feat(data): add as_cxpr_array

cth/incl/cth/data/cxpr.hpp

@@ -0,0 +1,24 @@
+#pragma once
+#include <algorithm>
+#include <array>
+#include <cstddef>
+#include <ranges>
+
+namespace cth::dt {
+
+/**
+ * copies the range returned by a constexpr generator into a std::array
+ * @details allows results computed with constexpr std::vector & co. to outlive constant evaluation
+ * @tparam Generator callable without arguments that returns a sized range
+ */
+template<auto Generator>
+[[nodiscard]] consteval auto as_cxpr_array() {
+    using value_type = std::ranges::range_value_t<decltype(Generator())>;
+    constexpr size_t SIZE = std::ranges::size(Generator());
+
+    std::array<value_type, SIZE> result{};
+    std::ranges::copy(Generator(), result.begin());
+    return result;
+}
+
+}

cth/tests/src/data/cxpr_test.cpp

@@ -0,0 +1,31 @@
+#include "test.hpp"
+
+#include "cth/data/cxpr.hpp"
+
+#include <string_view>
+#include <vector>
+
+namespace cth::dt {
+
+DATA_TEST(as_cxpr_array, main) {
+    constexpr auto array = as_cxpr_array<[] { return std::vector{1, 2, 3}; }>();
+
+    static_assert(array.size() == 3);
+    EXPECT_EQ(array, (std::array{1, 2, 3}));
+}
+
+DATA_TEST(as_cxpr_array, computed_size) {
+    static constexpr std::string_view CSV = "a,bb,ccc";
+
+    constexpr auto splits = as_cxpr_array<[] {
+        std::vector<size_t> result{};
+        for(size_t i = 0; i < CSV.size(); i++)
+            if(CSV[i] == ',')
+                result.push_back(i);
+        return result;
+    }>();
+
+    EXPECT_EQ(splits, (std::array<size_t, 2>{1, 4}));
+}
+
+}