# Crate fixed

Expand description

## §Fixed-point numbers

The fixed crate provides fixed-point numbers.

An n-bit fixed-point number has f = `Frac` fractional bits where 0 ≤ f ≤ n, and n − f integer bits. For example, `FixedI32<U24>` is a 32-bit signed fixed-point number with n = 32 total bits, f = 24 fractional bits, and n − f = 8 integer bits. `FixedI32<U0>` behaves like `i32`, and `FixedU32<U0>` behaves like `u32`.

The difference between any two successive representable numbers is constant throughout the possible range for a fixed-point number: Δ = 1/2f. When f = 0, like in `FixedI32<U0>`, Δ = 1 because representable numbers are integers, and the difference between two successive integers is 1. When f = n, Δ = 1/2n and the value lies in the range −0.5 ≤ x < 0.5 for signed numbers like `FixedI32<U32>`, and in the range 0 ≤ x < 1 for unsigned numbers like `FixedU32<U32>`.

In version 1 the typenum crate is used for the fractional bit count `Frac`; the plan is to to have a major version 2 with const generics when the Rust compiler’s `generic_const_exprs` feature is ready and stabilized. An alpha version is already available.

The main features are

• Representation of binary fixed-point numbers up to 128 bits wide.
• Conversions between fixed-point numbers and numeric primitives.
• Comparisons between fixed-point numbers and numeric primitives.
• Parsing from strings in decimal, binary, octal and hexadecimal.
• Display as decimal, binary, octal and hexadecimal.
• Arithmetic and logic operations.

This crate does not provide decimal fixed-point numbers. For example 0.001 cannot be represented exactly, as it is 1/103. It is binary fractions like 1/24 (0.0625) that can be represented exactly, provided there are enough fractional bits.

This crate does not provide general analytic functions.

• No algebraic functions are provided, for example no `pow`.
• No trigonometric functions are provided, for example no `sin` or `cos`.
• No other transcendental functions are provided, for example no `log` or `exp`.

These functions are not provided because different implementations can have different trade-offs, for example trading some correctness for speed. Implementations can be provided in other crates.

The conversions supported cover the following cases.

### §Quick examples

``````use fixed::types::I20F12;

// 19/3 = 6 1/3
let six_and_third = I20F12::from_num(19) / 3;
// four decimal digits for 12 binary digits
assert_eq!(six_and_third.to_string(), "6.3333");
// find the ceil and convert to i32
assert_eq!(six_and_third.ceil().to_num::<i32>(), 7);
// we can also compare directly to integers
assert_eq!(six_and_third.ceil(), 7);``````

The type `I20F12` is a 32-bit fixed-point signed number with 20 integer bits and 12 fractional bits. It is an alias to `FixedI32<U12>`. The unsigned counterpart would be `U20F12`. Aliases are provided for all combinations of integer and fractional bits adding up to a total of eight, 16, 32, 64 or 128 bits.

``````use fixed::types::{I4F4, I4F12};

// -8 ≤ I4F4 < 8 with steps of 1/16 (~0.06)
let a = I4F4::from_num(1);
// multiplication and division by integers are possible
let ans1 = a / 5 * 17;
// 1 / 5 × 17 = 3 2/5 (3.4), but we get 3 3/16 (~3.2)
assert_eq!(ans1, I4F4::from_bits((3 << 4) + 3));
assert_eq!(ans1.to_string(), "3.2");

// -8 ≤ I4F12 < 8 with steps of 1/4096 (~0.0002)
let wider_a = I4F12::from(a);
let wider_ans = wider_a / 5 * 17;
let ans2 = I4F4::from_num(wider_ans);
// now the answer is the much closer 3 6/16 (~3.4)
assert_eq!(ans2, I4F4::from_bits((3 << 4) + 6));
assert_eq!(ans2.to_string(), "3.4");``````

The second example shows some precision and conversion issues. The low precision of `a` means that `a / 5` is 3⁄16 instead of 1⁄5, leading to an inaccurate result `ans1` = 3 3⁄16 (~3.2). With a higher precision, we get `wider_a / 5` equal to 819⁄4096, leading to a more accurate intermediate result `wider_ans` = 3 1635⁄4096. When we convert back to four fractional bits, we get `ans2` = 3 6⁄16 (~3.4).

Note that we can convert from `I4F4` to `I4F12` using `From`, as the target type has the same number of integer bits and a larger number of fractional bits. Converting from `I4F12` to `I4F4` cannot use `From` as we have less fractional bits, so we use `from_num` instead.

