Rick Regan <exploringbin...@gmail.com> added the comment:
> if (!(dig = quorem(b,d))) {
> b = multadd(b, 10, 0); /* very unlikely */
> dig = quorem(b,d);
> }
>
> This code is part of the algorithm for strtod. Here b and d are
> Bigints, and b / d is a fraction that gives an approximation to
> the value of the input to strtod; the aim is to produce the
> digits of b / d one-by-one to compare them with the strtod input,
> and (eventually) use the result of that comparison work out whether
> to round up or down.
> If the condition of the 'if' block above is ever satisfied, b is
> multiplied by 10 (that's the multadd(b, 10, 0) call), so the
> fraction b / d is multiplied by 10 (with no corresponding correction
> for the strtod input string), and the wrong comparison is made!
Mark,
I think I know the motivation for this code, although I still don't know how it
could hit. The halfway value H is scaled by a power of ten to put it in the
form "d1.d2d3d4d5...". The power of ten exponent is derived from the input
decimal string S, instead of computing it from H using logarithms.
Now what if H's exponent does not match S's? I'm thinking of cases like S =
10^n and H = 9.99999999... * 10^(n-1). Scaling H by 10^-n would make it
0.999999999... . That leading 0 needs to be removed, by multiplying by 10, do
put it in the right form.
First of all, I don't know if a case like this is possible. Second of all, the
check would fail either way (1 against 0 vs 1 against 9).
BTW, b / d represents only the significant digits of H, so it shouldn't matter
that there's no corresponding adjustment to the input string.
To summarize: I'm not saying this code is necessary; I'm just saying it makes
you wonder.
----------
nosy: +DoctorBinary
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<http://bugs.python.org/issue9009>
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