filter/TwoFilters: add class PreparedTwoFilters

src/filter/plugins/TwoFilters.cxx

@@ -18,7 +18,10 @@
  */
 
 #include "TwoFilters.hxx"
+#include "pcm/AudioFormat.hxx"
 #include "util/ConstBuffer.hxx"
+#include "util/RuntimeError.hxx"
+#include "util/StringBuffer.hxx"
 
 ConstBuffer<void>
 TwoFilters::FilterPCM(ConstBuffer<void> src)
@@ -37,3 +40,21 @@ TwoFilters::Flush()
 
 	return second->Flush();
 }
+
+std::unique_ptr<Filter>
+PreparedTwoFilters::Open(AudioFormat &audio_format)
+{
+	auto a = first->Open(audio_format);
+
+	const auto &a_out_format = a->GetOutAudioFormat();
+	auto b_in_format = a_out_format;
+	auto b = second->Open(b_in_format);
+
+	if (b_in_format != a_out_format)
+		throw FormatRuntimeError("Audio format not supported by filter '%s': %s",
+					 second_name.c_str(),
+					 ToString(a_out_format).c_str());
+
+	return std::make_unique<TwoFilters>(std::move(a),
+					    std::move(b));
+}

src/filter/plugins/TwoFilters.hxx

@@ -21,8 +21,10 @@
 #define MPD_TWO_FILTERS_HXX
 
 #include "filter/Filter.hxx"
+#include "filter/Prepared.hxx"
 
 #include <memory>
+#include <string>
 
 /**
  * A #Filter implementation which chains two other filters.
@@ -46,4 +48,21 @@ public:
 	ConstBuffer<void> Flush() override;
 };
 
+/**
+ * Like #TwoFilters, but implements the #PreparedFilter interface.
+ */
+class PreparedTwoFilters final : public PreparedFilter {
+	std::unique_ptr<PreparedFilter> first, second;
+	std::string second_name;
+
+public:
+	template<typename F, typename S, typename N>
+	PreparedTwoFilters(F &&_first, S &&_second, N &&_second_name) noexcept
+		:first(std::forward<F>(_first)),
+		 second(std::forward<S>(_second)),
+		 second_name(std::forward<N>(_second_name)) {}
+
+	std::unique_ptr<Filter> Open(AudioFormat &audio_format) override;
+};
+
 #endif