[Libreoffice-bugs] [Bug 141752] New: calc: calculate: trap in rawsubtract, wrong calculation sequence, documentation imprecise

[bugzilla-daemon at bugs.documentfoundation.org]

https://bugs.documentfoundation.org/show_bug.cgi?id=141752

            Bug ID: 141752
           Summary: calc: calculate: trap in rawsubtract, wrong
                    calculation sequence, documentation imprecise
           Product: LibreOffice
           Version: 6.1.6.3 release
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: medium
         Component: Calc
          Assignee: libreoffice-bugs at lists.freedesktop.org
          Reporter: newbie-02 at gmx.de

Description:
rawsubtract() is a handy tool to check e.g. which deviations evolve from
fp-math vs. e.g. calc rounding, and to perform precise 'unrounded'
calculations,  

the documentation says, as well as the tooltip: 
'RAWSUBTRACT(Minuend, Subtrahend1, Subtrahend2, ...)'
<https://help.libreoffice.org/6.1/he/text/scalc/01/func_rawsubtract.html> 

one would expect to get: 
((Minuend - Subtrahend1) - Subtrahend2) ... - Subtrahendx    and not: 
((Minuend - Subtrahendx) - Subtrahendx-1) ... - Subtrahend1 
but what you get is indeed the second calculation, reverse sequence of
processing the operands. 

as the sequence matters (non associativity of fp-addition) and e.g. 
'=1E16 + 1 - 1E16' results in 0 while 
'=1E16 - 1E16 + 1' gives 1 it is: 

1. important to process the operands in a meaningful order (suppose to correct
that), and 
2. important to have the documentation (and tooltip) matching the operation,
which does not as it is now, 

practical sample: 
'=RAWSUBTRACT(1E+016;1E+016;1)' -> '0', should be '-1', 
'=RAWSUBTRACT(1E+016;1;1E+016)' -> '-1', should be '0' due to fp-math :-(  

assumptions: compiler optimizations? 

difficult: changing the order will break compatibility for existing sheets, not
changing the order but only the documentation will trap more users in the
future (as they normally don't check the documentation, but expect a program
that can be operated intuitively) 

besides rawsubtract being rarely used (quite rarely know to users), it does!
matter to work in a meaningful way because if, then it's used to check
difficult things where precise functioning makes big difference. 

Steps to Reproduce:
1. key '=RAWSUBTRACT(1E+016;1E+016;1)' in a cell, 
2. key '=RAWSUBTRACT(1E+016;1;1E+016)' in the cell below


Actual Results:
'0'
'-1'

Expected Results:
'-1'
'0'


Reproducible: Always


User Profile Reset: No



Additional Info:
didn't check other hardware than Intel (Xeon), assume 'all', 
didn't check Linux, assume 'all', 
didn't check Versions prior 6.1.6.3, assume all versions having rawsubtract, 
didn't reset user profile as clear repro in different independent installs,

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