fix: N-02 exact powers of 10 for log10

[immrsd]

Fixes N-02

Summary by CodeRabbit

• Bug Fixes

  • Improved log10 precision for integer powers of ten in fixed-point math, now returning exact results for these inputs.

• Documentation

  • Clarified log10 abort and rounding conditions, and documented the exactness guarantee for powers of ten calculations.

• Tests

  • Updated test cases to verify exact log10 results for powers of ten across supported fixed-point types.

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Walkthrough

The PR enhances log10 computation for fixed-point types UD30x9 and SD29x9 to return exact results when inputs are integer powers of ten, adds a utility import for UD30x9, updates corresponding test expectations from approximate-within-1-ULP to exact, and clarifies the documentation to reflect the new guarantee.

Changes

Fixed-point log10 exactness for powers of ten

Layer / File(s)	Summary
UD30x9 log10 fast-path for powers of ten math/fixed_point/sources/ud30x9/ud30x9_base.move, math/fixed_point/tests/ud30x9_tests/log10_tests.move	New import of openzeppelin_math::u128 enables exact log10 computation for powers of ten. The log10 implementation branches on u128::is_power_of_ten, applies u128::log10 with scale adjustment, and returns immediately for exact results; non-powers-of-ten fall back to log2-based computation. Test renamed to log10_of_powers_of_ten_is_exact() and assertions updated to expect k * SCALE instead of k * SCALE - 1.
SD29x9 log10 fast-path for powers of ten math/fixed_point/sources/sd29x9/sd29x9_base.move, math/fixed_point/tests/sd29x9_tests/log10_tests.move	New fast-path detects when the decomposed magnitude is an integer power of ten (including sub-unit 10^-k), uses u256::is_power_of_ten and u256::log10 to compute the exponent exactly, adjusts by the fixed-point scale, and returns; other inputs continue with log2-based fallback. Tests renamed to log10_of_positive_powers_of_ten_is_exact() and log10_of_negative_powers_of_ten_is_exact(), with expected values simplified to ±k * SCALE (removed prior 1-ULP adjustments). Boundary test for pos(1) adjusted to expect exactly -9 * SCALE.
Documentation update math/fixed_point/README.md	Clarified log10 behavioral notes to explicitly guarantee exactness on integer powers of ten (including sub-unit powers for SD29x9), replacing the prior general statement about round-down rounding causing up-to-1-ULP deviation at certain identity points.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• OpenZeppelin/contracts-sui#323: Main PR's new fast-paths rely on u128::is_power_of_ten and u256::is_power_of_ten, which are refactored in this PR to compute via log10_floor + pow instead of lookup tables.
• OpenZeppelin/contracts-sui#320: Main PR's changes modify existing log10 implementations in sd29x9_base.move and ud30x9_base.move, building on the log10 functions introduced in PR #320.

Suggested reviewers

• ericnordelo
• 0xNeshi

Poem

🐰 Ten to the power—exact and true!
log10's fast-path cuts through,
Powers of ten, no rounding blur,
Scale adjusts with perfect fur,
Fixed-point math, now crystal-clear! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description provides only a reference to the audit issue without including required PR checklist items (Tests, Documentation, Changelog) or detailed context about the changes.	Add the PR checklist section with status indicators and provide context explaining what was changed and why the fix addresses the N-02 issue.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title precisely references the audit issue N-02 and describes the main change: fixing log10 to return exact values at integer powers of ten.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fp-diff-audit-fix-N-02

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[immrsd]

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[ericnordelo]

LGTM!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.91%. Comparing base (9dbcdec) to head (259213f).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-v1.2     #344      +/-   ##
================================================
+ Coverage         95.89%   95.91%   +0.01%     
================================================
  Files                22       22              
  Lines              2215     2225      +10     
  Branches            603      605       +2     
================================================
+ Hits               2124     2134      +10     
  Misses               67       67              
  Partials             24       24              

Flag	Coverage Δ
contracts/access	65.40% <ø> (ø)
math/core	86.12% <ø> (ø)
math/fixed_point	59.00% <100.00%> (+3.34%)	⬆️

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[0xNeshi]

Nice