Re: draft-ietf-httpbis-header-structure-15, Serializing a Decimal

[Mark Nottingham <[e-mail]>]

On 30 Jan 2020, at 4:52 pm, Kari Hurtta <[e-mail]> wrote:
> 
>> Hi Kari,
>> 
>> Very good catch; I noticed this a while back but forgot to file an issue. Now:
>>  https://github.com/httpwg/http-extensions/issues/1043
> 
> | ... still isn't right, as Kari noticed. E.g., a value of 1.9944 won't get rounded to 2.0.
> 
> You mean:  E.g., a value of 1.9998  won't get rounded to 2.0.
> 
> Algorithm produces  three digits for fractional component.
> 
> And 1.994 is correct rounding for 1.9944, I think.

Corrected.

> 
> 
>> Cheers,
>> 
> 
> Another thing what can be issue when rounding is applied.
> 
>>> On 30 Jan 2020, at 5:17 am, Kari Hurtta <[e-mail]> wrote:
>>> 
>>> 4.1.5.  Serializing a Decimal
>>> https://tools.ietf.org/html/draft-ietf-httpbis-header-structure-15#section-4.1.5
>>> 
>>> 
>>> |   Given a decimal_number as input_decimal, return an ASCII string
>>> |   suitable for use in a HTTP header value.
>>> |
>>> |   1.  Let output be an empty string.
>>> |
>>> |   2.  If input_decimal is less than (but not equal to) 0, append "-" to
>>> |       output.
>>> |
>>> |   3.  Append input_decimal's integer component represented in base 10
>>> |       (using only decimal digits) to output; if it is zero, append "0".
>>> |
>>> |   4.  If the number of characters appended in the previous step is
>>> |       greater than 12, fail serialisation.
> 
> A decimal_number  number can be  -999 999 999 999.9997 or 999 999 999 999.9997
> 
> In step 3 there is  digits 999999999999 so no failure on step 4
> 
> 
>>> |   5.  Append "." to output.
>>> |
>>> |   6.  If input_decimal's fractional component is zero, append "0" to
>>> |       output.
>>> |
>>> |   7.  Else if input_decimal's fractional component has up to three
>>> |       digits, append them represented in base 10 (using only decimal
>>> |       digits) to output.
>>> |
>>> |   8.  Otherwise, append the first three digits of input_decimal's
>>> |       fractional component (represented in base 10, using only decimal
>>> |       digits) to output, rounding the final digit to the nearest value,
>>> |       or to the even value if it is equidistant.
>>> |
>>> |   9.  Return output.
> 
> First theree digits for fractional component are 999
> 
> "Correct" rounding for  -999 999 999 999.9997  is -1 000 000 000 000.0 and
> for 999 999 999 999.9997 is 1 000 000 000 000.0, but there are 13 digits 
> on integer component.
> 
> That is "solved" if 
> when decimal_number < -999 999 999 999.999 or
> decimal_number > 999 999 999 999.999, fractional component
> is truncated to 3 digits and rounding is not tried.
> 
> / Kari Hurtta

--
Mark Nottingham   https://www.mnot.net/