si-scale

Format value with units according to SI (système international d’unités).

Version requirement: rustc 1.50+

[dependencies]
si-scale = "0.1"

Getting started

This crate parses and formats numbers using the SI Scales: from 1 y (yocto, i.e. 1e-24) to 1 Y (Yotta, i.e. 1e24). It is essentially agnostic of units per-se; you can totally keep representing units with strings or uom, or something else.

Pre-defined helper functions

You can use one of the predefined helper functions to format numbers:

use si_scale::helpers::{seconds, seconds3};

let actual = format!("{}", seconds(1.3e-5));
let expected = "13 µs";
assert_eq!(actual, expected);

let actual = format!("{}", seconds3(1.3e-5));
let expected = "13.000 µs";
assert_eq!(actual, expected);

Currently the helper functions are:

• The prefix constraint UnitOnly means the provided value won't be scaled: if you provide a value larger than 1000, say 1234, it will be printed as 1234.
• Base B1000 means 1k = 1000, the base B1024 means 1k = 1024
• Groupings refer to "thousands groupings"; the provided char will be used (for instance 1234 is displayed as 1_234), if none, the value is displayed 1234.
• The mantissa format string acts only upon the mantissa: "{}" will display the value with all its digits or no digits if it is round, and "{:.3}" for instance will always display one decimal.

Custom helper functions

To define your own format function, use the scale_fn!() macro. All pre-defined helper functions from this crate are defined using this macro.

For instance, let's define a formatting function for bits per sec which prints the mantissa with 2 decimals, and also uses base 1024 (where 1 ki = 1024). Note that although we define the function in a separate module, this is not a requirement.

mod unit_fmt {
    use si_scale::scale_fn;
    use si_scale::prelude::Value;

    // defines the `bits_per_sec()` function
    scale_fn!(bits_per_sec,
              base: B1024,
              constraint: UnitAndAbove,
              mantissa_fmt: "{:.2}",
              groupings: '_',
              unit: "bit/s");
}

use unit_fmt::bits_per_sec;

fn main() {
    let x = 2.1 * 1024 as f32;
    let actual = format!("throughput: {:>15}", bits_per_sec(x));
    let expected = "throughput:    2.10 kibit/s";
    assert_eq!(actual, expected);

    let x = 2;
    let actual = format!("throughput: {}", bits_per_sec(x));
    let expected = "throughput: 2.00 bit/s";
    assert_eq!(actual, expected);
}

You can omit the groupings argument of the macro to not sepearate thousands.

SI Scales

With base = 1000, 1k = 1000, 1M = 1_000_000, 1m = 0.001, 1µ = 0.000_001, etc.

The base is usually 1000, but can also be 1024 (bibytes).

With base = 1024, 1ki = 1024, 1Mi = 1024 * 1024, etc.

Overview

The central representation is the Value type, which holds

• the mantissa,
• the SI unit prefix (such as "kilo", "Mega", etc),
• and the base which represents the cases where "1 k" means 1000 (most common) and the cases where "1 k" means 1024 (for kiB, MiB, etc).

This crate provides 2 APIs: a low-level API, and a high-level API for convenience.

For the low-level API, the typical use case is

• first parse a number into a Value. For doing this, you have to specify the base, and maybe some constraint on the SI scales. See Value::new() and Value::new_with()

• then display the Value either by yourself formatting the mantissa and prefix (implements the fmt::Display trait), or using the provided Formatter.

For the high-level API, the typical use cases are

1. parse and display a number using the provided functions such as bibytes(), bytes() or seconds(), they will choose for each number the most appropriate SI scale.

2. In case you want the same control granularity as the low-level API (e.g. constraining the scale in some way, using some base, specific mantissa formatting), then you can build a custom function using the provided macro scale_fn!(). The existing functions such as bibytes(), bytes(), seconds() are all built using this same macro.

The high-level API

The seconds3() function parses a number into a Value and displays it using 3 decimals and the appropriate scale for seconds (UnitAndBelow), so that non-sensical scales such as kilo-seconds can't be output. The seconds() function does the same but formats the mantissa with the default "{}", so no decimals are printed for integer mantissa.

use si_scale::helpers::{seconds, seconds3};

let actual = format!("result is {:>15}", seconds(1234.5678));
let expected = "result is     1234.5678 s";
assert_eq!(actual, expected);

let actual = format!("result is {:>10}", seconds3(12.3e-7));
let expected = "result is   1.230 µs";
assert_eq!(actual, expected);

The bytes() function parses a number into a Value using base 1000 and displays it using 1 decimal and the appropriate scale for bytes (UnitAndAbove), so that non-sensical scales such as milli-bytes may not appear.

use si_scale::helpers::{bytes, bytes1};

let actual = format!("result is {}", bytes1(12_345_678));
let expected = "result is 12.3 MB";
assert_eq!(actual, expected);

let actual = format!("result is {:>10}", bytes(16));
let expected = "result is       16 B";
assert_eq!(actual, expected);

let actual = format!("result is {}", bytes(0.12));
let expected = "result is 0.12 B";
assert_eq!(actual, expected);

The bibytes1() function parses a number into a Value using base 1024 and displays it using 1 decimal and the appropriate scale for bytes (UnitAndAbove), so that non-sensical scales such as milli-bytes may not appear.

use si_scale::helpers::{bibytes, bibytes1};

let actual = format!("result is {}", bibytes1(12_345_678));
let expected = "result is 11.8 MiB";
assert_eq!(actual, expected);

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