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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 20:36:41 +00:00
parent e37c0d2eb8
commit 2c4abf11e1
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 * IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.115 2005/10/15 02:49:28 momjian Exp $ * $PostgreSQL: pgsql/src/backend/utils/adt/float.c,v 1.115.2.1 2006/04/24 20:36:41 tgl Exp $
* *
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
@@ -2321,11 +2321,22 @@ float84ge(PG_FUNCTION_ARGS)
/* ========== PRIVATE ROUTINES ========== */ /* ========== PRIVATE ROUTINES ========== */
#ifndef HAVE_CBRT #ifndef HAVE_CBRT
static double static double
cbrt(double x) cbrt(double x)
{ {
int isneg = (x < 0.0); 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; return isneg ? -tmpres : tmpres;
} }