From 64e0cae2741c501e8558966096b4b139d945ca82 Mon Sep 17 00:00:00 2001 From: marcandre Date: Wed, 31 Aug 2011 04:13:00 +0000 Subject: * numeric.c (flo_round): Avoid overflow by optimizing for trivial cases [Bug #5227] git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@33140 b2dd03c8-39d4-4d8f-98ff-823fe69b080e --- ChangeLog | 5 +++++ numeric.c | 34 ++++++++++++++++++++++++++++------ 2 files changed, 33 insertions(+), 6 deletions(-) diff --git a/ChangeLog b/ChangeLog index 24ac06bb9a..67ae0bff0e 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,8 @@ +Wed Aug 31 13:09:41 2011 Marc-Andre Lafortune + + * numeric.c (flo_round): Avoid overflow by optimizing for trivial + cases [Bug #5227] + Wed Aug 31 00:50:01 2011 NAKAMURA Usaku * win32/win32.c (rb_w32_select_with_thread): and my typo. we all must diff --git a/numeric.c b/numeric.c index 96a3022e84..176b589b0b 100644 --- a/numeric.c +++ b/numeric.c @@ -1491,18 +1491,40 @@ flo_round(int argc, VALUE *argv, VALUE num) VALUE nd; double number, f; int ndigits = 0; + int binexp; long val; if (argc > 0 && rb_scan_args(argc, argv, "01", &nd) == 1) { ndigits = NUM2INT(nd); } number = RFLOAT_VALUE(num); - f = pow(10, abs(ndigits)); - - if (isinf(f)) { - if (ndigits < 0) number = 0; - } - else { + frexp (number , &binexp); + +/* Let `exp` be such that `number` is written as:"0.#{digits}e#{exp}", + i.e. such that 10 ** (exp - 1) <= |number| < 10 ** exp + Recall that up to 17 digits can be needed to represent a double, + so if ndigits + exp >= 17, the intermediate value (number * 10 ** ndigits) + will be an integer and thus the result is the original number. + If ndigits + exp <= 0, the result is 0 or "1e#{exp}", so + if ndigits + exp < 0, the result is 0. + We have: + 2 ** (binexp-1) <= |number| < 2 ** binexp + 10 ** ((binexp-1)/log_2(10)) <= |number| < 10 ** (binexp/log_2(10)) + If binexp >= 0, and since log_2(10) = 3.322259: + 10 ** (binexp/4 - 1) < |number| < 10 ** (binexp/3) + binexp/4 <= exp <= binexp/3 + If binexp <= 0, swap the /4 and the /3 + So if ndigits + binexp/(4 or 3) >= 17, the result is number + If ndigits + binexp/(3 or 4) < 0 the result is 0 +*/ + if (isinf(number) || isnan(number)) { + /* Do nothing */ + } + else if ((long)ndigits * (4 - (binexp > 0)) + binexp < 0) { + number = 0; + } + else if (((long)ndigits - 17) * (3 + (binexp > 0)) + binexp < 0) { + f = pow(10, abs(ndigits)); if (ndigits < 0) { double absnum = fabs(number); if (absnum < f) return INT2FIX(0); -- cgit v1.2.3