ;
  
  } /* smooth_mag_spectrum */
  
  static void
  linear_smooth (double *mag, PEAK_DATA *larger, PEAK_DATA *smaller)
  {	int k ;
  
  	if (smaller->index < larger->index)
  	{	for (k = smaller->index + 1 ; k < larger->index ; k++)
  			mag [k] = (mag [k] < mag [k - 1]) ? 0.999 * mag [k - 1] : mag [k] ;
  		}
  	else
  	{	for (k = smaller->index - 1 ; k >= larger->index ; k--)
  			mag [k] = (mag [k] < mag [k + 1]) ? 0.999 * mag [k + 1] : mag [k] ;
  		} ;
  
  } /* linear_smooth */
  
  /*==============================================================================
  */
  
  static int
  peak_compare (const void *vp1, const void *vp2)
  {	const PEAK_DATA *peak1, *peak2 ;
  
  	peak1 = (const PEAK_DATA*) vp1 ;
  	peak2 = (const PEAK_DATA*) vp2 ;
  
  	return (peak1->peak < peak2->peak) ? 1 : -1 ;
  } /* peak_compare */
  
  static double
  find_snr (const double *magnitude, int len, int expected_peaks)
  {	PEAK_DATA peaks [MAX_PEAKS] ;
  
  	int		k, peak_count = 0 ;
  	double	snr ;
  
  	memset (peaks, 0, sizeof (peaks)) ;
  
  	/* Find the MAX_PEAKS largest peaks. */
  	for (k = 1 ; k < len - 1 ; k++)
  	{	if (magnitude [k - 1] < magnitude [k] && magnitude [k] >= magnitude [k + 1])
  		{	if (peak_count < MAX_PEAKS)
  			{	peaks [peak_count].peak = magnitude [k] ;
  				peaks [peak_count].index = k ;
  				peak_count ++ ;
  				qsort (peaks, peak_count, sizeof (PEAK_DATA), peak_compare) ;
  				}
  			else if (magnitude [k] > peaks [MAX_PEAKS - 1].peak)
  			{	peaks [MAX_PEAKS - 1].peak = magnitude [k] ;
  				peaks [MAX_PEAKS - 1].index = k ;
  				qsort (peaks, MAX_PEAKS, sizeof (PEAK_DATA), peak_compare) ;
  				} ;
  			} ;
  		} ;
  
  	if (peak_count < expected_peaks)
  	{	printf ("\n%s : line %d : bad peak_count (%d), expected %d.\n\n", __FILE__, __LINE__, peak_count, expected_peaks) ;
  		return -1.0 ;
  		} ;
  
  	/* Sort the peaks. */
  	qsort (peaks, peak_count, sizeof (PEAK_DATA), peak_compare) ;
  
  	snr = peaks [0].peak ;
  	for (k = 1 ; k < peak_count ; k++)
  		if (fabs (snr - peaks [k].peak) > 10.0)
  			return fabs (peaks [k].peak) ;
  
  	return snr ;
  } /* find_snr */
  
  static void
  log_mag_spectrum (double *input, int len, double *magnitude)
  {	fftw_plan plan = NULL ;
  
  	double	maxval ;
  	int		k ;
  
  	if (input == NULL || magnitude == NULL)
  		return ;
  
  	plan = fftw_plan_r2r_1d (len, input, magnitude, FFTW_R2HC, FFTW_ESTIMATE | FFTW_PRESERVE_INPUT) ;
  	if (plan == NULL)
  	{	printf ("%s : line %d : create plan failed.\n", __FILE__, __LINE__) ;
  		exit (1) ;
  		} ;
  
  	fftw_execute (plan) ;
  
  	fftw_destroy_plan (plan) ;
  
  	maxval = 0.0 ;
  	for (k = 1 ; k < len / 2 ; k++)
  	{	/*
  		** From : http://www.fftw.org/doc/Real_002dto_002dReal-Transform-Kinds.html#Real_002dto_002dReal-Transform-Kinds
  		**
  		** FFTW_R2HC computes a real-input DFT with output in “halfcomplex” format, i.e. real and imaginary parts
  		** for a transform of size n stored as:
  		**
  		**      r0, r1, r2, ..., rn/2, i(n+1)/2-1, ..., i2, i1
  		*/
  		double re = magnitude [k] ;
  		double im = magnitude [len - k] ;
  		magnitude [k] = sqrt (re * re + im * im) ;
  		maxval = (maxval < magnitude [k]) ? magnitude [k] : maxval ;
  		} ;
  
  	memset (magnitude + len / 2, 0, len / 2 * sizeof (magnitude [0])) ;
  
  	/* Don't care about DC component. Make it zero. */
  	magnitude [0] = 0.0 ;
  
  	/* log magnitude. */
  	for (k = 0 ; k < len ; k++)
  	{	magnitude [k] = magnitude [k] / maxval ;
  		magnitude [k] = (magnitude [k] < 1e-15) ? -200.0 : 20.0 * log10 (magnitude [k]) ;
  		} ;
  
  	return ;
  } /* log_mag_spectrum */
  
  #else /* ! (HAVE_LIBFFTW && HAVE_LIBRFFTW) */
  
  double
  calculate_snr (float *data, int len, int expected_peaks)
  {	double snr = 200.0 ;
  
  	data = data ;
  	len = len ;
  	expected_peaks = expected_peaks ;
  
  	return snr ;
  } /* calculate_snr */
  
  #endif
  
  