stillwater-sc / universal / 26381695317

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Commit Message

fix(einteger): correct multiply carry propagation + shift canonicalization (#991) (#992)

operator*= kept a single carry accumulator across all outer-limb rows and
flushed it only once at block(ll+rl-1), misplacing carries at the limb
boundary: (2^32)*(2^32) != 2^64 and ~99.99% of 53-bit x 53-bit products were
wrong. Single-limb operands happened to work, so the existing test (which only
ever multiplied single-limb values < 2^20) never caught it. Rewrite with a
per-row carry that ripples into the higher limbs, and trim high zero limbs so
results are canonical for the limb-count-sensitive operator==.

Also fix operator<<=: when the shift is an exact multiple of the block width
it returned early, skipping remove_leading_zeros() and leaving the speculative
push_back(0) as a trailing zero limb -- which made equal values (e.g. a product
vs 1<<64) compare unequal.

Strengthen elastic/einteger/arithmetic/multiplication.cpp with multi-limb
coverage cross-checked against a native int64 reference (operands span several
limbs, product fits int64 -- portable, no __int128) and hand-checkable
power-of-two products across limb boundaries with canonical-form (limb count)
assertions. Passes gcc-13 and clang-18.

Resolves #991

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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