# Crate approx

Expand description

A crate that provides facilities for testing the approximate equality of floating-point based types, using either relative difference, or units in the last place (ULPs) comparisons.

You can also use the `*_{eq, ne}!` and `assert_*_{eq, ne}!` macros to test for equality using a more positional style:

``````#[macro_use]
extern crate approx;

use std::f64;

abs_diff_eq!(1.0, 1.0);
abs_diff_eq!(1.0, 1.0, epsilon = f64::EPSILON);

relative_eq!(1.0, 1.0);
relative_eq!(1.0, 1.0, epsilon = f64::EPSILON);
relative_eq!(1.0, 1.0, max_relative = 1.0);
relative_eq!(1.0, 1.0, epsilon = f64::EPSILON, max_relative = 1.0);
relative_eq!(1.0, 1.0, max_relative = 1.0, epsilon = f64::EPSILON);

ulps_eq!(1.0, 1.0);
ulps_eq!(1.0, 1.0, epsilon = f64::EPSILON);
ulps_eq!(1.0, 1.0, max_ulps = 4);
ulps_eq!(1.0, 1.0, epsilon = f64::EPSILON, max_ulps = 4);
ulps_eq!(1.0, 1.0, max_ulps = 4, epsilon = f64::EPSILON);``````

## Implementing approximate equality for custom types

The `*Eq` traits allow approximate equalities to be implemented on types, based on the fundamental floating point implementations.

For example, we might want to be able to do approximate assertions on a complex number type:

``````#[macro_use]
extern crate approx;

#[derive(Debug, PartialEq)]
struct Complex<T> {
x: T,
i: T,
}

let x = Complex { x: 1.2, i: 2.3 };

assert_relative_eq!(x, x);
assert_ulps_eq!(x, x, max_ulps = 4);``````

To do this we can implement `AbsDiffEq`, `RelativeEq` and `UlpsEq` generically in terms of a type parameter that also implements `AbsDiffEq`, `RelativeEq` and `UlpsEq` respectively. This means that we can make comparisons for either `Complex<f32>` or `Complex<f64>`:

``````impl<T: AbsDiffEq> AbsDiffEq for Complex<T> where
T::Epsilon: Copy,
{
type Epsilon = T::Epsilon;

fn default_epsilon() -> T::Epsilon {
T::default_epsilon()
}

fn abs_diff_eq(&self, other: &Self, epsilon: T::Epsilon) -> bool {
T::abs_diff_eq(&self.x, &other.x, epsilon) &&
T::abs_diff_eq(&self.i, &other.i, epsilon)
}
}

impl<T: RelativeEq> RelativeEq for Complex<T> where
T::Epsilon: Copy,
{
fn default_max_relative() -> T::Epsilon {
T::default_max_relative()
}

fn relative_eq(&self, other: &Self, epsilon: T::Epsilon, max_relative: T::Epsilon) -> bool {
T::relative_eq(&self.x, &other.x, epsilon, max_relative) &&
T::relative_eq(&self.i, &other.i, epsilon, max_relative)
}
}

impl<T: UlpsEq> UlpsEq for Complex<T> where
T::Epsilon: Copy,
{
fn default_max_ulps() -> u32 {
T::default_max_ulps()
}

fn ulps_eq(&self, other: &Self, epsilon: T::Epsilon, max_ulps: u32) -> bool {
T::ulps_eq(&self.x, &other.x, epsilon, max_ulps) &&
T::ulps_eq(&self.i, &other.i, epsilon, max_ulps)
}
}``````

Floating point is hard! Thanks goes to these links for helping to make things a little easier to understand:

## Macros

Approximate equality of using the absolute difference.

Approximate inequality of using the absolute difference.

An assertion that delegates to `abs_diff_eq!`, and panics with a helpful error on failure.

An assertion that delegates to `abs_diff_ne!`, and panics with a helpful error on failure.

An assertion that delegates to `relative_eq!`, and panics with a helpful error on failure.

An assertion that delegates to `relative_ne!`, and panics with a helpful error on failure.

An assertion that delegates to `ulps_eq!`, and panics with a helpful error on failure.

An assertion that delegates to `ulps_ne!`, and panics with a helpful error on failure.

Approximate equality using both the absolute difference and relative based comparisons.

Approximate inequality using both the absolute difference and relative based comparisons.

Approximate equality using both the absolute difference and ULPs (Units in Last Place).

Approximate inequality using both the absolute difference and ULPs (Units in Last Place).

## Structs

The requisite parameters for testing for approximate equality using a absolute difference based comparison.

The requisite parameters for testing for approximate equality using a relative based comparison.

The requisite parameters for testing for approximate equality using an ULPs based comparison.

## Traits

Equality that is defined using the absolute difference of two numbers.

Equality comparisons between two numbers using both the absolute difference and relative based comparisons.

Equality comparisons between two numbers using both the absolute difference and ULPs (Units in Last Place) based comparisons.