output/alsa, pcm: rename "DSD over USB" to "DoP"

The standard has been renamed since the early draft that was
implemented in MPD.

src/pcm/PcmDop.cxx

@@ -0,0 +1,96 @@
+/*
+ * Copyright (C) 2003-2014 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "PcmDop.hxx"
+#include "PcmBuffer.hxx"
+#include "AudioFormat.hxx"
+#include "util/ConstBuffer.hxx"
+
+#include <assert.h>
+
+constexpr
+static inline uint32_t
+pcm_two_dsd_to_dop_marker1(uint8_t a, uint8_t b)
+{
+	return 0xff050000 | (a << 8) | b;
+}
+
+constexpr
+static inline uint32_t
+pcm_two_dsd_to_dop_marker2(uint8_t a, uint8_t b)
+{
+	return 0xfffa0000 | (a << 8) | b;
+}
+
+ConstBuffer<uint32_t>
+pcm_dsd_to_dop(PcmBuffer &buffer, unsigned channels,
+	       ConstBuffer<uint8_t> _src)
+{
+	assert(audio_valid_channel_count(channels));
+	assert(!_src.IsNull());
+	assert(_src.size > 0);
+	assert(_src.size % channels == 0);
+
+	const unsigned num_src_samples = _src.size;
+	const unsigned num_src_frames = num_src_samples / channels;
+
+	/* this rounds down and discards the last odd frame; not
+	   elegant, but good enough for now */
+	const unsigned num_frames = num_src_frames / 2;
+	const unsigned num_samples = num_frames * channels;
+
+	uint32_t *const dest0 = (uint32_t *)buffer.GetT<uint32_t>(num_samples),
+		*dest = dest0;
+
+	auto src = _src.data;
+	for (unsigned i = num_frames / 2; i > 0; --i) {
+		for (unsigned c = channels; c > 0; --c) {
+			/* each 24 bit sample has 16 DSD sample bits
+			   plus the magic 0x05 marker */
+
+			*dest++ = pcm_two_dsd_to_dop_marker1(src[0], src[channels]);
+
+			/* seek the source pointer to the next
+			   channel */
+			++src;
+		}
+
+		/* skip the second byte of each channel, because we
+		   have already copied it */
+		src += channels;
+
+		for (unsigned c = channels; c > 0; --c) {
+			/* each 24 bit sample has 16 DSD sample bits
+			   plus the magic 0xfa marker */
+
+			*dest++ = pcm_two_dsd_to_dop_marker2(src[0], src[channels]);
+
+			/* seek the source pointer to the next
+			   channel */
+			++src;
+		}
+
+		/* skip the second byte of each channel, because we
+		   have already copied it */
+		src += channels;
+	}
+
+	return { dest0, num_samples };
+}