+namespace {
+
+void apply_filter_fade(StereoFilter *filter, float *data, unsigned num_samples, float cutoff_hz, float db, float last_db)
+{
+ // A granularity of 32 samples is an okay tradeoff between speed and
+ // smoothness; recalculating the filters is pretty expensive, so it's
+ // good that we don't do this all the time.
+ static constexpr unsigned filter_granularity_samples = 32;
+
+ const float cutoff_linear = cutoff_hz * 2.0 * M_PI / OUTPUT_FREQUENCY;
+ if (fabs(db - last_db) < 1e-3) {
+ // Constant over this frame.
+ if (fabs(db) > 0.01f) {
+ filter->render(data, num_samples, cutoff_linear, 0.5f, db / 40.0f);
+ }
+ } else {
+ // We need to do a fade. (Rounding up avoids division by zero.)
+ unsigned num_blocks = (num_samples + filter_granularity_samples - 1) / filter_granularity_samples;
+ const float inc_db_norm = (db - last_db) / 40.0f / num_blocks;
+ float db_norm = db / 40.0f;
+ for (size_t i = 0; i < num_samples; i += filter_granularity_samples) {
+ size_t samples_this_block = std::min<size_t>(num_samples - i, filter_granularity_samples);
+ filter->render(data + i * 2, samples_this_block, cutoff_linear, 0.5f, db_norm);
+ db_norm += inc_db_norm;
+ }
+ }
+}
+
+} // namespace
+