Prepare dedicated Disk-reader de-click gain-stage

This allows to specify a shorter fade-duration than default
Amp::apply_gain(), also allows to unroll and vectorize the loop

libs/ardour/ardour/disk_reader.h

@@ -94,7 +94,7 @@ public:
 	void reset_tracker ();
 
 	bool declick_in_progress () const {
-		return _declick_gain != 0; // declick-out
+		return _declick_amp.gain() != 0; // declick-out
 	}
 
 	static void set_midi_readahead_samples (samplecnt_t samples_ahead) { midi_readahead = samples_ahead; }
@@ -125,6 +125,22 @@ protected:
 
 	int add_channel_to (boost::shared_ptr<ChannelList>, uint32_t how_many);
 
+	class DeclickAmp
+	{
+		public:
+			DeclickAmp (samplecnt_t sample_rate);
+
+			void apply_gain (AudioBuffer& buf, samplecnt_t n_samples, const float target);
+
+			float gain () const { return _g; }
+			void set_gain (float g) { _g = g; }
+
+		private:
+			float _a;
+			float _l;
+			float _g;
+	};
+
 private:
 	/** The number of samples by which this diskstream's output should be delayed
 	    with respect to the transport sample.  This is used for latency compensation.
@@ -135,7 +151,8 @@ private:
 	IOChange      input_change_pending;
 	samplepos_t   file_sample[DataType::num_types];
 
-	gain_t        _declick_gain;
+	DeclickAmp     _declick_amp;
+	sampleoffset_t _declick_offs;
 
 	int _do_refill_with_alloc (bool partial_fill);
 

libs/ardour/disk_reader.cc

@@ -23,6 +23,7 @@
 #include "pbd/memento_command.h"
 #include "pbd/playback_buffer.h"
 
+#include "ardour/amp.h"
 #include "ardour/audioengine.h"
 #include "ardour/audioplaylist.h"
 #include "ardour/audio_buffer.h"
@@ -57,7 +58,8 @@ DiskReader::DiskReader (Session& s, string const & str, DiskIOProcessor::Flag f)
 	, overwrite_sample (0)
 	, _pending_overwrite (false)
 	, overwrite_queued (false)
-	, _declick_gain (0)
+	, _declick_amp (s.nominal_sample_rate ())
+	, _declick_offs (0)
 {
 	file_sample[DataType::AUDIO] = 0;
 	file_sample[DataType::MIDI] = 0;
@@ -1400,3 +1402,54 @@ DiskReader::set_no_disk_output (bool yn)
 	*/
 	_no_disk_output = yn;
 }
+
+DiskReader::DeclickAmp::DeclickAmp (samplecnt_t sample_rate)
+{
+	_a = 2200.f / (gain_t)sample_rate;
+	_l = -log1p (_a);
+	_g = 0;
+}
+
+void
+DiskReader::DeclickAmp::apply_gain (AudioBuffer& buf, samplecnt_t n_samples, const float target)
+{
+	if (n_samples == 0) {
+		return;
+	}
+	float g = _g;
+
+	if (g == target) {
+		Amp::apply_simple_gain (buf, n_samples, target, 0);
+		return;
+	}
+
+	const float a = _a;
+	Sample* const buffer = buf.data ();
+
+#define MAX_NPROC 16
+	uint32_t remain = n_samples;
+	uint32_t offset = 0;
+	while (remain > 0) {
+		uint32_t n_proc = remain > MAX_NPROC ? MAX_NPROC : remain;
+		for (uint32_t i = 0; i < n_proc; ++i) {
+			buffer[offset + i] *= g;
+		}
+#if 1
+		g += a * (target - g);
+#else /* accurate exponential fade */
+		if (n_proc == MAX_NPROC) {
+			g += a * (target - g);
+		} else {
+			g = target - (target - g) * expf (_l * n_proc / MAX_NPROC);
+		}
+#endif
+		remain -= n_proc;
+		offset += n_proc;
+	}
+
+	if (fabsf (g - target) < /* GAIN_COEFF_DELTA */ 1e-5) {
+		_g = target;
+	} else {
+		_g = g;
+	}
+}