pcm/Export: add GetSilence()

src/pcm/Export.cxx

@@ -21,10 +21,13 @@
 #include "AudioFormat.hxx"
 #include "Order.hxx"
 #include "Pack.hxx"
+#include "Silence.hxx"
 #include "util/ByteReverse.hxx"
 #include "util/ConstBuffer.hxx"
+#include "util/WritableBuffer.hxx"
 
 #include <assert.h>
+#include <string.h>
 
 void
 PcmExport::Open(SampleFormat sample_format, unsigned _channels,
@@ -89,6 +92,16 @@ PcmExport::Open(SampleFormat sample_format, unsigned _channels,
 		if (sample_size > 1)
 			reverse_endian = sample_size;
 	}
+
+	/* prepare a moment of silence for GetSilence() */
+	char buffer[sizeof(silence_buffer)];
+	const size_t buffer_size = GetInputBlockSize();
+	assert(buffer_size < sizeof(buffer));
+	PcmSilence({buffer, buffer_size}, src_sample_format);
+	auto s = Export({buffer, buffer_size});
+	assert(s.size < sizeof(silence_buffer));
+	silence_size = s.size;
+	memcpy(silence_buffer, s.data, s.size);
 }
 
 void
@@ -190,6 +203,12 @@ PcmExport::GetOutputBlockSize() const noexcept
 	return GetOutputFrameSize();
 }
 
+ConstBuffer<void>
+PcmExport::GetSilence() const noexcept
+{
+	return {silence_buffer, silence_size};
+}
+
 unsigned
 PcmExport::Params::CalcOutputSampleRate(unsigned sample_rate) const noexcept
 {

src/pcm/Export.hxx

@@ -79,6 +79,10 @@ class PcmExport {
 	 */
 	PcmBuffer reverse_buffer;
 
+	size_t silence_size;
+
+	uint8_t silence_buffer[64]; /* worst-case size */
+
 	/**
 	 * The sample format of input data.
 	 */
@@ -210,6 +214,14 @@ public:
 	gcc_pure
 	size_t GetOutputBlockSize() const noexcept;
 
+	/**
+	 * @return one block of silence output; its size is the same
+	 * as GetOutputBlockSize(); the pointer is valid as long as
+	 * this #PcmExport object exists and until the next Open()
+	 * call
+	 */
+	ConstBuffer<void> GetSilence() const noexcept;
+
 	/**
 	 * Export a PCM buffer.
 	 *