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tests/calc_snr.c: Fix indexing of FFTW output

--- a/tests/calc_snr.c

+++ b/tests/calc_snr.c

@@ -204,10 +204,19 @@

 	fftw_destroy_plan (plan) ;

-	/* (k < N/2 rounded up) */

 	maxval = 0.0 ;

 	for (k = 1 ; k < len / 2 ; k++)

-	{	magnitude [k] = sqrt (magnitude [k] * magnitude [k] + magnitude [len - k - 1] * magnitude [len - k - 1]) ;

+	{	/*

+		** 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 ;

 		} ;