float_eq
Explicit and deliberate comparison of IEEE floating point numbers.
Comparing floating point values for equality is really hard. To get it right requires careful thought and iteration based on the needs of each specific algorithm's inputs and error margins. This API provides a toolbox of components to make your options clear and your choices explicit to future maintainers.
Background
Given how widely algorithmic requirements can vary, float_eq
explores the
idea that there are no generally sensible default margins for comparisons.
This is in contrast to the approach taken by many existing crates, which often
provide default epsilon values in checks or implicitly favour particular
algorithms. The author's hope is that by exposing the inherent complexity in
a uniform way, programmers will find it easier to develop an intuition for how
to write effective comparisons. The trade-off is that each individual
comparison requires more iteration time and thought.
And yes, this is yet another crate built on the principles described in that Random ASCII floating point comparison article, which is highly recommended background reading 🙂.
Usage
Add this to your cargo.toml:
[dependencies]
float_eq = "0.1"
and, if you're using the 2015 edition, this to your crate root:
extern crate float_eq;
then, you can import items with use
:
use ;
Comparisons
This crate provides boolean comparison operations:
assert!;
// f32::EPSILON.sqrt()
const ROUNDING_ERROR: f32 = 0.00034526698;
assert!;
And asserts:
// 1.5 * 2_f32.powi(-12), as per SSE intrinsics documentation
const RECIP_REL_EPSILON: f32 = 0.00036621094;
let recip = 0.1_f32.recip;
assert_float_eq!;
assert_float_ne!;
See the API documentation for a long form introduction to the different kinds
of checks, their uses and limitations. Comparison of new types is supported by
implementing the FloatEq
and FloatDiff
traits.
Features
This crate can be used without the standard library (#![no_std]
) by disabling
the default std
feature. Use this in Cargo.toml
:
[dependencies.float_eq]
version = "0.1"
default-features = false
Related efforts
There are a number of existing crates that implement these kinds of comparisons
if you're looking for a more mature solution or simply a different approach.
The approx
, float-cmp
and almost
crates all provide a similar style
of general comparison operations, whereas assert_float_eq
focuses
specifically on assertions. In contrast, efloat
comes at the problem from a
different angle, instead tracking the error bounds of values as operations are
applied.
Changelog
Release information is available in CHANGELOG.md.
Future plans
-
#[derive]
support for comparison of custom types that are composed of already comparable floating point values. -
Support for
num::Complex
. -
Investigate the safety guarantees of the ulps check. Currently, it doesn't act like the default floating point checks when it comes to NaNs and other special values.
-
More exhaustive testing. Tests currently cover all basic functionality, but there are lots of edge cases that aren't being tested yet.
-
Benchmark performance, especially the implications of chaining multiple tests.
Contributing
Constructive feedback, suggestions and contributions welcomed, please open an issue.