refactor: add uint256::FromUserHex helper

FromUserHex will be used in future commits to construct
uint256 instances from user hex input without being
unnecessarily restrictive on formatting by allowing
0x-prefixed input that is shorter than 64 characters.

src/test/fuzz/hex.cpp

@@ -32,6 +32,10 @@ FUZZ_TARGET(hex)
         assert(random_hex_string.length() == 64);
         assert(Txid::FromHex(random_hex_string));
         assert(Wtxid::FromHex(random_hex_string));
+        assert(uint256::FromUserHex(random_hex_string));
+    }
+    if (const auto result{uint256::FromUserHex(random_hex_string)}) {
+        assert(uint256::FromHex(result->ToString()));
     }
     (void)uint256S(random_hex_string);
     try {

src/test/uint256_tests.cpp

@@ -386,6 +386,35 @@ BOOST_AUTO_TEST_CASE(from_hex)
     TestFromHex<Wtxid>();
 }
 
+BOOST_AUTO_TEST_CASE(from_user_hex)
+{
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("").value(), uint256::ZERO);
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("0x").value(), uint256::ZERO);
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("0").value(), uint256::ZERO);
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("00").value(), uint256::ZERO);
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("1").value(), uint256::ONE);
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("0x10").value(), uint256{0x10});
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("10").value(), uint256{0x10});
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("0xFf").value(), uint256{0xff});
+    BOOST_CHECK_EQUAL(uint256::FromUserHex("Ff").value(), uint256{0xff});
+    const std::string valid_hex_64{"0x0123456789abcdef0123456789abcdef0123456789ABDCEF0123456789ABCDEF"};
+    BOOST_REQUIRE_EQUAL(valid_hex_64.size(), 2 + 64); // 0x prefix and 64 hex digits
+    BOOST_CHECK_EQUAL(uint256::FromUserHex(valid_hex_64.substr(2)).value().ToString(), ToLower(valid_hex_64.substr(2)));
+    BOOST_CHECK_EQUAL(uint256::FromUserHex(valid_hex_64.substr(0)).value().ToString(), ToLower(valid_hex_64.substr(2)));
+
+    BOOST_CHECK(!uint256::FromUserHex("0x0 "));                       // no spaces at end,
+    BOOST_CHECK(!uint256::FromUserHex(" 0x0"));                       // or beginning,
+    BOOST_CHECK(!uint256::FromUserHex("0x 0"));                       // or middle,
+    BOOST_CHECK(!uint256::FromUserHex(" "));                          // etc.
+    BOOST_CHECK(!uint256::FromUserHex("0x0ga"));                      // invalid character
+    BOOST_CHECK(!uint256::FromUserHex("x0"));                         // broken prefix
+    BOOST_CHECK(!uint256::FromUserHex("0x0x00"));                     // two prefixes not allowed
+    BOOST_CHECK(!uint256::FromUserHex(valid_hex_64.substr(2) + "0")); // 1 hex digit too many
+    BOOST_CHECK(!uint256::FromUserHex(valid_hex_64 + "a"));           // 1 hex digit too many
+    BOOST_CHECK(!uint256::FromUserHex(valid_hex_64 + " "));           // whitespace after max length
+    BOOST_CHECK(!uint256::FromUserHex(valid_hex_64 + "z"));           // invalid character after max length
+}
+
 BOOST_AUTO_TEST_CASE( check_ONE )
 {
     uint256 one = uint256S("0000000000000000000000000000000000000000000000000000000000000001");

src/uint256.h

@@ -9,6 +9,7 @@
 #include <crypto/common.h>
 #include <span.h>
 #include <util/strencodings.h>
+#include <util/string.h>
 
 #include <algorithm>
 #include <array>
@@ -17,6 +18,7 @@
 #include <cstring>
 #include <optional>
 #include <string>
+#include <string_view>
 
 /** Template base class for fixed-sized opaque blobs. */
 template<unsigned int BITS>
@@ -106,6 +108,25 @@ std::optional<uintN_t> FromHex(std::string_view str)
     rv.SetHexDeprecated(str);
     return rv;
 }
+/**
+ * @brief Like FromHex(std::string_view str), but allows an "0x" prefix
+ *        and pads the input with leading zeroes if it is shorter than
+ *        the expected length of uintN_t::size()*2.
+ *
+ *        Designed to be used when dealing with user input.
+ */
+template <class uintN_t>
+std::optional<uintN_t> FromUserHex(std::string_view input)
+{
+    input = util::RemovePrefixView(input, "0x");
+    constexpr auto expected_size{uintN_t::size() * 2};
+    if (input.size() < expected_size) {
+        auto padded = std::string(expected_size, '0');
+        std::copy(input.begin(), input.end(), padded.begin() + expected_size - input.size());
+        return FromHex<uintN_t>(padded);
+    }
+    return FromHex<uintN_t>(input);
+}
 } // namespace detail
 
 /** 160-bit opaque blob.
@@ -127,6 +148,7 @@ public:
 class uint256 : public base_blob<256> {
 public:
     static std::optional<uint256> FromHex(std::string_view str) { return detail::FromHex<uint256>(str); }
+    static std::optional<uint256> FromUserHex(std::string_view str) { return detail::FromUserHex<uint256>(str); }
     constexpr uint256() = default;
     constexpr explicit uint256(uint8_t v) : base_blob<256>(v) {}
     constexpr explicit uint256(Span<const unsigned char> vch) : base_blob<256>(vch) {}