AudioCore: Implement time stretcher (#1737)

* AudioCore: Implement time stretcher

* fixup! AudioCore: Implement time stretcher

* fixup! fixup! AudioCore: Implement time stretcher

* fixup! fixup! fixup! AudioCore: Implement time stretcher

* fixup! fixup! fixup! fixup! AudioCore: Implement time stretcher

* fixup! fixup! fixup! fixup! fixup! AudioCore: Implement time stretcher

src/audio_core/time_stretch.h

@@ -0,0 +1,57 @@
+// Copyright 2016 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <cstddef>
+#include <memory>
+#include <vector>
+
+#include "common/common_types.h"
+
+namespace AudioCore {
+
+class TimeStretcher final {
+public:
+    TimeStretcher();
+    ~TimeStretcher();
+
+    /**
+     * Set sample rate for the samples that Process returns.
+     * @param sample_rate The sample rate.
+     */
+    void SetOutputSampleRate(unsigned int sample_rate);
+
+    /**
+     * Add samples to be processed.
+     * @param sample_buffer Buffer of samples in interleaved stereo PCM16 format.
+     * @param num_sample Number of samples.
+     */
+    void AddSamples(const s16* sample_buffer, size_t num_samples);
+
+    /// Flush audio remaining in internal buffers.
+    void Flush();
+
+    /// Resets internal state and clears buffers.
+    void Reset();
+
+    /**
+     * Does audio stretching and produces the time-stretched samples.
+     * Timer calculations use sample_delay to determine how much of a margin we have.
+     * @param sample_delay How many samples are buffered downstream of this module and haven't been played yet.
+     * @return Samples to play in interleaved stereo PCM16 format.
+     */
+    std::vector<s16> Process(size_t sample_delay);
+
+private:
+    struct Impl;
+    std::unique_ptr<Impl> impl;
+
+    /// INTERNAL: ratio = wallclock time / emulated time
+    double CalculateCurrentRatio();
+    /// INTERNAL: If we have too many or too few samples downstream, nudge ratio in the appropriate direction.
+    double CorrectForUnderAndOverflow(double ratio, size_t sample_delay) const;
+    /// INTERNAL: Gets the time-stretched samples from SoundTouch.
+    std::vector<s16> GetSamples();
+};
+
+} // namespace AudioCore