1 files changed, 79 insertions, 0 deletions
diff --git a/src/audio_core/algorithm/filter.cpp b/src/audio_core/algorithm/filter.cpp
new file mode 100644
index 000000000..403b8503f
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+++ b/src/audio_core/algorithm/filter.cpp
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+// Copyright 2018 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#define _USE_MATH_DEFINES
+
+#include <algorithm>
+#include <array>
+#include <cmath>
+#include <vector>
+#include "audio_core/algorithm/filter.h"
+#include "common/common_types.h"
+
+namespace AudioCore {
+
+Filter Filter::LowPass(double cutoff, double Q) {
+    const double w0 = 2.0 * M_PI * cutoff;
+    const double sin_w0 = std::sin(w0);
+    const double cos_w0 = std::cos(w0);
+    const double alpha = sin_w0 / (2 * Q);
+
+    const double a0 = 1 + alpha;
+    const double a1 = -2.0 * cos_w0;
+    const double a2 = 1 - alpha;
+    const double b0 = 0.5 * (1 - cos_w0);
+    const double b1 = 1.0 * (1 - cos_w0);
+    const double b2 = 0.5 * (1 - cos_w0);
+
+    return {a0, a1, a2, b0, b1, b2};
+}
+
+Filter::Filter() : Filter(1.0, 0.0, 0.0, 1.0, 0.0, 0.0) {}
+
+Filter::Filter(double a0, double a1, double a2, double b0, double b1, double b2)
+    : a1(a1 / a0), a2(a2 / a0), b0(b0 / a0), b1(b1 / a0), b2(b2 / a0) {}
+
+void Filter::Process(std::vector<s16>& signal) {
+    const size_t num_frames = signal.size() / 2;
+    for (size_t i = 0; i < num_frames; i++) {
+        std::rotate(in.begin(), in.end() - 1, in.end());
+        std::rotate(out.begin(), out.end() - 1, out.end());
+
+        for (size_t ch = 0; ch < channel_count; ch++) {
+            in[0][ch] = signal[i * channel_count + ch];
+
+            out[0][ch] = b0 * in[0][ch] + b1 * in[1][ch] + b2 * in[2][ch] - a1 * out[1][ch] -
+                         a2 * out[2][ch];
+
+            signal[i * 2 + ch] = std::clamp(out[0][ch], -32768.0, 32767.0);
+        }
+    }
+}
+
+/// Calculates the appropriate Q for each biquad in a cascading filter.
+/// @param total_count The total number of biquads to be cascaded.
+/// @param index 0-index of the biquad to calculate the Q value for.
+static double CascadingBiquadQ(size_t total_count, size_t index) {
+    const double pole = M_PI * (2 * index + 1) / (4.0 * total_count);
+    return 1.0 / (2.0 * std::cos(pole));
+}
+
+CascadingFilter CascadingFilter::LowPass(double cutoff, size_t cascade_size) {
+    std::vector<Filter> cascade(cascade_size);
+    for (size_t i = 0; i < cascade_size; i++) {
+        cascade[i] = Filter::LowPass(cutoff, CascadingBiquadQ(cascade_size, i));
+    }
+    return CascadingFilter{std::move(cascade)};
+}
+
+CascadingFilter::CascadingFilter() = default;
+CascadingFilter::CascadingFilter(std::vector<Filter> filters) : filters(std::move(filters)) {}
+
+void CascadingFilter::Process(std::vector<s16>& signal) {
+    for (auto& filter : filters) {
+        filter.Process(signal);
+    }
+}
+
+} // namespace AudioCore