[bug #68174] dlmread performance can be improved with modern C++

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Follow-up Comment #2, bug #68174 (group octave):

Found a bug. The old tmp_stream intrinsically ignored "\r", "\v", "\f" because
"operator>>" respects std::skipws (which in turn relies on std::isspace).
"std::from_chars does no do whitespace skipping at all.
  
In Octave, when the delimiter is set to whitespace, it strictly means spaces
and tabs (" \t"). If we mix the column separator logic with the token trimming
logic, a stray e.g. "\r" causes the parser to merge rows or incorrectly split
columns. 

Attached CSET (file bug68174_dlmread_fast_20260325_200.cset) has a fix for
that (and a BIST).

Dmitri.
-- 


(file #58398)

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