stillwater-sc / universal / 26012810689

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Commit Message

feat(efloat): implement parse() for decimal and scientific literals (#867)

* feat(efloat): implement parse() for decimal and scientific literals

efloat::parse was a stub that always returned false, so operator>>
extracted the token and immediately set failbit -- the elastic
binary float type couldn't ingest any literal.

Implement parse() to:
- route the "nan", "inf", and "infinity" tokens (case-insensitive,
  optional leading sign) to the matching FloatingPointState via new
  public setnan / setinf modifiers
- route every other input through
  sw::universal::decimal_to_binary::convert (Phase B2a, #841) with
  target_mantissa_bits sized to fill efloat's available limb storage
  (nlimbs * 32), capped at 2040 bits to stay under the d2b BigBits
  budget (2048)
- apply round-to-nearest-even using d2b's guard/sticky output; if a
  carry escapes the top of the mantissa, shift right and bump the
  binary scale
- pack the resulting normalized bigint mantissa into efloat's
  uint32_t limbs MSB-first: _limb[0] bit 31 holds the hidden bit,
  _limb[i] bit 0 holds mantissa bit (target - 32(i+1))

New efloat modifiers (public API):
- setinf(bool sign = false)
- setnan(bool quiet = true)
- setsign(bool)
- setexponent(std::int64_t)
- setlimb(unsigned, std::uint32_t)

operator>> hygiene (failbit + extraction guard) was shipped in #858;
the test file now pins both the new parse contract and that hygiene.

Test (elastic/efloat/conversion/string_parse.cpp) extended to 10 groups:
- canonical normals (1, -1, 2, 4, 0.5, 0.25, 1.5, -2.5) checked
  bit-exactly against hand-derived patterns
- scientific notation across +/-100 decimal exponent
- 3.14 cross-checked against the IEEE-754 double bit pattern after
  reducing efloat's 64-bit RTNE significand to 53 bits with RTNE
- signed zero (+0, -0, 0e10, -0.0e5)
- nan/inf/infinity case-insensitive
- malformed reject (empty, abc, 1.2.3, 1e, ., 1e3.5, 42x, 0xFF,
  nann, inff)
- operator>> failbit / success in whitespace / empty st... (continued)

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66 of 68 new or added lines in 1 file covered. (97.06%)

8 existing lines in 1 file now uncovered.

46827 of 55598 relevant lines covered (84.22%)

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