Add audio interpolation helpers

include/audio/audio_interpolation.hpp

@@ -0,0 +1,58 @@
+// Copyright 2016 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <array>
+#include <deque>
+
+#include "audio/hle_mixer.hpp"
+#include "helpers.hpp"
+
+namespace Audio::Interpolation {
+	// A variable length buffer of signed PCM16 stereo samples.
+	using StereoBuffer16 = std::deque<std::array<s16, 2>>;
+	using StereoFrame16 = Audio::DSPMixer::StereoFrame<s16>;
+
+	struct State {
+		// Two historical samples.
+		std::array<s16, 2> xn1 = {};  //< x[n-1]
+		std::array<s16, 2> xn2 = {};  //< x[n-2]
+		// Current fractional position.
+		u64 fposition = 0;
+	};
+
+	/**
+	 * No interpolation. This is equivalent to a zero-order hold. There is a two-sample predelay.
+	 * @param state Interpolation state.
+	 * @param input Input buffer.
+	 * @param rate Stretch factor. Must be a positive non-zero value.
+	 *             rate > 1.0 performs decimation and rate < 1.0 performs upsampling.
+	 * @param output The resampled audio buffer.
+	 * @param outputi The index of output to start writing to.
+	 */
+	void none(State& state, StereoBuffer16& input, float rate, StereoFrame16& output, usize& outputi);
+
+	/**
+	 * Linear interpolation. This is equivalent to a first-order hold. There is a two-sample predelay.
+	 * @param state Interpolation state.
+	 * @param input Input buffer.
+	 * @param rate Stretch factor. Must be a positive non-zero value.
+	 *             rate > 1.0 performs decimation and rate < 1.0 performs upsampling.
+	 * @param output The resampled audio buffer.
+	 * @param outputi The index of output to start writing to.
+	 */
+	void linear(State& state, StereoBuffer16& input, float rate, StereoFrame16& output, usize& outputi);
+
+	/**
+	 * Polyphase interpolation. This is currently stubbed to just perform linear interpolation
+	 * @param state Interpolation state.
+	 * @param input Input buffer.
+	 * @param rate Stretch factor. Must be a positive non-zero value.
+	 *             rate > 1.0 performs decimation and rate < 1.0 performs upsampling.
+	 * @param output The resampled audio buffer.
+	 * @param outputi The index of output to start writing to.
+	 */
+	void polyphase(State& state, StereoBuffer16& input, float rate, StereoFrame16& output, usize& outputi);
+}  // namespace Audio::Interpolation