hex: Add from_hex<T> for user types

include/evmc/hex.hpp

@@ -114,4 +114,21 @@ inline std::optional<bytes> from_hex(std::string_view hex)
         return {};
     return bs;
 }
+
+template <typename T>
+constexpr std::optional<T> from_hex(std::string_view s) noexcept
+{
+    // Omit the optional 0x prefix.
+    if (s.size() >= 2 && s[0] == '0' && s[1] == 'x')
+        s.remove_prefix(2);
+
+    T r{};  // The T must have .bytes array. This may be lifted if std::bit_cast is available.
+    constexpr auto num_out_bytes = std::size(r.bytes);
+    const auto num_in_bytes = s.length() / 2;
+    if (num_in_bytes > num_out_bytes)
+        return {};
+    if (!from_hex(s.begin(), s.end(), &r.bytes[num_out_bytes - num_in_bytes]))
+        return {};
+    return r;
+}
 }  // namespace evmc

test/unittests/hex_test.cpp

@@ -73,3 +73,33 @@ TEST(hex, validate_hex)
     EXPECT_FALSE(validate_hex("0"));
     EXPECT_FALSE(validate_hex("WXYZ"));
 }
+
+TEST(hex, from_hex_to_custom_type)
+{
+    struct X
+    {
+        uint8_t bytes[4];
+    };
+    constexpr auto test = [](std::string_view in) {
+        return evmc::hex({evmc::from_hex<X>(in).value().bytes, sizeof(X)});
+    };
+
+    static_assert(evmc::from_hex<X>("01").value().bytes[3] == 0x01);  // Works in constexpr.
+
+    EXPECT_EQ(test("01020304"), "01020304");
+    EXPECT_EQ(test("010203"), "00010203");
+    EXPECT_EQ(test("0102"), "00000102");
+    EXPECT_EQ(test("01"), "00000001");
+    EXPECT_EQ(test(""), "00000000");
+
+    EXPECT_EQ(test("0x01020304"), "01020304");
+    EXPECT_EQ(test("0x010203"), "00010203");
+    EXPECT_EQ(test("0x0102"), "00000102");
+    EXPECT_EQ(test("0x01"), "00000001");
+    EXPECT_EQ(test("0x"), "00000000");
+
+    EXPECT_FALSE(evmc::from_hex<X>("0"));
+    EXPECT_FALSE(evmc::from_hex<X>("0x "));
+    EXPECT_FALSE(evmc::from_hex<X>("0xf"));
+    EXPECT_FALSE(evmc::from_hex<X>("0x 00"));
+}