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Bug#11754279 SIGNIFICANT INACCURACY IN DECIMAL MULTIPLICATION CALCULATIONS

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frac is the number of decimal digits after the point
For each multiplication in the expression, decimal_mul() does this:
  to->frac= from1->frac + from2->frac;              /* store size in digits */
which will eventually overflow.
The code for handling the overflow, will truncate the two digits in "1.75" to "1"

Solution:
Truncate to 31 significant fractional digits, when doing decimal multiplication.
This commit is contained in:
Tor Didriksen 2012-11-29 17:21:36 +01:00
parent d68cba3413
commit b125abecb7
1 changed files with 17 additions and 16 deletions
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29
strings/decimal.c
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@ -1989,44 +1989,45 @@ int decimal_mul(const decimal_t *from1, const decimal_t *from2, decimal_t *to)
int intg1=ROUND_UP(from1->intg), intg2=ROUND_UP(from2->intg),
frac1=ROUND_UP(from1->frac), frac2=ROUND_UP(from2->frac),
intg0=ROUND_UP(from1->intg+from2->intg),
frac0=frac1+frac2, error, i, j, d_to_move;
frac0=frac1+frac2, error, iii, jjj, d_to_move;
dec1 *buf1=from1->buf+intg1, *buf2=from2->buf+intg2, *buf0,
*start2, *stop2, *stop1, *start0, carry;
sanity(to);
i=intg0; /* save 'ideal' values */
j=frac0;
iii= intg0; /* save 'ideal' values */
jjj= frac0;
FIX_INTG_FRAC_ERROR(to->len, intg0, frac0, error); /* bound size */
to->sign= from1->sign != from2->sign;
to->frac= from1->frac + from2->frac; /* store size in digits */
set_if_smaller(to->frac, NOT_FIXED_DEC);
to->intg=intg0*DIG_PER_DEC1;
if (unlikely(error))
{
set_if_smaller(to->frac, frac0*DIG_PER_DEC1);
set_if_smaller(to->intg, intg0*DIG_PER_DEC1);
if (unlikely(i > intg0)) /* bounded integer-part */
if (unlikely(iii > intg0)) /* bounded integer-part */
{
i-=intg0;
j=i >> 1;
intg1-= j;
intg2-=i-j;
iii-=intg0;
jjj= iii >> 1;
intg1-= jjj;
intg2-=iii-jjj;
frac1=frac2=0; /* frac0 is already 0 here */
}
else /* bounded fract part */
{
j-=frac0;
i=j >> 1;
jjj-=frac0;
iii=jjj >> 1;
if (frac1 <= frac2)
{
frac1-= i;
frac2-=j-i;
frac1-= iii;
frac2-=jjj-iii;
}
else
{
frac2-= i;
frac1-=j-i;
frac2-= iii;
frac1-=jjj-iii;
}
}
}