1a7d20509f
73e3fa10b4dd63b7767d6b6f270df66971067341 doc + test: Correct uint256 hex string endianness (Hodlinator) Pull request description: This PR is a follow-up to #30436. Only changes test-code and modifies/adds comments. Byte order of hex string representation was wrongfully documented as little-endian, but are in fact closer to "big-endian" (endianness is a memory-order concept rather than a numeric concept). `[arith_]uint256` both store their data in arrays with little-endian byte order (`arith_uint256` has host byte order within each `uint32_t` element). **uint256_tests.cpp** - Avoid using variable from the left side of the condition in the right side. Credits to @maflcko: https://github.com/bitcoin/bitcoin/pull/30436#discussion_r1688273553 **setup_common.cpp** - Skip needless ArithToUint256-conversion. Credits to @stickies-v: https://github.com/bitcoin/bitcoin/pull/30436#discussion_r1688621638 --- <details> <summary> ## Logical reasoning for endianness </summary> 1. Comparing an `arith_uint256` (`base_uint<256>`) to a `uint64_t` compares the beginning of the array, and verifies the remaining elements are zero. ```C++ template <unsigned int BITS> bool base_uint<BITS>::EqualTo(uint64_t b) const { for (int i = WIDTH - 1; i >= 2; i--) { if (pn[i]) return false; } if (pn[1] != (b >> 32)) return false; if (pn[0] != (b & 0xfffffffful)) return false; return true; } ``` ...that is consistent with little endian ordering of the array. 2. They have the same endianness (but `arith_*` has host-ordering of each `uint32_t` element): ```C++ arith_uint256 UintToArith256(const uint256 &a) { arith_uint256 b; for(int x=0; x<b.WIDTH; ++x) b.pn[x] = ReadLE32(a.begin() + x*4); return b; } ``` ### String conversions The reversal of order which happens when converting hex-strings <=> uint256 means strings are actually closer to big-endian, see the end of `base_blob<BITS>::SetHexDeprecated`: ```C++ unsigned char* p1 = m_data.data(); unsigned char* pend = p1 + WIDTH; while (digits > 0 && p1 < pend) { *p1 = ::HexDigit(trimmed[--digits]); if (digits > 0) { *p1 |= ((unsigned char)::HexDigit(trimmed[--digits]) << 4); p1++; } } ``` Same reversal here: ```C++ template <unsigned int BITS> std::string base_blob<BITS>::GetHex() const { uint8_t m_data_rev[WIDTH]; for (int i = 0; i < WIDTH; ++i) { m_data_rev[i] = m_data[WIDTH - 1 - i]; } return HexStr(m_data_rev); } ``` It now makes sense to me that `SetHexDeprecated`, upon receiving a shorter hex string that requires zero-padding, would pad as if the missing hex chars where towards the end of the little-endian byte array, as they are the most significant bytes. "Big-endian" string representation is also consistent with the case where `SetHexDeprecated` receives too many hex digits and discards the leftmost ones, as a form of integer narrowing takes place. ### How I got it wrong in #30436 Previously I used the less than (`<`) comparison to prove endianness, but for `uint256` it uses `memcmp` and thereby gives priority to the *lower* bytes at the beginning of the array. ```C++ constexpr int Compare(const base_blob& other) const { return std::memcmp(m_data.data(), other.m_data.data(), WIDTH); } ``` `arith_uint256` is different in that it begins by comparing the bytes from the end, as it is using little endian representation, where the bytes toward the end are more significant. ```C++ template <unsigned int BITS> int base_uint<BITS>::CompareTo(const base_uint<BITS>& b) const { for (int i = WIDTH - 1; i >= 0; i--) { if (pn[i] < b.pn[i]) return -1; if (pn[i] > b.pn[i]) return 1; } return 0; } ``` (The commit documents that `base_blob::Compare()` is doing lexicographic ordering unlike the `arith_*`-variant which is doing numeric ordering). </details> ACKs for top commit: paplorinc: ACK 73e3fa10b4dd63b7767d6b6f270df66971067341 ryanofsky: Code review ACK 73e3fa10b4dd63b7767d6b6f270df66971067341 Tree-SHA512: 121630c37ab01aa7f7097f10322ab37da3cbc0696a6bbdbf2bbd6db180dc5938c7ed91003aaa2df7cf4a4106f973f5118ba541b5e077cf3588aa641bbd528f4e
Bitcoin Core integration/staging tree
For an immediately usable, binary version of the Bitcoin Core software, see https://bitcoincore.org/en/download/.
What is Bitcoin Core?
Bitcoin Core connects to the Bitcoin peer-to-peer network to download and fully validate blocks and transactions. It also includes a wallet and graphical user interface, which can be optionally built.
Further information about Bitcoin Core is available in the doc folder.
License
Bitcoin Core is released under the terms of the MIT license. See COPYING for more information or see https://opensource.org/licenses/MIT.
Development Process
The master branch is regularly built (see doc/build-*.md for instructions) and tested, but it is not guaranteed to be
completely stable. Tags are created
regularly from release branches to indicate new official, stable release versions of Bitcoin Core.
The https://github.com/bitcoin-core/gui repository is used exclusively for the development of the GUI. Its master branch is identical in all monotree repositories. Release branches and tags do not exist, so please do not fork that repository unless it is for development reasons.
The contribution workflow is described in CONTRIBUTING.md and useful hints for developers can be found in doc/developer-notes.md.
Testing
Testing and code review is the bottleneck for development; we get more pull requests than we can review and test on short notice. Please be patient and help out by testing other people's pull requests, and remember this is a security-critical project where any mistake might cost people lots of money.
Automated Testing
Developers are strongly encouraged to write unit tests for new code, and to
submit new unit tests for old code. Unit tests can be compiled and run
(assuming they weren't disabled in configure) with: make check. Further details on running
and extending unit tests can be found in /src/test/README.md.
There are also regression and integration tests, written
in Python.
These tests can be run (if the test dependencies are installed) with: test/functional/test_runner.py
The CI (Continuous Integration) systems make sure that every pull request is built for Windows, Linux, and macOS, and that unit/sanity tests are run automatically.
Manual Quality Assurance (QA) Testing
Changes should be tested by somebody other than the developer who wrote the code. This is especially important for large or high-risk changes. It is useful to add a test plan to the pull request description if testing the changes is not straightforward.
Translations
Changes to translations as well as new translations can be submitted to Bitcoin Core's Transifex page.
Translations are periodically pulled from Transifex and merged into the git repository. See the translation process for details on how this works.
Important: We do not accept translation changes as GitHub pull requests because the next pull from Transifex would automatically overwrite them again.