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Ryan Ofsky 1a7d20509f
Merge bitcoin/bitcoin#30526: doc: Correct uint256 hex string endianness 
73e3fa10b4 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 73e3fa10b4
  ryanofsky:
    Code review ACK 73e3fa10b4

Tree-SHA512: 121630c37ab01aa7f7097f10322ab37da3cbc0696a6bbdbf2bbd6db180dc5938c7ed91003aaa2df7cf4a4106f973f5118ba541b5e077cf3588aa641bbd528f4e
2024-08-04 22:27:10 -04:00
.github ci: enable berkley db on test each commit job 2024-07-29 11:43:47 +01:00
.tx qt: Bump Transifex slug for 28.x 2024-07-30 16:14:19 +01:00
build-aux/m4 qt, build: Drop QT_STATICPLUGIN macro 2024-08-01 14:01:07 +01:00
build_msvc qt, build: Drop QT_STATICPLUGIN macro 2024-08-01 14:01:07 +01:00
ci Merge bitcoin/bitcoin#30522: ci: Add missing qttools5-dev install to Asan task 2024-07-25 12:06:55 +01:00
contrib Merge bitcoin/bitcoin#30452: guix: bump time-machine to efc26826400762207cde9f23802cfe75a737963c 2024-07-31 09:50:10 +01:00
depends depends: Fix zeromq build on OpenBSD 2024-07-31 23:37:50 +01:00
doc Merge bitcoin/bitcoin#30352: policy: Add PayToAnchor(P2A), OP_1 <0x4e73> as a standard output script for spending 2024-08-02 15:49:44 +01:00
share contrib: rpcauth.py - Add new option (-j/--json) to output text in json format 2024-04-25 08:32:28 -05:00
src Merge bitcoin/bitcoin#30526: doc: Correct uint256 hex string endianness 2024-08-04 22:27:10 -04:00
test Merge bitcoin/bitcoin#30352: policy: Add PayToAnchor(P2A), OP_1 <0x4e73> as a standard output script for spending 2024-08-02 15:49:44 +01:00
.cirrus.yml ci: forks can opt-out of CI branch push (Cirrus only) 2024-06-25 20:03:44 +02:00
.editorconfig ci: Drop AppVeyor CI integration 2021-09-07 06:12:53 +03:00
.gitattributes Separate protocol versioning from clientversion 2014-10-29 00:24:40 -04:00
.gitignore Merge bitcoin/bitcoin#29733: build, macos: Drop unused osx_volname target 2024-04-02 14:57:22 +01:00
.python-version Bump .python-version from 3.9.17 to 3.9.18 2023-10-24 18:51:24 +02:00
.style.yapf Update .style.yapf 2023-06-01 23:35:10 +05:30
autogen.sh build: make sure we can overwrite config.{guess,sub} 2023-06-13 14:58:43 +02:00
configure.ac Merge bitcoin/bitcoin#28893: Fix SSE4.1-related issues 2024-07-17 16:58:54 +01:00
CONTRIBUTING.md doc: Correct pull request prefix for scripts and tools 2024-05-22 09:59:58 +02:00
COPYING doc: upgrade Bitcoin Core license to 2024 2024-01-10 16:29:01 -06:00
INSTALL.md doc: Added hyperlink for doc/build 2021-09-09 19:53:12 +05:30
Makefile.am contrib: use c++ rather than c for binary tests 2024-07-04 20:16:16 +00:00
README.md doc: Explain Bitcoin Core in README.md 2022-05-10 07:49:09 +02:00
SECURITY.md Update security.md contact for achow101 2023-12-14 18:14:54 -05:00

README.md

Bitcoin Core integration/staging tree

https://bitcoincore.org

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.