369 lines
17 KiB
Markdown
369 lines
17 KiB
Markdown
<!-- Copyright © 2018–2020 Trevor Spiteri -->
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<!-- Copying and distribution of this file, with or without
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modification, are permitted in any medium without royalty provided the
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copyright notice and this notice are preserved. This file is offered
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as-is, without any warranty. -->
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# Fixed-point numbers
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The [*fixed* crate] provides fixed-point numbers.
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* [`FixedI8`] and [`FixedU8`] are eight-bit fixed-point numbers.
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* [`FixedI16`] and [`FixedU16`] are 16-bit fixed-point numbers.
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* [`FixedI32`] and [`FixedU32`] are 32-bit fixed-point numbers.
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* [`FixedI64`] and [`FixedU64`] are 64-bit fixed-point numbers.
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* [`FixedI128`] and [`FixedU128`] are 128-bit fixed-point numbers.
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These types can have `Frac` fractional bits, where
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0 ≤ `Frac` ≤ <i>n</i> and <i>n</i> is the total number of bits. When
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`Frac` = 0, the fixed-point number behaves like an <i>n</i>-bit
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integer. When `Frac` = <i>n</i>, the value <i>x</i> lies in the range
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−0.5 ≤ <i>x</i> < 0.5 for signed numbers, and in the range
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0 ≤ <i>x</i> < 1 for unsigned numbers.
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In version 1 the [*typenum* crate] is used for the fractional bit
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count `Frac`; the plan is to to have a major version 2 with [const
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generics] when they are supported by the Rust compiler.
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The main features are
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* Representation of fixed-point numbers up to 128 bits wide.
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* Conversions between fixed-point numbers and numeric primitives.
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* Comparisons between fixed-point numbers and numeric primitives.
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* Parsing from strings in decimal, binary, octal and hexadecimal.
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* Display as decimal, binary, octal and hexadecimal.
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* Arithmetic and logic operations.
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This crate does *not* provide general analytic functions.
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* No algebraic functions are provided, for example no `sqrt` or
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`pow`.
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* No trigonometric functions are provided, for example no `sin` or
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`cos`.
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* No other transcendental functions are provided, for example no
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`log` or `exp`.
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These functions are not provided because different implementations can
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have different trade-offs, for example trading some correctness for
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speed. Implementations can be provided in other crates.
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* The [*fixed-sqrt* crate] provides the square root operation.
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The conversions supported cover the following cases.
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* Infallible lossless conversions between fixed-point numbers and
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numeric primitives are provided using [`From`] and [`Into`]. These
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never fail (infallible) and do not lose any bits (lossless).
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* Infallible lossy conversions between fixed-point numbers and
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numeric primitives are provided using the [`LossyFrom`] and
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[`LossyInto`] traits. The source can have more fractional bits
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than the destination.
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* Checked lossless conversions between fixed-point numbers and
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numeric primitives are provided using the [`LosslessTryFrom`] and
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[`LosslessTryInto`] traits. The source cannot have more fractional
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bits than the destination.
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* Checked conversions between fixed-point numbers and numeric
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primitives are provided using the [`FromFixed`] and [`ToFixed`]
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traits, or using the [`from_num`] and [`to_num`] methods and
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[their checked versions][`checked_from_num`].
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* Fixed-point numbers can be parsed from decimal strings using
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[`FromStr`], and from binary, octal and hexadecimal strings using
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the [`from_str_binary`], [`from_str_octal`] and [`from_str_hex`]
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methods. The result is rounded to the nearest, with ties rounded
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to even.
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* Fixed-point numbers can be converted to strings using [`Display`],
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[`Binary`], [`Octal`], [`LowerHex`] and [`UpperHex`]. The output
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is rounded to the nearest, with ties rounded to even.
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## What’s new
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### Version 1.5.0 news (unreleased)
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* The [`widening_mul`][f-wm-1-5] method was added to all fixed-point
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numbers up to 64 bits wide.
