stillwater-sc / universal / 26486254675

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Commit Message

fix(ereal): parse high-precision decimals without precision loss or NaN (#1006) (#1011)

ereal::parse() built the mantissa as a giant integer M and multiplied by
10^(-e). Two failures:

- NaN for inputs needing |exponent| >= 309: 10^e (and M for >308-digit integers)
  exceed DBL_MAX, so the leading component became inf and the EFT error term NaN.
- Precision decayed ~1 decimal digit per power of ten: 10^(-e) is a
  subnormal-range value (1/10^300 ~ 1e-300, whose 2nd component ~1e-316 is below
  DBL_MIN), so it cannot hold more than ~1 normal component -- capping results
  near 16 digits regardless of maxlimbs.

Root primitive fix (expansion_quotient): scale the divisor to near-unit
magnitude by an exact power of two (ldexp) before the Newton reciprocal, form
the product in normal range, then apply the exact 2^-k. The reciprocal now
operates on a normal, near-unit operand, so division keeps full precision
whenever the quotient is in normal range. Renormalize after the ldexp to strip
any components that underflowed to zero (restores Priest canonical form). This
also benefits every other ereal division of a large/small-magnitude operand.

parse() changes:
- Apply a negative exponent by DIVIDING by the normal-range 10^e (full precision
  via the fixed quotient) instead of multiplying by the subnormal 10^-e.
- Cap significant-digit accumulation at MAX_SIG_DIGITS (<= 307) so M stays below
  DBL_MAX; dropped integer digits scale the exponent, sub-precision fraction
  digits are dropped. Skip leading zeros.
- Apply |exponent| in chunks of <= 10^308 so out-of-range values saturate to
  inf (overflow) or 0 (underflow) rather than NaN.
- Cap the result to maxlimbs components (the leading, canonical-order ones carry
  the full value; the rest would be subnormal, violating the EFT invariant).

Result: parse precision scales with input length to ereal<19>'s ~306 digits
(verified 7*parse("0.1428...") == 1 to 100-300 digits); 1e400 -> +inf,
1e-400 -> 0, no NaN for an... (continued)

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