Add optional support for digit separators and cpp prefixes

[zaewc]

Add optional support in from_chars_advanced to:

• skip a configurable digit separator (e.g. ')
• skip standard cpp prefixes (0x/0X, 0b/0B) before parsing (decimal-only; no hex/binary floats)

Resolves #124

no measurable regression on standard inputs (canada.txt / mesh.txt).

[lemire]

Thanks, to be reviewed.

[lemire]

I rebased the PR here : https://github.com/fastfloat/fast_float/tree/pr-369-rebase

I get that this PR slows down the performance by 5% to 10%.

We could hack things so that the it is performance neutral.

But I am not convinced. I don't know who needs this feature. I am concerned about adding a function that nobody would use but that we would need to maintain.

[zaewc]

@lemire

I think the useful precedent here is rust-lexical. Its float parser uses an Eisel-Lemire implementation, so this is not a random general-purpose parser with unrelated tradeoffs.

lexical explicitly supports this class of use case: configurable digit separators, base prefixes/radices, and predefined number formats for data formats and language literal grammars such as JSON, TOML, YAML, Rust, Python, C++, C#, FORTRAN, COBOL, and others.

“who would use this”

-> users parsing numeric literals or config/data formats whose grammar is intentionally wider than std::from_chars.

[zaewc]

@lemire

The performance concern is also handled in the same spirit. In lexical, the number format is a compile-time format specification; in this PR, the separator path is similarly isolated behind a compile-time has_separator template parameter. The default has_separator == false instantiation used by normal callers does not put a separator comparison in the digit loop, preserves the existing SIMD fast path, and in the current revision generates the same assembly for the default from_chars<double> path as main.

So I agree this feature should not regress the default parser. My point is that the current design confines the extra behavior to an opt-in cold instantiation, while keeping the standard-number hot path unchanged. That seems to address both concerns: no runtime cost for default users, and a bounded maintenance surface for users who actually need language/config literal parsing.

[lemire]

Look. I understand you have good intentions, and that you have put in some work into this, and I do invite contributions... But you need to tell us what the motivation is.

My view is that having code in the library that nobody uses is a net negative. It makes the library larger, it increases the maintenance burden. So who is the consumer here ?

Let us look at the PR:

Firstly, I am measuring a performance degradation following this PR in our benchmarks using GCC 14 and an x64 processor. I am sure we can alleviate this with some engineering effort.

Secondly why does it need to be in fast_float. It is effectively trivial for someone to preprocessing their strings so that fast_float can parse them. This would only entail a relatively minor penalty. Basically, scan for ' and prune them. Skip any leading component. This can be written in under 5 lines of code. And then you pass it off to fast_float.

I have never encountered a case where I need to parse lots of floats with separators. I know that some programming languages allow it in code, but you rarely have lots and lots of floats to parse in code... and the compilers already have their own number parsers... can you point at one that would adopt fast_float if we add this feature?

[zaewc]

@lemire

Thanks for taking the time to lay this out, I think your core position is right, and I want to agree with it up front: carrying code that no clear consumer needs is a net negative, and the maintenance/size cost is real regardless of how cheap the runtime path is. So I won’t argue that this must go in.

1. On performance: I believe the default path is now performance-neutral in the configurations I tested.
   The feature is gated on a compile-time template parameter (has_separator). The false instantiation that every default caller uses is intended to be the unmodified parser, and parse_options_t is kept at 16 bytes so the calling convention is unchanged.
   In my local checks against main, isolating just this commit:

   • GCC 14 and GCC 12 on x86-64: from_chars for double/float/int/long long, from_chars_advanced with default options, and the scientific path matched the generated assembly from main in the cases I tested.
   • clang-18 on x86-64: the translation unit matched as well.
     So if you still measure a regression on GCC 14 / x64, it may be against an earlier revision of this branch - HEAD is 1d35d26. The only path that should gain an out-of-loop, well-predicted branch is when a caller passes runtime-varying options, which no 3-arg from_chars caller hits.

2. On “who is the consumer”: you’re right that source-literal parsing is not a strong motivation.
   Compilers have their own parsers and don’t parse floats in bulk. The only place I can honestly point to is bulk data ingestion.
   DuckDB already vendors/uses fast_float in its numeric parsing path, and users have asked for parsing thousands-separated numbers such as "100,000.00" in CSV input. pandas exposes the same need through read_csv(thousands=...), which is a heavily-used API surface for real-world datasets.
   But I don’t want to oversell this: I cannot point to a project that has committed to adopting this fast_float feature if it lands. This is recurring user demand around bulk numeric ingestion, not a signed-up consumer.

Given that, I’m completely fine with you closing this at your discretion. The work wasn’t wasted, the perf-neutral approach was a worthwhile exercise, and the branch will stay available if a concrete consumer ever shows up. Thanks for the thoughtful review either way