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.
 	 *

test/test_pcm_export.cxx

@@ -49,6 +49,12 @@ TEST(PcmTest, ExportShift8)
 	auto dest = e.Export({src, sizeof(src)});
 	EXPECT_EQ(sizeof(expected), dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected, dest.size) == 0);
+
+	const auto silence = e.GetSilence();
+	constexpr uint8_t expected_silence[8]{};
+	EXPECT_EQ(silence.size, sizeof(expected_silence));
+	EXPECT_EQ(memcmp(silence.data, expected_silence,
+			 sizeof(expected_silence)), 0);
 }
 
 TEST(PcmTest, ExportPack24)
@@ -92,6 +98,12 @@ TEST(PcmTest, ExportPack24)
 	auto dest = e.Export({src, sizeof(src)});
 	EXPECT_EQ(expected_size, dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected, dest.size) == 0);
+
+	const auto silence = e.GetSilence();
+	constexpr uint8_t expected_silence[6]{};
+	EXPECT_EQ(silence.size, sizeof(expected_silence));
+	EXPECT_EQ(memcmp(silence.data, expected_silence,
+			 sizeof(expected_silence)), 0);
 }
 
 TEST(PcmTest, ExportReverseEndian)
@@ -147,6 +159,12 @@ TEST(PcmTest, ExportReverseEndian)
 	dest = e.Export({src, sizeof(src)});
 	EXPECT_EQ(sizeof(expected4), dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected4, dest.size) == 0);
+
+	const auto silence = e.GetSilence();
+	constexpr uint8_t expected_silence[8]{};
+	EXPECT_EQ(silence.size, sizeof(expected_silence));
+	EXPECT_EQ(memcmp(silence.data, expected_silence,
+			 sizeof(expected_silence)), 0);
 }
 
 #ifdef ENABLE_DSD
@@ -205,6 +223,12 @@ TEST(PcmTest, ExportDsdU16)
 	dest = e.Export({src4, sizeof(src4)});
 	EXPECT_EQ(sizeof(expected4), dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected4, dest.size) == 0);
+
+	const auto silence = e.GetSilence();
+	constexpr uint8_t expected_silence[]{0x69, 0x69, 0x69, 0x69};
+	EXPECT_EQ(silence.size, sizeof(expected_silence));
+	EXPECT_EQ(memcmp(silence.data, expected_silence,
+			 sizeof(expected_silence)), 0);
 }
 
 TEST(PcmTest, ExportDsdU32)
@@ -261,6 +285,12 @@ TEST(PcmTest, ExportDsdU32)
 	dest = e.Export({src4, sizeof(src4)});
 	EXPECT_EQ(sizeof(expected4), dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected4, dest.size) == 0);
+
+	const auto silence = e.GetSilence();
+	constexpr uint8_t expected_silence[]{0x69, 0x69, 0x69, 0x69, 0x69, 0x69, 0x69, 0x69};
+	EXPECT_EQ(silence.size, sizeof(expected_silence));
+	EXPECT_EQ(memcmp(silence.data, expected_silence,
+			 sizeof(expected_silence)), 0);
 }
 
 TEST(PcmTest, ExportDop)
@@ -329,6 +359,12 @@ TEST(PcmTest, ExportDop)
 	dest = e.Export({src6, sizeof(src6)});
 	ASSERT_EQ(sizeof(expected6), dest.size);
 	ASSERT_TRUE(memcmp(dest.data, expected6, dest.size) == 0);
+
+	const auto silence = e.GetSilence();
+	constexpr uint32_t expected_silence[]{0xff056969, 0xff056969, 0xfffa6969, 0xfffa6969};
+	EXPECT_EQ(silence.size, sizeof(expected_silence));
+	EXPECT_EQ(memcmp(silence.data, expected_silence,
+			 sizeof(expected_silence)), 0);
 }
 
 #endif
@@ -391,6 +427,12 @@ TestAlsaChannelOrder71()
 	auto dest = e.Export({src, sizeof(src)});
 	EXPECT_EQ(sizeof(expected), dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected, dest.size) == 0);
+
+	const auto silence = e.GetSilence();
+	constexpr value_type expected_silence[8]{};
+	EXPECT_EQ(silence.size, sizeof(expected_silence));
+	EXPECT_EQ(memcmp(silence.data, expected_silence,
+			 sizeof(expected_silence)), 0);
 }
 
 TEST(PcmTest, ExportAlsaChannelOrder)