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Enumerate all cases in the overflow detection strategy in mpd_qlog10().

This commit is contained in:
Stefan Krah 2012-06-09 00:01:28 +02:00
parent f6f56183ee
commit 5248a2d3c1
1 changed files with 17 additions and 4 deletions
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21
Modules/_decimal/libmpdec/mpdecimal.c
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@ -4793,12 +4793,25 @@ mpd_qlog10(mpd_t *result, const mpd_t *a, const mpd_context_t *ctx,
mpd_qfinalize(result, &workctx, status);
return;
}
/* Check if the result will overflow.
/*
* Check if the result will overflow (0 < x, x != 1):
* 1) log10(x) < 0 iff adjexp(x) < 0
* 2) 0 < x /\ x <= y ==> adjexp(x) <= adjexp(y)
* 3) adjexp(x) <= log10(x) < adjexp(x) + 1
*
* 1) adjexp(a) + 1 > log10(a) >= adjexp(a)
* Case adjexp(x) >= 0:
* 4) adjexp(x) <= abs(log10(x))
* Case adjexp(x) > 0:
* 5) adjexp(adjexp(x)) <= adjexp(abs(log10(x)))
* Case adjexp(x) == 0:
* mpd_exp_digits(t)-1 == 0 <= emax (the shortcut is not triggered)
*
* 2) |log10(a)| >= adjexp(a), if adjexp(a) >= 0
* |log10(a)| > -adjexp(a)-1, if adjexp(a) < 0
* Case adjexp(x) < 0:
* 6) -adjexp(x) - 1 < abs(log10(x))
* Case adjexp(x) < -1:
* 7) adjexp(-adjexp(x) - 1) <= adjexp(abs(log(x)))
* Case adjexp(x) == -1:
* mpd_exp_digits(t)-1 == 0 <= emax (the shortcut is not triggered)
*/
adjexp = mpd_adjexp(a);
t = (adjexp < 0) ? -adjexp-1 : adjexp;