Add all-pass echo filter

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

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <psemek/audio/stream.hpp>
+#include <psemek/audio/duration.hpp>
+
+namespace psemek::audio
+{
+
+	// All-pass filter
+	// Turns x[n] into y[n] = - gain * x[n] + x[n - delay] + gain * y[n - delay]
+	stream_ptr all_pass(stream_ptr stream, duration delay, float gain);
+
+}

libs/audio/source/effect/all_pass.cpp

@@ -0,0 +1,68 @@
+#include <psemek/audio/effect/all_pass.hpp>
+
+#include <vector>
+
+namespace psemek::audio
+{
+
+	namespace
+	{
+
+		struct all_pass_impl
+			: stream
+		{
+			all_pass_impl(stream_ptr stream, duration delay, float gain)
+				: stream_(std::move(stream))
+				, buffer_(2 * std::max(1l, delay.samples()), 0.f)
+				, gain_(gain)
+			{}
+
+			std::optional<std::size_t> length() const override
+			{
+				return std::nullopt;
+			}
+
+			// y[n] = (−g·x[n]) + x[n−M] + (g·y[n−M])
+			std::size_t read(util::span<float> samples) override
+			{
+				auto count = stream_->read(samples);
+
+				for (std::size_t i = 0; i < count; ++i)
+				{
+					float input = samples[i];
+					samples[i] = - gain_ * input + buffer_[buffer_pos_];
+					buffer_[buffer_pos_] = input + gain_ * samples[i];
+					++buffer_pos_;
+					buffer_pos_ %= buffer_.size();
+				}
+
+				for (std::size_t i = count; i < samples.size(); ++i)
+				{
+					samples[i] = buffer_[buffer_pos_];
+					buffer_[buffer_pos_] = gain_ * buffer_[buffer_pos_];
+					++buffer_pos_;
+					buffer_pos_ %= buffer_.size();
+				}
+
+				return samples.size();
+			}
+
+			std::size_t played() const override
+			{
+				return stream_->played();
+			}
+
+		private:
+			stream_ptr stream_;
+			std::vector<float> buffer_;
+			std::size_t buffer_pos_ = 0;
+			float gain_;
+		};
+
+	}
+
+	stream_ptr all_pass(stream_ptr stream, duration delay, float gain)
+	{
+		return std::make_shared<all_pass_impl>(std::move(stream), delay, gain);
+	}
+}