### §Writing fixed-point constants and values literally

The `lit` method, which is available as a `const` function, can be used to parse literals. It supports

• underscores as separators;
• prefixes “`0b`”, “`0o`” and “`0x`” for binary, octal and hexadecimal numbers;
• an optional decimal exponent with separator “`e`” or “`E`” for decimal, binary and octal numbers, or with separator “`@`” for all supported radices including hexadecimal.
``````use fixed::types::I16F16;

// 0.1275e2 is 12.75
const TWELVE_POINT_75: I16F16 = I16F16::lit("0.127_5e2");
// 1.8 hexadecimal is 1.5 decimal, and 18@-1 is 1.8
const ONE_POINT_5: I16F16 = I16F16::lit("0x_18@-1");
// 12.75 + 1.5 = 14.25
let sum = TWELVE_POINT_75 + ONE_POINT_5;
assert_eq!(sum, 14.25);``````

### §Using the fixed crate

The fixed crate is available on crates.io. To use it in your crate, add it as a dependency inside Cargo.toml:

``````[dependencies]
fixed = "1.28"
``````

The fixed crate requires rustc version 1.79.0 or later.

### §Optional features

The fixed crate has these optional feature:

1. `arbitrary`, disabled by default. This provides the generation of arbitrary fixed-point numbers from raw, unstructured data. This feature requires the arbitrary crate.
2. `borsh`, disabled by default. This implements serialization and deserialization using the borsh crate.
3. `serde`, disabled by default. This provides serialization support for the fixed-point types. This feature requires the serde crate.
4. `std`, disabled by default. This is for features that are not possible under `no_std`: currently the implementation of the `Error` trait for `ParseFixedError`.
5. `serde-str`, disabled by default. Fixed-point numbers are serialized as strings showing the value when using human-readable formats. This feature requires the `serde` and the `std` optional features. Warning: numbers serialized when this feature is enabled cannot be deserialized when this feature is disabled, and vice versa.

To enable features, you can add the dependency like this to Cargo.toml:

``````[dependencies.fixed]
features = ["serde"]
version = "1.28"
``````

### §Experimental optional features

It is not considered a breaking change if the following experimental features are removed. The removal of experimental features would however require a minor version bump. Similarly, on a minor version bump, optional dependencies can be updated to an incompatible newer version.

1. `num-traits`, disabled by default. This implements some traits from the num-traits crate. (The plan is to promote this to an optional feature once the num-traits crate reaches version 1.0.0.)
2. `nightly-float`, disabled by default. This requires the nightly compiler, and implements conversions and comparisons with the experimental `f16` and `f128` primitives. (The plan is to always implement the conversions and comparisons and remove this experimental feature once the primitives are stabilized.)

### §Deprecated optional features

The following optional features are deprecated and will be removed in the next major version of the crate.

1. `az`, has no effect. Previously required for the `az` cast traits. Now these cast traits are always provided.
2. `f16`, has no effect. Previously required for conversion to/from `half::f16` and `half::bf16`. Now these conversions are always provided.

This crate is free software: you can redistribute it and/or modify it under the terms of either

#### §Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache License, Version 2.0, shall be dual licensed as above, without any additional terms or conditions.

## Modules§

• Mathematical constants.
• Constants specific to the `F128` quadruple-precision floating-point type.
• A prelude to import useful traits.
• Traits for conversions and for generic use of fixed-point numbers.
• Type aliases for all supported fixed-point numbers.

## Structs§

• A binary128 floating-point number (`f128`).
• F128BitsDeprecated
The bit representation of a binary128 floating-point number (`f128`).
• An eight-bit signed number with `Frac` fractional bits.
• A 16-bit signed number with `Frac` fractional bits.
• A 32-bit signed number with `Frac` fractional bits.
• A 64-bit signed number with `Frac` fractional bits.
• A 128-bit signed number with `Frac` fractional bits.
• An eight-bit unsigned number with `Frac` fractional bits.
• A 16-bit unsigned number with `Frac` fractional bits.
• A 32-bit unsigned number with `Frac` fractional bits.
• A 64-bit unsigned number with `Frac` fractional bits.
• A 128-bit unsigned number with `Frac` fractional bits.
• An error which can be returned when parsing a fixed-point number.
• Provides saturating arithmetic on fixed-point numbers.
• Provides arithmetic operations that panic on overflow even when debug assertions are disabled.
• Provides intentionally wrapped arithmetic on fixed-point numbers.

## Enums§

• An error which can be returned when parsing a fixed-point number with a given radix.