stillwater-sc / universal / 35352604718

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fix: ereal integer conversion and pown, posit text functions (#1552)

Two ereal defects and three posit ones, batched for one CI round.

ereal's convert_signed() and convert_unsigned() were `// TBD` for every
non-zero value, and all ten integer constructors and assignment operators go
through them, so ereal(5), ereal(-7) and `e = 42` were all zero. Below 2^53 an
integer goes into one limb exactly; above it, it is split at 32 bits into two
exact doubles whose sum is the integer. The magnitude of a signed value is
taken unsigned, so the most negative value converts like any other, and a
signed plain char goes to the signed path rather than being read as a value
near 2^64 (#1460).

pown(ereal, int) negated a negative exponent in signed arithmetic, which is
undefined for INT_MIN. The magnitude is now taken unsigned and the reciprocal
applied once at the end (#1466).

posit's quadrant() classified by sign before looking for zero or NaR, so zero
came back SE and NaR NW, and to_base2_scientific() decoded both as if they had
fields. hex_format() wrote "16.1x0x40'00p", which is not the documented
nbits.esxNN...NNp form and which parse() rejected, so the native format did not
round-trip at any width; parse() also read the digits through a uint64_t that
dropped every bit above 63. It now writes the documented form, parse() reads
the full width, and the old form still parses (#1391).

The three field operator>> overloads were defined on a const reference where
their friends were declared on a non-const one, and relied on an input
operator and a member that do not exist. They compiled only because nothing
instantiated them; deleted (#1411). operator<<(positFraction) pre-incremented
where to_string() post-increments, and dropped the last valid bit (#1412).

Nine mutations of the fixed code were each caught by the new tests.

BEHAVIOUR-CHANGE: an ereal constructed or assigned from a non-zero integer
holds that integer instead of zero; posit quadrant() returns "0" an... (continued)

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57 of 60 new or added lines in 3 files covered. (95.0%)

3 existing lines in 1 file now uncovered.

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