Support retrieving length of a stream

libs/audio/include/psemek/audio/stream.hpp

@@ -2,13 +2,18 @@
 
 #include <cstddef>
 #include <memory>
+#include <optional>
 
 namespace psemek::audio
 {
 
 	struct stream
 	{
+		// The length of the stream in samples, or nullopt if the stream is infinite
+		virtual std::optional<std::size_t> length() const = 0;
+
 		// Return value less than sample count means end of stream
+		// Must be called from mixing thread
 		virtual std::size_t read(float * data, std::size_t sample_count) = 0;
 
 		virtual ~stream() {}

libs/audio/include/psemek/audio/wave/generator.hpp

@@ -14,6 +14,11 @@ namespace psemek::audio
 			: func_(std::move(func))
 		{}
 
+		std::optional<std::size_t> length() const override
+		{
+			return std::nullopt;
+		}
+
 		std::size_t read(float * data, std::size_t sample_count) override
 		{
 			auto end = data + sample_count;

libs/audio/source/duplicate.cpp

@@ -27,6 +27,11 @@ namespace psemek::audio
 				: common_(std::move(common))
 			{}
 
+			std::optional<std::size_t> length() const override
+			{
+				return common_->stream->length();
+			}
+
 			std::size_t read(float * data, std::size_t sample_count) override
 			{
 				if (counter_ == common_->counter)

libs/audio/source/effect/fade_in.cpp

@@ -18,6 +18,11 @@ namespace psemek::audio
 				, start_samples_(2 * std::round(start * frequency))
 			{}
 
+			std::optional<std::size_t> length() const override
+			{
+				return stream_->length();
+			}
+
 			std::size_t read(float * data, std::size_t sample_count) override
 			{
 				auto const result = stream_->read(data, sample_count);

libs/audio/source/effect/fade_out.cpp

@@ -18,6 +18,12 @@ namespace psemek::audio
 				, start_samples_(2 * std::round(start * frequency))
 			{}
 
+			std::optional<std::size_t> length() const override
+			{
+				// TODO: compute fade_out length using the number of samples already played
+				return stream_->length();
+			}
+
 			std::size_t read(float * data, std::size_t sample_count) override
 			{
 				if (current_ >= length_)

libs/audio/source/effect/volume.cpp

@@ -21,6 +21,11 @@ namespace psemek::audio
 			float smoothness() const override { return base_.smoothness(); }
 			float smoothness(float value) override { return base_.smoothness(value); }
 
+			std::optional<std::size_t> length() const override
+			{
+				return stream_->length();
+			}
+
 			std::size_t read(float * data, std::size_t sample_count) override
 			{
 				auto result = stream_->read(data, sample_count);

libs/audio/source/mixer.cpp

@@ -18,6 +18,11 @@ namespace psemek::audio
 
 			std::size_t read(float * data, std::size_t sample_count) override;
 
+			std::optional<std::size_t> length() const override
+			{
+				return std::nullopt;
+			}
+
 		private:
 			std::vector<channel_ptr> channels_;
 			std::vector<channel_ptr> alive_channels_;

libs/audio/source/stereo.cpp

@@ -16,6 +16,19 @@ namespace psemek::audio
 				, right_(std::move(right))
 			{}
 
+			std::optional<std::size_t> length() const override
+			{
+				auto left = left_->length();
+				auto right = right_->length();
+
+				if (left && right)
+					return std::min(*left, *right);
+				else if (left)
+					return left;
+				else
+					return right;
+			}
+
 			std::size_t read(float * data, std::size_t sample_count) override
 			{
 				if (buffer_.size() < sample_count)

libs/audio/source/wave/silence.cpp

@@ -14,6 +14,11 @@ namespace psemek::audio
 				std::fill(data, data + sample_count, 0.f);
 				return sample_count;
 			}
+
+			std::optional<std::size_t> length() const override
+			{
+				return std::nullopt;
+			}
 		};
 
 	}