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[f-wm-1-5]: https://tspiteri.gitlab.io/fixed/dev/fixed/struct.FixedI32.html#method.widening_mul
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### Version 1.4.1 news (unreleased)
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* For the experimental feature [`num-traits`][feat-exp-1-4], some
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missing supertraits were added to
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[`FixedOptionalFeatures`][tfof-1-4].
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### Version 1.4.0 news (2020-10-22)
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* The following methods were added to all fixed-point types, to the
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[`Fixed`][tf-1-4] trait, and to the [`Wrapping`][w-1-4] wrapper:
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* [`recip`][f-rec-1-4], [`checked_recip`][f-crec-1-4],
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[`saturating_recip`][f-srec-1-4],
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[`wrapping_recip`][f-wrec-1-4],
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[`overflowing_recip`][f-orec-1-4]
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* For the experimental feature [`num-traits`][feat-exp-1-4], the
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following traits were implemented where applicable ([issue 23]):
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* [`Num`][nt-0-2-num]
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* [`Signed`][nt-0-2-signed], [`Unsigned`][nt-0-2-unsigned]
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* [`Inv`][nt-0-2-inv]
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* [`MulAdd`][nt-0-2-ma], [`MulAddAssign`][nt-0-2-maa]
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* [`FloatConst`][nt-0-2-fc]
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* [`ToPrimitive`][nt-0-2-tp], [`FromPrimitive`][nt-0-2-fp]
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* For the experimental feature [`serde-str`][feat-exp-1-4],
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serialization in human-readable formats was made more convenient
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to write manually ([issue 24]). This makes it incompatible with
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the version in 1.3.0.
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[f-crec-1-4]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.checked_recip
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[f-orec-1-4]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.overflowing_recip
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[f-rec-1-4]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.recip
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[f-srec-1-4]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.saturating_recip
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[f-wrec-1-4]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.wrapping_recip
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[feat-exp-1-4]: https://docs.rs/fixed/~1.4/fixed/#experimental-optional-features
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[issue 23]: https://gitlab.com/tspiteri/fixed/-/issues/23
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[issue 24]: https://gitlab.com/tspiteri/fixed/-/issues/24
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[nt-0-2-fc]: https://docs.rs/num-traits/^0.2/num_traits/float/trait.FloatConst.html
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[nt-0-2-fp]: https://docs.rs/num-traits/^0.2/num_traits/cast/trait.FromPrimitive.html
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[nt-0-2-inv]: https://docs.rs/num-traits/^0.2/num_traits/ops/inv/trait.Inv.html
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[nt-0-2-ma]: https://docs.rs/num-traits/^0.2/num_traits/ops/mul_add/trait.MulAdd.html
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[nt-0-2-maa]: https://docs.rs/num-traits/^0.2/num_traits/ops/mul_add/trait.MulAddAssign.html
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[nt-0-2-num]: https://docs.rs/num-traits/^0.2/num_traits/trait.Num.html
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[nt-0-2-signed]: https://docs.rs/num-traits/^0.2/num_traits/sign/trait.Signed.html
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[nt-0-2-tp]: https://docs.rs/num-traits/^0.2/num_traits/cast/trait.ToPrimitive.html
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[nt-0-2-unsigned]: https://docs.rs/num-traits/^0.2/num_traits/sign/trait.Unsigned.html
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[tf-1-4]: https://docs.rs/fixed/~1.4/fixed/traits/trait.Fixed.html
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[tfof-1-4]: https://docs.rs/fixed/~1.4/fixed/traits/trait.FixedOptionalFeatures.html
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[w-1-4]: https://docs.rs/fixed/~1.4/fixed/struct.Wrapping.html
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### Version 1.3.0 news (2020-10-15)
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* The [`MulAssign`] implementation on fixed-point numbers now
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accepts an rhs fixed-point number with a different number of
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fractional bits from `self`.
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* The following methods were added to all fixed-point types, to the
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[`Fixed`][tf-ma-1-3] trait, and to the [`Wrapping`][w-ma-1-3] wrapper:
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* [`mul_add`][f-ma-1-3], [`checked_mul_add`][f-cma-1-3],
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[`saturating_mul_add`][f-sma-1-3],
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[`wrapping_mul_add`][f-wma-1-3],
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[`overflowing_mul_add`][f-oma-1-3]
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* The new experimental feature [`unwrapped`][feat-exp-1-3] was
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added, providing arithmetic methods that panic on overflow even
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when debug assertions are disabled.
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* The new experimental feature [`serde-str`][feat-exp-1-3] was
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added, which makes serialization use the number’s value in
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human-readable formats.
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[`MulAssign`]: https://doc.rust-lang.org/nightly/core/ops/trait.MulAssign.html
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[f-cma-1-3]: https://docs.rs/fixed/~1.3/fixed/struct.FixedI32.html#method.checked_mul_add
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[f-ma-1-3]: https://docs.rs/fixed/~1.3/fixed/struct.FixedI32.html#method.mul_add
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[f-oma-1-3]: https://docs.rs/fixed/~1.3/fixed/struct.FixedI32.html#method.overflowing_mul_add
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[f-sma-1-3]: https://docs.rs/fixed/~1.3/fixed/struct.FixedI32.html#method.saturating_mul_add
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[f-wma-1-3]: https://docs.rs/fixed/~1.3/fixed/struct.FixedI32.html#method.wrapping_mul_add
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[feat-exp-1-3]: https://docs.rs/fixed/~1.3/fixed/#experimental-optional-features
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[tf-ma-1-3]: https://docs.rs/fixed/~1.3/fixed/traits/trait.Fixed.html#tymethod.mul_add
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[w-ma-1-3]: https://docs.rs/fixed/~1.3/fixed/struct.Wrapping.html#method.mul_add
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### Other releases
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Details on other releases can be found in [*RELEASES.md*].
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[*RELEASES.md*]: https://gitlab.com/tspiteri/fixed/blob/master/RELEASES.md
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## Quick examples
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```rust
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use fixed::types::I20F12;
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// 19/3 = 6 1/3
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let six_and_third = I20F12::from_num(19) / 3;
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// four decimal digits for 12 binary digits
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assert_eq!(six_and_third.to_string(), "6.3333");
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// find the ceil and convert to i32
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assert_eq!(six_and_third.ceil().to_num::<i32>(), 7);
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// we can also compare directly to integers
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assert_eq!(six_and_third.ceil(), 7);
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```
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The type [`I20F12`] is a 32-bit fixed-point signed number with 20
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integer bits and 12 fractional bits. It is an alias to
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<code>[FixedI32][`FixedI32`]<[U12][`U12`]></code>. The unsigned
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counterpart would be [`U20F12`]. Aliases are provided for all
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combinations of integer and fractional bits adding up to a total of
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eight, 16, 32, 64 or 128 bits.
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```rust
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use fixed::types::{I4F4, I4F12};
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// −8 ≤ I4F4 < 8 with steps of 1/16 (~0.06)
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let a = I4F4::from_num(1);
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// multiplication and division by integers are possible
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let ans1 = a / 5 * 17;
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// 1 / 5 × 17 = 3 2/5 (3.4), but we get 3 3/16 (~3.2)
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assert_eq!(ans1, I4F4::from_bits((3 << 4) + 3));
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assert_eq!(ans1.to_string(), "3.2");
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// −8 ≤ I4F12 < 8 with steps of 1/4096 (~0.0002)
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let wider_a = I4F12::from(a);
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let wider_ans = wider_a / 5 * 17;
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let ans2 = I4F4::from_num(wider_ans);
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// now the answer is the much closer 3 6/16 (~3.4)
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assert_eq!(ans2, I4F4::from_bits((3 << 4) + 6));
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assert_eq!(ans2.to_string(), "3.4");
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```
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The second example shows some precision and conversion issues. The low
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precision of `a` means that `a / 5` is 3⁄16 instead of 1⁄5, leading to
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an inaccurate result `ans1` = 3 3⁄16 (~3.2). With a higher precision,
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we get `wider_a / 5` equal to 819⁄4096, leading to a more accurate
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intermediate result `wider_ans` = 3 1635⁄4096. When we convert back to
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four fractional bits, we get `ans2` = 3 6⁄16 (~3.4).
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Note that we can convert from [`I4F4`] to [`I4F12`] using [`From`], as
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the target type has the same number of integer bits and a larger
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number of fractional bits. Converting from [`I4F12`] to [`I4F4`]
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cannot use [`From`] as we have less fractional bits, so we use
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[`from_num`] instead.
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## Writing fixed-point constants and values literally
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The [*fixed-macro* crate] provides a convenient macro to write down
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fixed-point constants literally in the code.
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```rust
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use fixed::types::I16F16;
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use fixed_macro::fixed;
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const NUM1: I16F16 = fixed!(12.75: I16F16);
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let num2 = NUM1 + fixed!(13.125: I16F16);
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assert_eq!(num2, 25.875);
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```
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## Using the *fixed* crate
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The *fixed* crate is available on [crates.io][*fixed* crate]. To use
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it in your crate, add it as a dependency inside [*Cargo.toml*]:
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```toml
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[dependencies]
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fixed = "1.4"
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```
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The *fixed* crate requires rustc version 1.44.0 or later.
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## Optional features
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The *fixed* crate has four optional feature:
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1. `az`, disabled by default. This implements the cast traits
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provided by the [*az* crate].
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2. `f16`, disabled by default. This provides conversion to/from
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[`f16`] and [`bf16`]. This features requires the [*half* crate].
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3. `serde`, disabled by default. This provides serialization support
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for the fixed-point types. This feature requires the
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[*serde* crate].
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4. `std`, disabled by default. This is for features that are not
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possible under `no_std`: currently the implementation of the
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[`Error`] trait for [`ParseFixedError`].
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To enable features, you can add the dependency like this to
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[*Cargo.toml*]:
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```toml
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[dependencies.fixed]
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version = "1.4"
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features = ["f16", "serde"]
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```
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## Experimental optional features
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It is not considered a breaking change if experimental features are
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removed. The removal of experimental features would however require a
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minor version bump. Similarly, on a minor version bump, optional
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dependencies can be updated to an incompatible newer version.
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There are three experimental feature:
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1. `num-traits`, disabled by default. This implements some traits
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from the [*num-traits* crate]. (The plan is to upgrade this to an
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optional feature once *num-traits* reaches version 1.0.0.)
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2. `unwrapped`, disabled by default. This adds methods for arithmetic
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that panic on overflow even when debug assertions are disabled,
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similar to how wrapping methods will wrap even when debug
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assertions are enabled. (The plan is to always enable this
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functionality but remove the experimental feature in version
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1.5.0.)
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3. `serde-str`, disabled by default. Fixed-point numbers are
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serialized as strings showing the value when using human-readable
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formats. This feature requires the `serde` and the `std` optional
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features. (The plan is to upgrade this to an optional feature in
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version 1.5.0.) **Warning:** numbers serialized when this feature
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is enabled cannot be deserialized when this feature is disabled,
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and vice versa.
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## License
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This crate is free software: you can redistribute it and/or modify it
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under the terms of either
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* the [Apache License, Version 2.0][LICENSE-APACHE] or
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* the [MIT License][LICENSE-MIT]
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at your option.
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### Contribution
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Unless you explicitly state otherwise, any contribution intentionally
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submitted for inclusion in the work by you, as defined in the Apache
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License, Version 2.0, shall be dual licensed as above, without any
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additional terms or conditions.
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[*Cargo.toml*]: https://doc.rust-lang.org/cargo/guide/dependencies.html
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[*az* crate]: https://crates.io/crates/az
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[*fixed* crate]: https://crates.io/crates/fixed
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[*fixed-macro* crate]: https://crates.io/crates/fixed-macro
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[*fixed-sqrt* crate]: https://crates.io/crates/fixed-sqrt
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[*half* crate]: https://crates.io/crates/half
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[*num-traits* crate]: https://crates.io/crates/num-traits
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[*serde* crate]: https://crates.io/crates/serde
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[*typenum* crate]: https://crates.io/crates/typenum
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[LICENSE-APACHE]: https://www.apache.org/licenses/LICENSE-2.0
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[LICENSE-MIT]: https://opensource.org/licenses/MIT
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[`Binary`]: https://doc.rust-lang.org/nightly/core/fmt/trait.Binary.html
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[`Display`]: https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html
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[`Error`]: https://doc.rust-lang.org/nightly/std/error/trait.Error.html
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[`FixedI128`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI128.html
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[`FixedI16`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI16.html
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[`FixedI32`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html
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[`FixedI64`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI64.html
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[`FixedI8`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI8.html
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[`FixedU128`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedU128.html
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[`FixedU16`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedU16.html
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[`FixedU32`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedU32.html
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[`FixedU64`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedU64.html
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[`FixedU8`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedU8.html
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[`FromFixed`]: https://docs.rs/fixed/~1.4/fixed/traits/trait.FromFixed.html
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[`FromStr`]: https://doc.rust-lang.org/nightly/core/str/trait.FromStr.html
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[`From`]: https://doc.rust-lang.org/nightly/core/convert/trait.From.html
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[`I20F12`]: https://docs.rs/fixed/~1.4/fixed/types/type.I20F12.html
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[`I4F12`]: https://docs.rs/fixed/~1.4/fixed/types/type.I4F12.html
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[`I4F4`]: https://docs.rs/fixed/~1.4/fixed/types/type.I4F4.html
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[`Into`]: https://doc.rust-lang.org/nightly/core/convert/trait.Into.html
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[`LosslessTryFrom`]: https://docs.rs/fixed/~1.4/fixed/traits/trait.LosslessTryFrom.html
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[`LosslessTryInto`]: https://docs.rs/fixed/~1.4/fixed/traits/trait.LosslessTryInto.html
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[`LossyFrom`]: https://docs.rs/fixed/~1.4/fixed/traits/trait.LossyFrom.html
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[`LossyInto`]: https://docs.rs/fixed/~1.4/fixed/traits/trait.LossyInto.html
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[`LowerHex`]: https://doc.rust-lang.org/nightly/core/fmt/trait.LowerHex.html
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[`Octal`]: https://doc.rust-lang.org/nightly/core/fmt/trait.Octal.html
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[`ParseFixedError`]: https://docs.rs/fixed/~1.4/fixed/struct.ParseFixedError.html
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[`ToFixed`]: https://docs.rs/fixed/~1.4/fixed/traits/trait.ToFixed.html
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[`U12`]: https://docs.rs/fixed/~1.4/fixed/types/extra/type.U12.html
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[`U20F12`]: https://docs.rs/fixed/~1.4/fixed/types/type.U20F12.html
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[`UpperHex`]: https://doc.rust-lang.org/nightly/core/fmt/trait.UpperHex.html
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[`bf16`]: https://docs.rs/half/^1/half/struct.bf16.html
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[`checked_from_num`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.checked_from_num
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[`f16`]: https://docs.rs/half/^1/half/struct.f16.html
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[`from_num`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.from_num
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[`from_str_binary`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.from_str_binary
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[`from_str_hex`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.from_str_hex
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[`from_str_octal`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.from_str_octal
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[`to_num`]: https://docs.rs/fixed/~1.4/fixed/struct.FixedI32.html#method.to_num
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[const generics]: https://github.com/rust-lang/rust/issues/44580
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