Implement first-order feedback audio filter

libs/audio/include/psemek/audio/effect/filter.hpp

@@ -5,9 +5,12 @@
 namespace psemek::audio
 {
 
-	// Turns x[n] into a0*x[n] + a1*x[n-1]
+	// Turns x[n] into y[n] = a0*x[n] + a1*x[n-1]
 	stream_ptr feedforward_filter(stream_ptr stream, float a0, float a1);
 
+	// Turns x[n] into y[n] = a0*x[n] + b1*y[n-1]
+	stream_ptr feedback_filter(stream_ptr stream, float a0, float b1);
+
 	stream_ptr low_pass_filter(stream_ptr stream);
 	stream_ptr high_pass_filter(stream_ptr stream);
 

libs/audio/source/effect/filter.cpp

@@ -48,6 +48,47 @@ namespace psemek::audio
 			float prev_[2]{0.f};
 		};
 
+		struct feedback_filter_impl
+			: stream
+		{
+			feedback_filter_impl(stream_ptr stream, float a0, float b1)
+				: stream_(std::move(stream))
+				, a0_(a0)
+				, b1_(b1)
+			{}
+
+			std::optional<std::size_t> length() const override
+			{
+				return stream_->length();
+			}
+
+			std::size_t read(util::span<float> samples) override
+			{
+				std::size_t count = stream_->read(samples);
+
+				for (std::size_t i = 0; i < count; i += 2)
+				{
+					samples[i + 0] = prev_[0] * b1_ + samples[i + 0] * a0_;
+					samples[i + 1] = prev_[1] * b1_ + samples[i + 1] * a0_;
+					prev_[0] = samples[i + 0];
+					prev_[1] = samples[i + 1];
+				}
+
+				return count;
+			}
+
+			std::size_t played() const override
+			{
+				return stream_->played();
+			}
+
+		private:
+			stream_ptr stream_;
+			float a0_;
+			float b1_;
+			float prev_[2]{0.f};
+		};
+
 	}
 
 	stream_ptr feedforward_filter(stream_ptr stream, float a0, float a1)
@@ -55,6 +96,11 @@ namespace psemek::audio
 		return std::make_shared<feedforward_filter_impl>(std::move(stream), a0, a1);
 	}
 
+	stream_ptr feedback_filter(stream_ptr stream, float a0, float b1)
+	{
+		return std::make_shared<feedback_filter_impl>(std::move(stream), a0, b1);
+	}
+
 	stream_ptr low_pass_filter(stream_ptr stream)
 	{
 		return feedforward_filter(std::move(stream), 1.f, 1.f);