refactor: Move unhex lookup table out of function (#7104)

include/xrpl/basics/StringUtilities.h

@@ -7,9 +7,11 @@
 #include <boost/utility/string_view.hpp>
 
 #include <array>
+#include <concepts>
 #include <cstdint>
 #include <optional>
 #include <string>
+#include <type_traits>
 
 namespace xrpl {
 
@@ -26,28 +28,39 @@ namespace xrpl {
 std::string
 sqlBlobLiteral(Blob const& blob);
 
+namespace detail {
+
+template <typename T>
+concept SomeChar = std::same_as<std::remove_cvref_t<T>, int8_t> ||
+    std::same_as<std::remove_cvref_t<T>, char> || std::same_as<std::remove_cvref_t<T>, uint8_t>;
+
+inline constexpr std::array<std::optional<int>, 256> const kDIGIT_LOOKUP_TABLE = []() {
+    std::array<std::optional<int>, 256> t{};
+
+    for (int i = 0; i < 10; ++i)
+        t['0' + i] = i;
+
+    for (int i = 0; i < 6; ++i)
+    {
+        t['A' + i] = 10 + i;
+        t['a' + i] = 10 + i;
+    }
+
+    return t;
+}();
+
+inline std::optional<int>
+hexCharToInt(SomeChar auto hexChar)
+{
+    return kDIGIT_LOOKUP_TABLE[static_cast<uint8_t>(hexChar)];
+}
+
+}  // namespace detail
+
 template <class Iterator>
 std::optional<Blob>
 strUnHex(std::size_t strSize, Iterator begin, Iterator end)
 {
-    static constexpr std::array<int, 256> const kDIGIT_LOOKUP_TABLE = []() {
-        std::array<int, 256> t{};
-
-        for (auto& x : t)
-            x = -1;
-
-        for (int i = 0; i < 10; ++i)
-            t['0' + i] = i;
-
-        for (int i = 0; i < 6; ++i)
-        {
-            t['A' + i] = 10 + i;
-            t['a' + i] = 10 + i;
-        }
-
-        return t;
-    }();
-
     Blob out;
 
     out.reserve((strSize + 1) / 2);
@@ -56,27 +69,26 @@ strUnHex(std::size_t strSize, Iterator begin, Iterator end)
 
     if (strSize & 1)
     {
-        int c = kDIGIT_LOOKUP_TABLE[*iter++];
-
-        if (c < 0)
+        auto const c = detail::hexCharToInt(*iter++);
+        if (!c.has_value())
             return {};
 
-        out.push_back(c);
+        out.push_back(static_cast<unsigned char>(*c));
     }
 
     while (iter != end)
     {
-        int const cHigh = kDIGIT_LOOKUP_TABLE[*iter++];
+        auto const cHigh = detail::hexCharToInt(*iter++);
 
-        if (cHigh < 0)
+        if (!cHigh.has_value())
             return {};
 
-        int const cLow = kDIGIT_LOOKUP_TABLE[*iter++];
+        auto const cLow = detail::hexCharToInt(*iter++);
 
-        if (cLow < 0)
+        if (!cLow.has_value())
             return {};
 
-        out.push_back(static_cast<unsigned char>((cHigh << 4) | cLow));
+        out.push_back(static_cast<unsigned char>((*cHigh << 4) | *cLow));
     }
 
     return {std::move(out)};