Add wavetable selection by frequency

src/sfizz/Wavetables.cpp

@@ -5,6 +5,7 @@
 // If not, contact the sfizz maintainers at https://github.com/sfztools/sfizz
 
 #include "Wavetables.h"
+#include "MathHelpers.h"
 #include <kiss_fftr.h>
 #include <memory>
 
@@ -112,4 +113,33 @@ const HarmonicProfile& HarmonicProfile::getSquare()
     return squareProfile;
 }
 
+//------------------------------------------------------------------------------
+constexpr unsigned WavetableRange::countOctaves;
+constexpr float WavetableRange::frequencyScaleFactor;
+
+unsigned WavetableRange::getOctaveForFrequency(float f)
+{
+    int oct = fp_exponent(frequencyScaleFactor * f);
+    return clamp<int>(oct, 0, countOctaves - 1);
+}
+
+WavetableRange WavetableRange::getRangeForOctave(int o)
+{
+    WavetableRange range;
+
+    Fraction<uint64_t> mant = fp_mantissa(0.0f);
+    float k = 1.0f / frequencyScaleFactor;
+
+    range.minFrequency = k * fp_from_parts<float>(0, o, 0);
+    range.maxFrequency = k * fp_from_parts<float>(0, o, mant.den - 1);
+
+    return range;
+}
+
+WavetableRange WavetableRange::getRangeForFrequency(float f)
+{
+    int oct = getOctaveForFrequency(f);
+    return getRangeForOctave(oct);
+}
+
 } // namespace sfz

src/sfizz/Wavetables.h

@@ -38,4 +38,36 @@ public:
     void generate(absl::Span<float> table, double amplitude, double cutoff) const;
 };
 
+/**
+   A helper to select ranges of a multi-sampled oscillator, according to the
+   frequency of an oscillator.
+
+   The ranges are identified by octave numbers; not octaves in a musical sense,
+   but as logarithmic divisions of the frequency range.
+ */
+class WavetableRange {
+public:
+    float minFrequency = 0;
+    float maxFrequency = 0;
+
+    static constexpr unsigned countOctaves = 10;
+    static constexpr float frequencyScaleFactor = 0.05;
+
+    static unsigned getOctaveForFrequency(float f);
+    static WavetableRange getRangeForOctave(int o);
+    static WavetableRange getRangeForFrequency(float f);
+
+    // Note: using the frequency factor 0.05, octaves are as follows:
+    //     octave 0: 20 Hz - 40 Hz
+    //     octave 1: 40 Hz - 80 Hz
+    //     octave 2: 80 Hz - 160 Hz
+    //     octave 3: 160 Hz - 320 Hz
+    //     octave 4: 320 Hz - 640 Hz
+    //     octave 5: 640 Hz - 1280 Hz
+    //     octave 6: 1280 Hz - 2560 Hz
+    //     octave 7: 2560 Hz - 5120 Hz
+    //     octave 8: 5120 Hz - 10240 Hz
+    //     octave 9: 10240 Hz - 20480 Hz
+};
+
 } // namespace sfz

tests/CMakeLists.txt

@@ -26,6 +26,7 @@ set(SFIZZ_TEST_SOURCES
     CurveT.cpp
     RegionTriggersT.cpp
     FloatHelpersT.cpp
+    WavetablesT.cpp
 )
 
 add_executable(sfizz_tests ${SFIZZ_TEST_SOURCES})

tests/WavetablesT.cpp

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: BSD-2-Clause
+
+// This code is part of the sfizz library and is licensed under a BSD 2-clause
+// license. You should have receive a LICENSE.md file along with the code.
+// If not, contact the sfizz maintainers at https://github.com/sfztools/sfizz
+
+#include "sfizz/Wavetables.h"
+#include "sfizz/MathHelpers.h"
+#include "catch2/catch.hpp"
+#include <algorithm>
+
+TEST_CASE("[Wavetables] Frequency ranges")
+{
+    int cur_oct = std::numeric_limits<int>::min();
+    int min_oct = std::numeric_limits<int>::max();
+    int max_oct = std::numeric_limits<int>::min();
+
+    for (int note = 0; note < 128; ++note) {
+        double f = midiNoteFrequency(note);
+
+        int oct = sfz::WavetableRange::getOctaveForFrequency(f);
+
+        REQUIRE(oct >= 0);
+        REQUIRE(oct < sfz::WavetableRange::countOctaves);
+
+        REQUIRE(oct >= cur_oct);
+        cur_oct = oct;
+
+        min_oct = std::min(min_oct, oct);
+        max_oct = std::max(max_oct, oct);
+
+        sfz::WavetableRange range = sfz::WavetableRange::getRangeForOctave(oct);
+        REQUIRE((f >= range.minFrequency || oct == 0));
+        REQUIRE((f <= range.maxFrequency || oct == sfz::WavetableRange::countOctaves - 1));
+    }
+
+    // check ranges to be decently adjusted to the MIDI frequency range
+    REQUIRE(min_oct == 0);
+    REQUIRE(max_oct == sfz::WavetableRange::countOctaves - 1);
+}