From: nobu.nokada@...
Date: 2003-04-18T01:14:07+09:00
Subject: Re: Roundoff problem with Float and Marshal

Hi,

At Fri, 18 Apr 2003 00:22:02 +0900,
Yukihiro Matsumoto wrote:
> |Everything is working fine for 3.95601527633861
> |PROBLEM: a is 3.7517675036461267e+17, b is 3.7517675036461261e+17, 
> |and ma is "\004\010f\e3.7517675036461267e+17"
> 
> This is caused by bad implementation of strtod that Ruby 1.8 uses (in
> util.c).  I will seek for the better one.

Hmmm, it seems so.

$ ruby -e 'p 3.7517675036461267e+17'
3.7517675036461261e+17


Index: util.c
===================================================================
RCS file: /pub/cvs/ruby/src/ruby/util.c,v
retrieving revision 1.32
diff -u -2 -p -w -r1.32 util.c
--- util.c	16 Jan 2003 07:34:03 -0000	1.32
+++ util.c	17 Apr 2003 16:10:57 -0000
@@ -16,4 +16,5 @@
 #include <stdio.h>
 #include <errno.h>
+#include <math.h>
 
 #ifdef _WIN32
@@ -807,5 +808,4 @@ ruby_strtod(string, endPtr)
 	fraction = 0.0;
 	p = string;
-	goto done;
     }
     else {
@@ -831,6 +831,4 @@ ruby_strtod(string, endPtr)
 	    frac2 = 10*frac2 + (c - '0');
 	}
-	fraction = (1.0e9 * frac1) + frac2;
-    }
 
     /*
@@ -880,5 +878,9 @@ ruby_strtod(string, endPtr)
 	exp = maxExponent;
 	errno = ERANGE;
+	    fraction = HUGE;
     }
+	else {
+	    fracExp = exp;
+	    exp += 9;
     dblExp = 1.0;
     for (d = powersOf10; exp != 0; exp >>= 1, d += 1) {
@@ -888,11 +890,25 @@ ruby_strtod(string, endPtr)
     }
     if (expSign) {
-	fraction /= dblExp;
+		fraction = frac1 / dblExp;
     }
     else {
-	fraction *= dblExp;
+		fraction = frac1 * dblExp;
+	    }
+	    exp = fracExp;
+	    dblExp = 1.0;
+	    for (d = powersOf10; exp != 0; exp >>= 1, d += 1) {
+		if (exp & 01) {
+		    dblExp *= *d;
+		}
+	    }
+	    if (expSign) {
+		fraction += frac2 / dblExp;
+	    }
+	    else {
+		fraction += frac2 * dblExp;
+	    }
+	}
     }
 
-done:
     if (endPtr != NULL) {
 	*endPtr = (char *) p;


-- 
Nobu Nakada