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

Subject: Re: [Boost-users] [Spirit] parsing double precision issues
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2018-11-14 22:05:25

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:

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;

>   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