Add simple 1st order feedforward filters

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

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <psemek/audio/stream.hpp>
+
+namespace psemek::audio
+{
+
+	// Turns x[n] into a0*x[n] + a1*x[n-1]
+	stream_ptr feedforward_filter(stream_ptr stream, float a0, float a1);
+
+	stream_ptr low_pass_filter(stream_ptr stream);
+	stream_ptr high_pass_filter(stream_ptr stream);
+
+}

libs/audio/source/effect/filter.cpp

@@ -0,0 +1,68 @@
+#include <psemek/audio/effect/filter.hpp>
+
+namespace psemek::audio
+{
+
+	namespace
+	{
+
+		struct feedforward_filter_impl
+			: stream
+		{
+			feedforward_filter_impl(stream_ptr stream, float a0, float a1)
+				: stream_(std::move(stream))
+				, a0_(a0)
+				, a1_(a1)
+			{}
+
+			std::optional<std::size_t> length() const override
+			{
+				return stream_->length();
+			}
+
+			std::size_t read(float * data, std::size_t sample_count) override
+			{
+				std::size_t count = stream_->read(data, sample_count);
+
+				for (std::size_t i = 0; i < count; i += 2)
+				{
+					std::swap(prev_[0], data[i + 0]);
+					std::swap(prev_[1], data[i + 1]);
+
+					data[i + 0] = prev_[0] * a0_ + data[i + 0] * a1_;
+					data[i + 1] = prev_[1] * a0_ + data[i + 1] * a1_;
+				}
+
+				return count;
+			}
+
+			std::size_t played() const override
+			{
+				return stream_->played();
+			}
+
+		private:
+			stream_ptr stream_;
+			float a0_;
+			float a1_;
+			float prev_[2]{0.f};
+		};
+
+	}
+
+	stream_ptr feedforward_filter(stream_ptr stream, float a0, float a1)
+	{
+		return std::make_shared<feedforward_filter_impl>(std::move(stream), a0, a1);
+	}
+
+	stream_ptr low_pass_filter(stream_ptr stream)
+	{
+		return feedforward_filter(std::move(stream), 1.f, 1.f);
+	}
+
+	stream_ptr high_pass_filter(stream_ptr stream)
+	{
+		return feedforward_filter(std::move(stream), 1.f, -1.f);
+	}
+
+}