From 1e3a33b0e7678289e6e86b2f552b751b6263ca91 Mon Sep 17 00:00:00 2001
From: Tobias Hienzsch <post@tobias-hienzsch.de>
Date: Sat, 10 Feb 2024 03:11:06 +0100
Subject: [PATCH] Add StaticLookupTable

---
 CMakeLists.txt                             |  1 +
 lib/grit/math/static_lookup_table.hpp      | 56 ++++++++++++++++++++++
 lib/grit/math/static_lookup_table_test.cpp | 20 ++++++++
 3 files changed, 77 insertions(+)
 create mode 100644 lib/grit/math/static_lookup_table.hpp
 create mode 100644 lib/grit/math/static_lookup_table_test.cpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 30a566f..f77b705 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -56,5 +56,6 @@ if(NOT CMAKE_CROSSCOMPILING)
             "lib/grit/fft/fft_test.cpp"
             "lib/grit/math/ilog2_test.cpp"
             "lib/grit/math/ipow_test.cpp"
+            "lib/grit/math/static_lookup_table_test.cpp"
     )
 endif()
diff --git a/lib/grit/math/static_lookup_table.hpp b/lib/grit/math/static_lookup_table.hpp
new file mode 100644
index 0000000..46dba4f
--- /dev/null
+++ b/lib/grit/math/static_lookup_table.hpp
@@ -0,0 +1,56 @@
+#pragma once
+
+#include <grit/math/linear_interpolation.hpp>
+
+#include <etl/algorithm.hpp>
+#include <etl/array.hpp>
+#include <etl/concepts.hpp>
+
+namespace grit {
+
+template<etl::floating_point Float, etl::size_t Size>
+struct StaticLookupTable
+{
+    constexpr StaticLookupTable() = default;
+
+    template<etl::regular_invocable<etl::size_t> FunctionToApproximate>
+        requires(etl::same_as<etl::invoke_result_t<FunctionToApproximate, etl::size_t>, Float>)
+    explicit constexpr StaticLookupTable(FunctionToApproximate func)
+    {
+        initialize(func);
+    }
+
+    template<etl::regular_invocable<etl::size_t> FunctionToApproximate>
+        requires(etl::same_as<etl::invoke_result_t<FunctionToApproximate, etl::size_t>, Float>)
+    constexpr auto initialize(FunctionToApproximate func) -> void
+    {
+        for (auto i = etl::size_t{0}; i < Size; ++i) {
+            auto value = func(i);
+            _buffer[i] = value;
+        }
+        _buffer[Size] = _buffer[Size - 1];
+    }
+
+    [[nodiscard]] constexpr auto atUnchecked(Float index) const -> Float
+    {
+        auto const i  = static_cast<etl::size_t>(index);
+        auto const f  = index - Float(i);
+        auto const x0 = _buffer[i];
+        auto const x1 = _buffer[i + 1];
+        return linearInterpolation(x0, x1, f);
+    }
+
+    [[nodiscard]] constexpr auto at(Float index) const -> Float
+    {
+        return atUnchecked(etl::clamp(index, Float(0), Float(Size - 1)));
+    }
+
+    [[nodiscard]] constexpr auto operator[](Float index) const -> Float { return atUnchecked(index); }
+
+    [[nodiscard]] static constexpr auto size() -> etl::size_t { return Size; }
+
+private:
+    etl::array<Float, Size + 1> _buffer;
+};
+
+}  // namespace grit
diff --git a/lib/grit/math/static_lookup_table_test.cpp b/lib/grit/math/static_lookup_table_test.cpp
new file mode 100644
index 0000000..6c48ad8
--- /dev/null
+++ b/lib/grit/math/static_lookup_table_test.cpp
@@ -0,0 +1,20 @@
+#include "static_lookup_table.hpp"
+
+#include <catch2/catch_approx.hpp>
+#include <catch2/catch_template_test_macros.hpp>
+
+TEMPLATE_TEST_CASE("grit/math: StaticLookupTable", "", float, double)
+{
+    using Float = TestType;
+
+    auto lut = grit::StaticLookupTable<Float, 127>{[](auto index) { return static_cast<Float>(index); }};
+    REQUIRE(lut[Float(0)] == Catch::Approx(0.0));
+    REQUIRE(lut[Float(55)] == Catch::Approx(55.0));
+    REQUIRE(lut[Float(126)] == Catch::Approx(126.0));
+
+    REQUIRE(lut.at(Float(100)) == Catch::Approx(100.0));
+    REQUIRE(lut.at(Float(125)) == Catch::Approx(125.0));
+    REQUIRE(lut.at(Float(126)) == Catch::Approx(126.0));
+    REQUIRE(lut.at(Float(127)) == Catch::Approx(126.0));
+    REQUIRE(lut.at(Float(128)) == Catch::Approx(126.0));
+}