Re: [Boost-users] [Spirit] parsing double precision issues

Subject: Re: [Boost-users] [Spirit] parsing double precision issues
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2018-11-17 20:38:11

On Wed, Nov 14, 2018 at 6:35 PM Michael Powell <mwpowellhtx_at_[hidden]> wrote:
>
> On Wed, Nov 14, 2018 at 5:05 PM Michael Powell <mwpowellhtx_at_[hidden]> wrote:
> >
> > On Wed, Nov 14, 2018 at 1:58 PM Michael Powell <mwpowellhtx_at_[hidden]> wrote:
> > >
> > > Hello,
> > >
> > > I am continuing to work through my Protobuf parsing with
> > > Constant/Floating-Point tests. Double precision is rearing its ugly
> > > head and I am curious what's the best way to handle potentially
> > > determining a healthy epsilon given the expected parsed value. For the
> > > most part, I think it's going to work, but for the precision issues.
> > > Catch:

No response? Does anyone have any insights as to the real parser policies?

The documented grammar would lead me to believe that "fixed format",
literally, (*digits  >> '.' >> +digits) | (+digits  >> '.' >>
*digits), in Qi phraseology, might be possible. However, I am finding
potentially this is not the case.

> > After doing some background reading on floating point comparison ...
> > it's not so simple as a==b, but I kind of knew that already. Even the
> > simple absolute difference epsilon check is not so robust. I did a
> > combination of nearly, relatively, essentially, etc, checks, and I
> > think I have gotten past that immediate issue.
> >
> > Now I actually have a parsing issue... According to the Protobuf v2
> > spec, this should parse, I think. However, it is failing to parse.
> >
> >   CHECK( parse_tried == expected_tried )
> > with expansion:
> >   false == true
> > with message:
> >   Source: syntax = 'proto2';option a =
> > 933252761176835091419019014852578134454945897134240121039056292642745934626544776679309792643426912681734775655456099849022472225507009671635655261635494179939591695082743337844489696175391517474159955183289891072214770215979051082304343590361421008538352849939802258131368544983961387362613078430270357504.000000;
> >   REQUIRE( tried )
> > with expansion:
> >   false
> > with message:
> >   Source: syntax = 'proto2';option a =
> > 933252761176835091419019014852578134454945897134240121039056292642745934626544776679309792643426912681734775655456099849022472225507009671635655261635494179939591695082743337844489696175391517474159955183289891072214770215979051082304343590361421008538352849939802258131368544983961387362613078430270357504.000000;
>
> This much I can say at the moment, it does not appear to be a max
> double issue, literally:
>
> 179769313486231570814527423731704356798070567525844996598917476803157260780028538760589558632766878171540458953514382464234321326889464182768467546703537516986049910576551282076245490090389328944075868508455133942304583236903222948165808559332123348274797826204144723168738177180919299881250404026184124858368.0000000000000000
>
> That would stand to reason, though, since my test data is randomly
> generated within max double boundaries.
>
> I am scratching my head on the strict real policies, however. If the
> docs are accurate, I do not see why that would not accept the fixed
> double format...
>
> But it could be a concern that parsing is done apart from Qi rules. It
> seems like some rigid logic embedded there.
>
> https://www.boost.org/doc/libs/1_68_0/libs/spirit/doc/html/spirit/qi/reference/numeric/real.html
>
> > >   REQUIRE( actual == expected )
> > > with expansion:
> > >   struct my::protobuf::ast::floating_point_t { 'val': 1.16624e+306,
> > > 'opt_sign': null }
> > >   ==
> > >   struct my::protobuf::ast::floating_point_t { 'val': 1.16624e+306,
> > > 'opt_sign': null }
> > > with messages:
> > >   Source: syntax = 'proto2';option L = 1.16624e+306;
> > >   Delta was: 2.35309e+300
> > >
> > > There was clearly a delta involved, most like a precision issue
> > > encountered during the parse. Without going into a great deal of
> > > depth, if possible:
> > >
> > > namespace my { namespace protobuf { namespace ast {
> > > using float_t = double;
> > > enum num_sign_t {
> > >     sign_none
> > >     /// '-'
> > >     , sign_minus
> > >     /// '+'
> > >     , sign_plus
> > > };
> > >
> > > // Which includes
> > > struct floating_point_t : numeric_t<float_t> { /* ... */ };
> > > } } }
> > >
> > > // Without sign, dot required.
> > > // >>> TODO: TBD: Precision issue? Or an actual double parsing issue? <<<
> > > float_lit %= real_parser<ast::float_t, strict_ureal_policies<ast::float_t>>{};
> > > floating_point_lit %= -num_lit_sign >> float_lit;
> > >
> > > qi::rule<It, ast::num_sign_t()> num_lit_sign;
> > >
> > > qi::rule<It, ast::float_t()> float_lit;
> > > qi::rule<It, ast::floating_point_t()> floating_point_lit;
> > >
> > > https://developers.google.com/protocol-buffers/docs/reference/proto2-spec#floating-point-literals
> > > https://developers.google.com/protocol-buffers/docs/reference/proto2-spec#constant
> > > https://www.boost.org/doc/libs/1_68_0/libs/spirit/doc/html/spirit/qi/reference/numeric/real.html
> > >
> > > Cheers,
> > >
> > > Michael Powell