pcm/Dsd{16,32}: stash odd frames away for the next call

Similar to commit 32380d1db0, these are
the final parts for really fixing
https://github.com/MusicPlayerDaemon/MPD/issues/469

src/pcm/Dsd16.cxx

@@ -20,6 +20,8 @@
 #include "Dsd16.hxx"
 #include "util/ConstBuffer.hxx"
 
+#include <functional>
+
 /**
  * Construct a 16 bit integer from two bytes.
  */
@@ -54,18 +56,14 @@ void
 Dsd16Converter::Open(unsigned _channels) noexcept
 {
 	channels = _channels;
+
+	rest_buffer.Open(channels);
 }
 
 ConstBuffer<uint16_t>
-Dsd16Converter::Convert(ConstBuffer<uint8_t> _src) noexcept
+Dsd16Converter::Convert(ConstBuffer<uint8_t> src) noexcept
 {
-	const size_t in_frames = _src.size / channels;
+	using namespace std::placeholders;
-	const size_t out_frames = in_frames / 2;
+	return rest_buffer.Process<uint16_t>(buffer, src, channels,
-	const size_t out_samples = out_frames * channels;
+					     std::bind(Dsd8To16, _1, _2, _3, channels));
-
-	const uint8_t *src = _src.data;
-	const auto dest = buffer.GetT<uint16_t>(out_samples);
-	Dsd8To16(dest, src, out_frames, channels);
-
-	return {dest, out_samples};
 }

src/pcm/Dsd16.hxx

@@ -21,6 +21,7 @@
 #define MPD_PCM_DSD_16_HXX
 
 #include "Buffer.hxx"
+#include "RestBuffer.hxx"
 
 #include <stdint.h>
 
@@ -34,10 +35,13 @@ class Dsd16Converter {
 
 	PcmBuffer buffer;
 
+	PcmRestBuffer<uint8_t, 2> rest_buffer;
+
 public:
 	void Open(unsigned _channels) noexcept;
 
 	void Reset() noexcept {
+		rest_buffer.Reset();
 	}
 
 	ConstBuffer<uint16_t> Convert(ConstBuffer<uint8_t> src) noexcept;

src/pcm/Dsd32.cxx

@@ -18,9 +18,10 @@
  */
 
 #include "Dsd32.hxx"
-#include "Buffer.hxx"
 #include "util/ConstBuffer.hxx"
 
+#include <functional>
+
 /**
  * Construct a 32 bit integer from four bytes.
  */
@@ -57,18 +58,14 @@ void
 Dsd32Converter::Open(unsigned _channels) noexcept
 {
 	channels = _channels;
+
+	rest_buffer.Open(channels);
 }
 
 ConstBuffer<uint32_t>
-Dsd32Converter::Convert(ConstBuffer<uint8_t> _src) noexcept
+Dsd32Converter::Convert(ConstBuffer<uint8_t> src) noexcept
 {
-	const size_t in_frames = _src.size / channels;
+	using namespace std::placeholders;
-	const size_t out_frames = in_frames / 4;
+	return rest_buffer.Process<uint32_t>(buffer, src, channels,
-	const size_t out_samples = out_frames * channels;
+					     std::bind(Dsd8To32, _1, _2, _3, channels));
-
-	const uint8_t *src = _src.data;
-	const auto dest = buffer.GetT<uint32_t>(out_samples);
-	Dsd8To32(dest, src, out_frames, channels);
-
-	return {dest, out_samples};
 }

src/pcm/Dsd32.hxx

@@ -21,6 +21,7 @@
 #define MPD_PCM_DSD_32_HXX
 
 #include "Buffer.hxx"
+#include "RestBuffer.hxx"
 
 #include <stdint.h>
 
@@ -34,10 +35,13 @@ class Dsd32Converter {
 
 	PcmBuffer buffer;
 
+	PcmRestBuffer<uint8_t, 4> rest_buffer;
+
 public:
 	void Open(unsigned _channels) noexcept;
 
 	void Reset() noexcept {
+		rest_buffer.Reset();
 	}
 
 	ConstBuffer<uint32_t> Convert(ConstBuffer<uint8_t> src) noexcept;

test/test_pcm_export.cxx

@@ -152,6 +152,27 @@ TEST(PcmTest, ExportDsdU16)
 	auto dest = e.Export({src, sizeof(src)});
 	EXPECT_EQ(sizeof(expected), dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected, dest.size) == 0);
+
+	/* no output, 2/4 remains */
+	static constexpr uint8_t src2[] = { 0x11, 0x22 };
+	static constexpr uint16_t expected2[] = {};
+	dest = e.Export({src2, sizeof(src2)});
+	EXPECT_EQ(sizeof(expected2), dest.size);
+	EXPECT_TRUE(memcmp(dest.data, expected2, dest.size) == 0);
+
+	/* one full frame and 2/4 remains */
+	static constexpr uint8_t src3[] = { 0x33, 0x44, 0x55, 0x66 };
+	static constexpr uint16_t expected3[] = { 0x1133, 0x2244 };
+	dest = e.Export({src3, sizeof(src3)});
+	EXPECT_EQ(sizeof(expected3), dest.size);
+	EXPECT_TRUE(memcmp(dest.data, expected3, dest.size) == 0);
+
+	/* two full frames and 2/4 remains again */
+	static constexpr uint8_t src4[] = { 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee };
+	static constexpr uint16_t expected4[] = { 0x5577, 0x6688, 0x99bb, 0xaacc };
+	dest = e.Export({src4, sizeof(src4)});
+	EXPECT_EQ(sizeof(expected4), dest.size);
+	EXPECT_TRUE(memcmp(dest.data, expected4, dest.size) == 0);
 }
 
 TEST(PcmTest, ExportDsdU32)
@@ -182,6 +203,27 @@ TEST(PcmTest, ExportDsdU32)
 	auto dest = e.Export({src, sizeof(src)});
 	EXPECT_EQ(sizeof(expected), dest.size);
 	EXPECT_TRUE(memcmp(dest.data, expected, dest.size) == 0);
+
+	/* no output, 4/8 remains */
+	static constexpr uint8_t src2[] = { 0x11, 0x22, 0x33, 0x44 };
+	static constexpr uint32_t expected2[] = {};
+	dest = e.Export({src2, sizeof(src2)});
+	EXPECT_EQ(sizeof(expected2), dest.size);
+	EXPECT_TRUE(memcmp(dest.data, expected2, dest.size) == 0);
+
+	/* one full frame and 4/8 remains */
+	static constexpr uint8_t src3[] = { 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc };
+	static constexpr uint32_t expected3[] = { 0x11335577, 0x22446688 };
+	dest = e.Export({src3, sizeof(src3)});
+	EXPECT_EQ(sizeof(expected3), dest.size);
+	EXPECT_TRUE(memcmp(dest.data, expected3, dest.size) == 0);
+
+	/* two full frames and 2/4 remains again */
+	static constexpr uint8_t src4[] = { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 };
+	static constexpr uint32_t expected4[] = { 0x99bb0103, 0xaacc0204 };
+	dest = e.Export({src4, sizeof(src4)});
+	EXPECT_EQ(sizeof(expected4), dest.size);
+	EXPECT_TRUE(memcmp(dest.data, expected4, dest.size) == 0);
 }
 
 TEST(PcmTest, ExportDop)