Added comments

gnuradio/gr-droneid/lib/normalized_xcorr_estimate_impl.cc

@@ -73,9 +73,11 @@ namespace gr {
                                                 gr_vector_int &ninput_items,
                                                 gr_vector_const_void_star &input_items,
                                                 gr_vector_void_star &output_items) {
+            // Get handles to the input and output arrays
             const auto *in = (const gr_complex *) input_items[0];
             auto *out = (gr_complex *) output_items[0];
 
+            // Always tell GNU Radio that all samples were accepted even if not this many samples were written out
             consume_each(noutput_items);
 
             // This is how the remaining samples are buffered between calls.  It's important to realize that this algo
@@ -154,18 +156,19 @@ namespace gr {
             // Multiply each variance by the tap variances then take the reciprocal
             volk_32f_s32f_multiply_32f(&vars_[0], &vars_[0], taps_var_, num_steps);
 
-            // Take the square root of the product of the two variances
-            volk_32f_sqrt_32f(&vars_[0], &vars_[0], num_steps);
-
-            // There's no VOLK function for the reciprocal operation.  This is being done so that a multiply can be
-            // used next to divide the dot product results by the sqrt calculated above
-            for (auto & var : vars_) {
-                var = 1.0f / var;
-            }
+            // Calculate the inverse square root (1/sqrt(vars_[x]))
+            volk_32f_invsqrt_32f(&vars_[0], &vars_[0], num_steps);
 
             // Divide by the square root above
             volk_32fc_32f_multiply_32fc(&out[0], &out[0], &vars_[0], num_steps);
 
+            // Go through all outputs and replace NaN's with zeros.  This isn't strictly required, but nice to have
+            for (uint32_t idx = 0; idx < num_steps; idx++) {
+                if (out[idx].real() == FP_NAN || out[idx].imag() == FP_NAN) {
+                    out[idx] = zero_complex_;
+                }
+            }
+
             // Remove all the samples that have been processed from the buffer.  Leaving just the last <window_size_-1>
             // samples for the next call
             buffer_.erase(buffer_.begin(), buffer_.begin() + num_steps);

gnuradio/gr-droneid/lib/normalized_xcorr_estimate_impl.h

@@ -37,6 +37,7 @@ namespace gr {
             std::vector<float> vars_;
             std::vector<float> abs_squared_;
             std::vector<gr_complex> buffer_;
+            const gr_complex zero_complex_ = gr_complex{0, 0};
             // Nothing to declare in this block.
 
         public: