Fixed-point numbers

Alpha: This is an alpha release of the new major version 2.0.0 that makes use of const generics instead of the typenum crate. This version requires the nightly compiler with the [generic_const_exprs feature] enabled. The stable version 2.0.0 itself will not be released before the [generic_const_exprs feature] is stabilized. See the documentation for [porting from version 1 to version 2].

The [fixed crate] provides fixed-point numbers.

• [FixedI8] and [FixedU8] are eight-bit fixed-point numbers.
• [FixedI16] and [FixedU16] are 16-bit fixed-point numbers.
• [FixedI32] and [FixedU32] are 32-bit fixed-point numbers.
• [FixedI64] and [FixedU64] are 64-bit fixed-point numbers.
• [FixedI128] and [FixedU128] are 128-bit fixed-point numbers.

An n-bit fixed-point number has f = FRAC fractional bits, and n − f integer bits. For example, [FixedI32]\<24> is a 32-bit signed fixed-point number with n = 32 total bits, f = 24 fractional bits, and n − f = 8 integer bits. [FixedI32]\<0> behaves like [i32], and [FixedU32]\<0> behaves like [u32].

The difference between any two successive representable numbers is constant throughout the possible range for a fixed-point number: Δ = 1/2^f. When f = 0, like in [FixedI32]\<0>, 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]\<32>, and in the range 0 ≤ x < 1 for unsigned numbers like [FixedU32]\<32>.

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 sqrt or 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 [fixed-sqrt crate] provides the square root operation.
• The [cordic crate] provides various functions implemented using the [CORDIC] algorithm.

The conversions supported cover the following cases.

• Infallible lossless conversions between fixed-point numbers and numeric primitives are provided using [From] and [Into]. These never fail (infallible) and do not lose any bits (lossless).
• Infallible lossy conversions between fixed-point numbers and numeric primitives are provided using the [LossyFrom] and [LossyInto] traits. The source can have more fractional bits than the destination.
• Checked lossless conversions between fixed-point numbers and numeric primitives are provided using the [LosslessTryFrom] and [LosslessTryInto] traits. The source cannot have more fractional bits than the destination.
• Checked conversions between fixed-point numbers and numeric primitives are provided using the [FromFixed] and [ToFixed] traits, or using the [from_num] and [to_num] methods and [their checked versions][checked_from_num].
• Additionally, [az] casts are implemented for conversion between fixed-point numbers and numeric primitives.
• Fixed-point numbers can be parsed from decimal strings using [FromStr], and from binary, octal and hexadecimal strings using the [from_str_binary], [from_str_octal] and [from_str_hex] methods. The result is rounded to the nearest, with ties rounded to even.
• Fixed-point numbers can be converted to strings using [Display], [Binary], [Octal], [LowerHex], [UpperHex], [LowerExp] and [UpperExp]. The output is rounded to the nearest, with ties rounded to even.
• All fixed-point numbers are plain old data, so [bytemuck] bit casting conversions can be used.

What’s new

Version 2.0.0-alpha.10 news (2023-02-06)

• The crate now requires the nightly compiler with the [generic_const_exprs feature] enabled.
• The crate now uses generic constant expressions to specify the number of fractional bits.
• The Fixed trait constraints have been relaxed, and the methods which needed the strict constraints have been moved to the subtrait FixedStrict.
• The INT_NBITS and FRAC_NBITS associated constants were replaced with INT_BITS and FRAC_BITS which can be negative.
• The Unwrapped methods from_str_binary, from_str_octal and from_str_hex return the value directly instead of a [Result].
• The deprecated F128Bits struct has been removed. It was replaced by F128 in version 1.18.0.
• The deprecated const_fixed_from_int macro has been removed. It was replaced by the const_from_int method in version 1.20.0.
• The deprecated optional features az and f16 were removed. These features had no effect, as the functionality they enabled is now always enabled.
• The new generic associated type Fixed was added to the FixedBits trait.
• The following methods of the Fixed trait and of the Wrapping and Unwrapped wrappers now have some parameters and return types that can be generic:
  • mul_add, add_prod, mul_acc
• The following methods of the Fixed trait now have some parameters and return types that can be generic:
  • checked_mul_add, checked_add_prod, checked_mul_acc
  • saturating_mul_add, saturating_add_prod, saturating_mul_acc
  • wrapping_mul_add, wrapping_add_prod, wrapping_mul_acc
  • unwrapped_mul_add, unwrapped_add_prod, unwrapped_mul_acc
  • overflowing_mul_add, overflowing_add_prod, overflowing_mul_acc

Version 1.23.0 news (2023-02-06)

• String parsing of binary, octal and hexadecimal strings now supports base-2 exponents starting with the separator “p” or “P”.
• Bug fix: int_log10, int_log, checked_int_log10 and checked_int_log were panicking or returning incorrect values for fixed-point numbers with no integer bits.

Other releases

Details on other releases can be found in [RELEASES.md].

Quick examples

```rust

![feature(genericconstexprs)]

use fixed::types::I20F12;

// 19/3 = 6 1/3 let sixandthird = I20F12::fromnum(19) / 3; // four decimal digits for 12 binary digits asserteq!(sixandthird.tostring(), "6.3333"); // find the ceil and convert to i32 asserteq!(sixandthird.ceil().tonum::(), 7); // we can also compare directly to integers asserteq!(sixandthird.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]\<12>. 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.

```rust

![feature(genericconstexprs)]

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

// -8 ≤ I4F4 < 8 with steps of 1/16 (~0.06) let a = I4F4::fromnum(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) asserteq!(ans1, I4F4::frombits((3 << 4) + 3)); asserteq!(ans1.to_string(), "3.2");

// -8 ≤ I4F12 < 8 with steps of 1/4096 (~0.0002) let widera = I4F12::from(a); let widerans = widera / 5 * 17; let ans2 = I4F4::fromnum(widerans); // now the answer is the much closer 3 6/16 (~3.4) asserteq!(ans2, I4F4::frombits((3 << 4) + 6)); asserteq!(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.

```rust

![feature(genericconstexprs)]

use fixed::types::I16F16;

// 0.1275e2 is 12.75 const TWELVEPOINT75: I16F16 = I16F16::lit("0.1275e2"); // 1.8 hexadecimal is 1.5 decimal, and 18@-1 is 1.8 const ONEPOINT5: I16F16 = I16F16::lit("0x18@-1"); // 12.75 + 1.5 = 14.25 let sum = TWELVEPOINT75 + ONEPOINT5; 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]:

toml [dependencies] fixed = "2.0.0-alpha.10"

This alpha version of the fixed crate requires the nightly compiler with the [generic_const_exprs feature] enabled.

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. serde, disabled by default. This provides serialization support for the fixed-point types. This feature requires the [serde crate].
3. std, disabled by default. This is for features that are not possible under no_std: currently the implementation of the [Error] trait for [ParseFixedError].
4. 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. With this feature, serialization is only supported for fixed-point numbers where the number of fractional bits is from zero to the total number of bits. 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]:

toml [dependencies.fixed] version = "2.0.0-alpha.10" features = ["serde"]

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. borsh, disabled by default. This implements serialization and deserialization using the [borsh crate]. (The plan is to promote this to an optional feature once the [borsh crate] reaches version 1.0.0.)
2. 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.)

Porting from version 1 to version 2

To port from version 1 to version 2, the following is required:

• Temporary change required until the [generic_const_exprs feature] are stabilized: use the nightly compiler and enable the [generic_const_exprs feature] using

  ```rust

  ![feature(genericconstexprs)]

  ```

• Use integer literals instead of typenum integer constants, for example [FixedI32][FixedI32]<8> instead of [FixedI32][FixedI32]<[U8]>.

  [U8]: https://docs.rs/fixed/1/fixed/types/extra/type.U8.html

• The [Fixed] trait constraints have been relaxed, and the methods which needed the strict constraints have been moved to the subtrait [FixedStrict]. For code that uses these trait methods, [Fixed] should be replaced by [FixedStrict].

  [Fixed]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/traits/trait.Fixed.html [FixedStrict]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/traits/trait.FixedStrict.html

• The [FRAC_NBITS] and [INT_NBITS] associated constants of type [u32] were replaced by [FRAC_BITS] and [INT_BITS] of type [i32].

  [FRAC_BITS]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.FixedI32.html#associatedconstant.FRACBITS [FRAC_NBITS]: https://docs.rs/fixed/1/fixed/struct.FixedI32.html#associatedconstant.FRACNBITS [INT_BITS]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.FixedI32.html#associatedconstant.INTBITS [INT_NBITS]: https://docs.rs/fixed/1/fixed/struct.FixedI32.html#associatedconstant.INTNBITS

• For the [Unwrapped] wrapper, the methods [from_str_binary][u-fsb], [from_str_octal][u-fso] and [from_str_hex][u-fsh] return the value directly instead of a [Result].

  [Result]: https://doc.rust-lang.org/nightly/core/result/enum.Result.html [Unwrapped]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.Unwrapped.html [u-fsb]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.Unwrapped.html#method.fromstrbinary [u-fsh]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.Unwrapped.html#method.fromstrhex [u-fso]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.Unwrapped.html#method.fromstroctal

• The deprecated [F128Bits] struct has been removed. It was replaced by [F128] in version 1.18.0

  [F128Bits]: https://docs.rs/fixed/1/fixed/struct.F128Bits.html [F128]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.F128.html

• The deprecated [const_fixed_from_int] macro has been removed. It was replaced by the [const_from_int][f-cfi] method in version 1.20.0.

  [const_fixed_from_int]: https://docs.rs/fixed/1/fixed/macro.constfixedfromint.html [f-cfi]: https://docs.rs/fixed/2.0.0-alpha.10/fixed/struct.FixedI32.html#method.constfrom_int

• The deprecated optional features az and f16 were removed. These features had no effect, as their functionality has been unconditionally enabled since version 1.7.0.

License

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

• the Apache License, Version 2.0 or
• the MIT License

at your option.

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.