Precalculate FFT window

Before this calculating FFT window was taking much more CPU time than calculating the FFT itself.

Author: tejeez

csdr.c

@@ -1245,6 +1245,8 @@ int main(int argc, char *argv[])
 		FFT_PLAN_T* plan=make_fft_c2c(fft_size, windowed, output, 1, benchmark);
 		if(benchmark) fprintf(stderr," done\n");
 		if(octave) printf("setenv(\"GNUTERM\",\"X11 noraise\");y=zeros(1,%d);semilogy(y,\"ydatasource\",\"y\");\n",fft_size);
+		float *windowt;
+		windowt = precalculate_window(fft_size, window);
 		for(;;)
 		{
 			FEOF_CHECK;
@@ -1263,7 +1265,8 @@ int main(int argc, char *argv[])
 				for(int i=0;i<fft_size-every_n_samples;i++) input[i]=input[i+every_n_samples];
 				fread(input+fft_size-every_n_samples, sizeof(complexf), every_n_samples, stdin);
 			}
-			apply_window_c(input,windowed,fft_size,window);
+			//apply_window_c(input,windowed,fft_size,window);
+			apply_precalculated_window_c(input,windowed,fft_size,windowt);
 			fft_execute(plan);
 			if(octave)
 			{

libcsdr.c

@@ -930,6 +930,29 @@ void apply_window_c(complexf* input, complexf* output, int size, window_t window
 	}
 }
 
+float *precalculate_window(int size, window_t window)
+{
+	float (*window_function)(float)=firdes_get_window_kernel(window);
+	float *windowt;
+	windowt = malloc(sizeof(float) * size);
+	for(int i=0;i<size;i++) //@precalculate_window
+	{
+		float rate=(float)i/(size-1);
+		windowt[i] = window_function(2.0*rate+1.0);
+	}
+	return windowt;
+}
+
+void apply_precalculated_window_c(complexf* input, complexf* output, int size, float *windowt)
+{
+	for(int i=0;i<size;i++) //@apply_precalculated_window_c
+	{
+		iof(output,i)=iof(input,i)*windowt[i];
+		qof(output,i)=qof(input,i)*windowt[i];
+	}
+}
+
+
 void apply_window_f(float* input, float* output, int size, window_t window)
 {
 	float (*window_function)(float)=firdes_get_window_kernel(window);

libcsdr.h

@@ -135,7 +135,9 @@ typedef struct rational_resampler_ff_s
 rational_resampler_ff_t rational_resampler_ff(float *input, float *output, int input_size, int interpolation, int decimation, float *taps, int taps_length, int last_taps_delay);
 void rational_resampler_get_lowpass_f(float* output, int output_size, int interpolation, int decimation, window_t window);
 
+float *precalculate_window(int size, window_t window);
 void apply_window_c(complexf* input, complexf* output, int size, window_t window);
+void apply_precalculated_window_c(complexf* input, complexf* output, int size, float *windowt);
 void apply_window_f(float* input, float* output, int size, window_t window);
 void logpower_cf(complexf* input, float* output, int size, float add_db);
 void accumulate_power_cf(complexf* input, float* output, int size);