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Improve our private implementation of cbrt() to give results of the

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accuracy expected by the regression tests.  Per suggestion from
Martijn van Oosterhout.
This commit is contained in:
Tom Lane 2006-04-24 20:36:32 +00:00
parent 7e97b419df
commit 332ea60d23
1 changed files with 13 additions and 2 deletions
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15
src/backend/utils/adt/float.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.123 2006/03/11 01:19:22 neilc Exp $
* $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.124 2006/04/24 20:36:32 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -2373,11 +2373,22 @@ float84ge(PG_FUNCTION_ARGS)
/* ========== PRIVATE ROUTINES ========== */
#ifndef HAVE_CBRT
static double
cbrt(double x)
{
int isneg = (x < 0.0);
double tmpres = pow(fabs(x), (double) 1.0 / (double) 3.0);
double absx = fabs(x);
double tmpres = pow(absx, (double) 1.0 / (double) 3.0);
/*
* The result is somewhat inaccurate --- not really pow()'s fault,
* as the exponent it's handed contains roundoff error. We can improve
* the accuracy by doing one iteration of Newton's formula. Beware of
* zero input however.
*/
if (tmpres > 0.0)
tmpres -= (tmpres - absx/(tmpres*tmpres)) / (double) 3.0;
return isneg ? -tmpres : tmpres;
}