From 52e589974d3bb03e47c28d2d92112b981aefbb76 Mon Sep 17 00:00:00 2001
From: Sam Hocevar <sam@hocevar.net>
Date: Thu, 25 Jul 2024 11:04:57 +0200
Subject: [PATCH] audio: make the Lanczos interpolator a separate class

---
 include/lol/interp                 | 18 +++++++++
 include/lol/private/audio/stream.h | 41 +++----------------
 include/lol/private/math/interp.h  | 63 ++++++++++++++++++++++++++++++
 3 files changed, 87 insertions(+), 35 deletions(-)
 create mode 100644 include/lol/interp
 create mode 100644 include/lol/private/math/interp.h

diff --git a/include/lol/interp b/include/lol/interp
new file mode 100644
index 00000000..09546f32
--- /dev/null
+++ b/include/lol/interp
@@ -0,0 +1,18 @@
+//
+//  Lol Engine
+//
+//  Copyright © 2010–2024 Sam Hocevar <sam@hocevar.net>
+//
+//  Lol Engine is free software. It comes without any warranty, to
+//  the extent permitted by applicable law. You can redistribute it
+//  and/or modify it under the terms of the Do What the Fuck You Want
+//  to Public License, Version 2, as published by the WTFPL Task Force.
+//  See http://www.wtfpl.net/ for more details.
+//
+
+#pragma once
+
+#include "private/push_macros.h"
+#include "private/math/interp.h"
+#include "private/pop_macros.h"
+
diff --git a/include/lol/private/audio/stream.h b/include/lol/private/audio/stream.h
index 33c021d0..0085c102 100644
--- a/include/lol/private/audio/stream.h
+++ b/include/lol/private/audio/stream.h
@@ -19,13 +19,14 @@
 //
 
 #include <cmath> // std::floor
-#include "../math/constants.h" // F_PI
 #include <functional> // std::function
 #include <limits> // std::numeric_limits
 #include <memory> // std::shared_ptr
 #include <type_traits> // std::is_same_v
 #include <unordered_set> // std::unordered_set
 
+#include "../math/interp.h" // lol::interp::lanczos
+
 namespace lol::audio
 {
 
@@ -338,32 +339,10 @@ public:
         if (in_rate == out_rate)
             return m_in->get(buf, frames);
 
-        // 16 taps lanczos filter
-        const size_t window_size = 16;
-        const float window_center = 8.0f;
-
-        const size_t constexpr lanczos_size = window_size * 64;
-        const float lanczos_lut_scale = (lanczos_size - 1) / (window_center + 1.0f);
-
-        static auto build_lanczos = [&]()
-        {
-            std::array<float, lanczos_size> ret;
-            for (int k = 0; k < lanczos_size; ++k)
-            {
-                float dist = float(k) * F_PI / lanczos_lut_scale;
-                float lanczos = 1.0f;
-                if (dist > 0.0f) lanczos = window_center * std::sin(dist) * std::sin(dist / window_center) / (dist * dist);
-                ret[k] = lanczos;
-            }
-            return ret;
-        };
-
-        static auto const lanczos_lut = build_lanczos();
-
         for (size_t n = 0; n < frames; ++n, m_pos += in_rate)
         {
             // Fill internal buffer if we don’t have enough data
-            while (m_cache.size() / channels < m_pos / out_rate + window_size)
+            while (m_cache.size() / channels < m_pos / out_rate + m_lanczos.size())
             {
                 // Remove obsolete frames on the left
                 size_t todelete = std::min(m_pos / out_rate, m_cache.size() / channels);
@@ -380,17 +359,7 @@ public:
             float alpha = float(m_pos % out_rate) / out_rate;
 
             for (size_t ch = 0; ch < channels; ++ch)
-            {
-                // lanczos upsampling
-                T value = T();
-                for (size_t wi = 0; wi < window_size; ++wi)
-                {
-                    float dist = std::abs(wi - window_center - alpha);
-                    float lanczos = lanczos_lut[int(dist * lanczos_lut_scale)];
-                    value += m_cache[(n0 + wi) * channels + ch] * lanczos;
-                }
-                *buf++ = value;
-            }
+                *buf++ = m_lanczos.get(&m_cache[n0 * channels + ch], channels, alpha);
         }
 
         return frames;
@@ -399,6 +368,8 @@ public:
 protected:
     std::shared_ptr<stream<T>> m_in;
 
+    interp::lanczos<T> m_lanczos;
+
     std::vector<T> m_cache;
 
     size_t m_pos;
diff --git a/include/lol/private/math/interp.h b/include/lol/private/math/interp.h
new file mode 100644
index 00000000..07a8843f
--- /dev/null
+++ b/include/lol/private/math/interp.h
@@ -0,0 +1,63 @@
+//
+//  Lol Engine
+//
+//  Copyright © 2010–2024 Sam Hocevar <sam@hocevar.net>
+//
+//  Lol Engine is free software. It comes without any warranty, to
+//  the extent permitted by applicable law. You can redistribute it
+//  and/or modify it under the terms of the Do What the Fuck You Want
+//  to Public License, Version 2, as published by the WTFPL Task Force.
+//  See http://www.wtfpl.net/ for more details.
+//
+
+#pragma once
+
+//
+// Interpolation classes
+// —————————————————————
+//
+
+#include <cmath> // std::sin
+#include "../math/constants.h" // F_PI
+
+namespace lol::interp
+{
+
+template<typename T, size_t SIZE = 16, size_t PRECISION = 64>
+class lanczos
+{
+public:
+    lanczos()
+    {
+        T const window_center = SIZE / 2;
+        T const lut_scale = (SIZE * PRECISION - 1) / (window_center + 1);
+        for (size_t k = 0; k < SIZE * PRECISION; ++k)
+        {
+            T dist = T(k) * F_PI / lut_scale;
+            m_lut[k] = dist ? window_center * std::sin(dist) * std::sin(dist / window_center) / (dist * dist) : T(1);
+        }
+    }
+
+    T get(T *data, size_t stride, T alpha)
+    {
+        T const window_center = SIZE / 2;
+        T const lut_scale = (SIZE * PRECISION - 1) / (window_center + 1);
+        T ret(0);
+        for (size_t k = 0; k < SIZE; ++k)
+        {
+            float dist = std::abs(k - window_center - alpha);
+            float lanczos = m_lut[size_t(dist * lut_scale)];
+            ret += data[k * stride] * lanczos;
+        }
+        return ret;
+    }
+
+    size_t const size() { return SIZE; }
+
+private:
+    static size_t const center = T(SIZE) / 2;
+
+    std::array<T, SIZE * PRECISION> m_lut;
+};
+
+} // namespace lol::interp