From 332ea60d2378ef8d567c93502fa7b95eccd962e5 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 24 Apr 2006 20:36:32 +0000
Subject: [PATCH] Improve our private implementation of cbrt() to give results
of the accuracy expected by the regression tests. Per suggestion from
Martijn van Oosterhout.
---
src/backend/utils/adt/float.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/src/backend/utils/adt/float.c b/src/backend/utils/adt/float.c
index 3382b86ba0d..b5147c9aa45 100644
--- a/src/backend/utils/adt/float.c
+++ b/src/backend/utils/adt/float.c
@@ -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;
}
--
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