[KennyMcSimpson]

SymmetricMatrix and DistanceMatrix keep their class when cloned. Algorithms that use those clones as workspaces consequently mirror writes into the opposite triangle; DistanceMatrix also resets each diagonal write to zero. For the matrix in #222, determinant() returns 757.1567106010962 instead of 442.75, and a nonsingular distance matrix can incorrectly fail inversion as singular.

Use plain Matrix copies for LU/QR decomposition workspaces, QR/Cholesky right-hand sides, echelon reduction, and the two-block NIPALS residuals. The public structured clone() behavior stays intact. Regressions check factor reconstruction, determinants, inverse/solve identities, right-hand sides, row reduction, residual reconstruction, and unchanged inputs.

Fixes #222.

Validation on Windows with Node.js 24.11.0:

• New tests on the unchanged source: 22 failures, 2 passing controls. Patched full suite: 328 tests pass; ESLint and Prettier pass.
• Independent NumPy 2.4.6 and reconstruction checks over 50 matrices: 1,400 checks, 552 failures on the original build and zero after the fix, including ordinary-matrix and public-clone controls.
• Package build, clean installation of the npm archive, the same 1,400 installed-package checks, and 39 CommonJS/ESM/UMD checks pass.

The existing one-block NIPALS path (Y omitted) mutates its input and retains its separate structured-residual limitation; this change only addresses the two-block path identified in #222. It does not change the clone() declaration or other algorithms.

Prepared with AI assistance (OpenAI Codex).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.08%. Comparing base (da8f1f6) to head (2253c78).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #225      +/-   ##
==========================================
+ Coverage   68.88%   70.08%   +1.20%     
==========================================
  Files          49       49              
  Lines        5904     5904              
  Branches     1060     1093      +33     
==========================================
+ Hits         4067     4138      +71     
+ Misses       1824     1753      -71     
  Partials       13       13              

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