output/alsa: add "allowed_formats" setting

Allows defining a list of supported audio formats, and allows
switching on and off DoP with certain formats.

This is a first rough draft.  The setting syntax and its semantics may
still be redesigned.

src/lib/alsa/AllowedFormat.cxx

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2003-2017 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 "AllowedFormat.hxx"
+#include "AudioParser.hxx"
+#include "util/IterableSplitString.hxx"
+
+#include <stdexcept>
+
+namespace Alsa {
+
+AllowedFormat::AllowedFormat(StringView s)
+{
+#ifdef ENABLE_DSD
+	const StringView dop_tail("=dop");
+	if (s.EndsWith(dop_tail)) {
+		dop = true;
+		s.size -= dop_tail.size;
+	} else
+		dop = false;
+#endif
+
+	char buffer[64];
+	if (s.size >= sizeof(buffer))
+		throw std::runtime_error("Failed to parse audio format");
+
+	memcpy(buffer, s.data, s.size);
+	buffer[s.size] = 0;
+
+	format = ParseAudioFormat(buffer, true);
+
+#ifdef ENABLE_DSD
+	if (dop && format.format != SampleFormat::DSD)
+		throw std::runtime_error("DoP works only with DSD");
+#endif
+}
+
+std::forward_list<AllowedFormat>
+AllowedFormat::ParseList(StringView s)
+{
+	std::forward_list<AllowedFormat> list;
+	auto tail = list.before_begin();
+
+	for (const auto i : IterableSplitString(s, ' '))
+		if (!i.empty())
+			tail = list.emplace_after(tail, i);
+
+	return list;
+}
+
+} // namespace Alsa

src/lib/alsa/AllowedFormat.hxx

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2003-2017 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.
+ */
+
+#ifndef MPD_ALSA_ALLOWED_FORMAT_HXX
+#define MPD_ALSA_ALLOWED_FORMAT_HXX
+
+#include "check.h"
+#include "AudioFormat.hxx"
+
+#include <forward_list>
+
+struct StringView;
+
+namespace Alsa {
+
+struct AllowedFormat {
+	AudioFormat format;
+#ifdef ENABLE_DSD
+	bool dop;
+#endif
+
+	explicit AllowedFormat(StringView s);
+
+	static std::forward_list<AllowedFormat> ParseList(StringView s);
+};
+
+} // namespace Alsa
+
+#endif