[php-maint] Bug#613982: printf generates scientific notation inaccurately

[Ronan]

Package: php5-cli
Version: 5.3.3-7
Severity: normal


printf, when outputting a number in scientific
notation, allows the precision to be specified.
However, the precision is capped at 40.  If a cap
has to be set at all, then it should be set at 52,
because this is the smallest value which allows all
IEEE double-precision floating-point numbers to be
output with perfect accuracy.

I've included a terminal transcript, in the file
sprintf-transcript, which shows a small PHP program
and its output.  The important point is that the
second number gives us 12 more bits of precision
than the first.

*** sprintf-transcript
Script started on Fri 18 Feb 2011 16:57:47 GMT
bash$ PS1='>>>>>>>>>>>> ' # make the prompt stand out more
>>>>>>>>>>>> cat demonstrate 
#!/usr/bin/php5 -n
<?php
error_reporting(-1);

/*
  PHP will emit a warning about truncating
  the precision to 40 digits
*/
printf("%.52e\n", 1 + pow(2, -52));

bcscale(52);
/*
  this scale is large enough to allow the
  calculation below to be performed without
  any loss of precision
*/

print bcadd(1, bcpow(2, -52)) . "\n";
>>>>>>>>>>>> php5 -n demonstrate 

Notice: printf(): Requested precision of 52 digits was truncated to PHP maximum of 40 digits in /home/ronan/work/php-bugs/sprintf/demonstrate on line 9
1.0000000000000002220446049250313080847263e+0
1.0000000000000002220446049250313080847263336181640625
>>>>>>>>>>>> exit

Script done on Fri 18 Feb 2011 16:58:00 GMT


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