Number Formatter of Fixed Significance with Metric Prefix

Formats a given number in one of the two Signifix notations as defined below by

1. selecting the appropriate metric prefix and
2. moving the decimal point position in a way to sustain a fixed number of four significant figures.

Definition

The Signifix notation comprises

• a metrically normalized signed significand, that is a decimal of four significant figures from ±1.000 to ±999.9 to cover the three powers of ten of a particular metric prefix with the three different decimal point positions between these four figures, and
• a metric prefix symbol or its placeholder in case of no prefix
  • either being appended along with a whitespace as in ±1.234␣k, that is the default notation,
  • or replacing the decimal point of the significand as in ±1k234, that is the alternate notation.

In default notation the placeholder is another whitespace as in ±1.234␣␣ to align consistently, while in alternate notation it is a number sign as in ±1#234 to conspicuously separate the integer from the fractional part of the significand.

The plus sign of positive numbers is optional.

Usage

This crate is on crates.io and can be used by adding signifix to the dependencies in your project's Cargo.toml:

[dependencies]
signifix = "0.4.0"

and this to your crate root:

#![feature(try_from)] // Until stabilized. Requires nightly Rust.

extern crate signifix;

Examples

Both the default and alternate notation result in a fixed number of characters preventing jumps to the left or right:

# #![feature(try_from)]
use std::convert::TryFrom; // Until stabilized.

use signifix::{Signifix, Result};

let format = |number| -> Result<(String, String)> {
	Signifix::try_from(number)
		.map(|number| (format!("{}", number), format!("{:#}", number)))
};

assert_eq!(format(1e-04), Ok(("100.0 µ".into(), "100µ0".into())));
assert_eq!(format(1e-03), Ok(("1.000 m".into(), "1m000".into())));
assert_eq!(format(1e-02), Ok(("10.00 m".into(), "10m00".into())));
assert_eq!(format(1e-01), Ok(("100.0 m".into(), "100m0".into())));
assert_eq!(format(1e+00), Ok(("1.000  ".into(), "1#000".into())));
assert_eq!(format(1e+01), Ok(("10.00  ".into(), "10#00".into())));
assert_eq!(format(1e+02), Ok(("100.0  ".into(), "100#0".into())));
assert_eq!(format(1e+03), Ok(("1.000 k".into(), "1k000".into())));
assert_eq!(format(1e+04), Ok(("10.00 k".into(), "10k00".into())));
assert_eq!(format(1e+05), Ok(("100.0 k".into(), "100k0".into())));
assert_eq!(format(1e+06), Ok(("1.000 M".into(), "1M000".into())));

This is useful to smoothly refresh a transfer rate within a terminal:

#![feature(i128_type)] // Until stabilized.

# #![feature(try_from)]
use std::convert::TryFrom; // Until stabilized.

use std::f64;
use signifix::{Signifix, Error, DEF_MIN_LEN};

let format_rate = |bytes: u128, nanoseconds: u128| -> String {
	let bytes_per_second = bytes as f64 / nanoseconds as f64 * 1e+09;
	let unit = "B/s";
	let rate = match Signifix::try_from(bytes_per_second) {
		Ok(rate) => if rate.factor() < 1e+00 {
			" - slow - ".into() // instead of mB/s, µB/s, ...
		} else {
			format!("{}{}", rate, unit) // normal rate
		},
		Err(case) => match case {
			Error::OutOfLowerBound(rate) => if rate == 0f64 {
				" - idle - " // no progress at all
			} else {
				" - slow - " // almost no progress
			},
			Error::OutOfUpperBound(rate) => if rate == f64::INFINITY {
				" - ---- - " // zero nanoseconds
			} else {
				" - fast - " // awkwardly fast
			},
			Error::Nan => " - ---- - ", // zero bytes in zero nanoseconds
		}.into(),
	};
	assert_eq!(rate.chars().count(), DEF_MIN_LEN + unit.chars().count());
	rate
};

assert_eq!(format_rate(42_667, 300_000_000_000), "142.2  B/s");
assert_eq!(format_rate(42_667, 030_000_000_000), "1.422 kB/s");
assert_eq!(format_rate(42_667, 003_000_000_000), "14.22 kB/s");
assert_eq!(format_rate(00_001, 003_000_000_000), " - slow - ");
assert_eq!(format_rate(00_000, 003_000_000_000), " - idle - ");
assert_eq!(format_rate(42_667, 000_000_000_000), " - ---- - ");

Or to monitor a measured quantity like an electrical current including its direction with positive numbers being padded to align with negative ones:

# #![feature(try_from)]
use std::convert::TryFrom; // Until stabilized.

use signifix::{Signifix, Result, DEF_MAX_LEN};

let format_load = |amps| -> Result<String> {
	if let Some(amps) = amps {
		Signifix::try_from(amps)
			.map(|amps| format!("{:>1$}A", amps, DEF_MAX_LEN))
	} else {
		Ok("     0  A".into())
	}
};

assert_eq!(format_load(Some( 1.476e-06)), Ok(" 1.476 µA".into()));
assert_eq!(format_load(None),             Ok("     0  A".into()));
assert_eq!(format_load(Some(-2.927e-06)), Ok("-2.927 µA".into()));

While to visualize a change in file size, a plus sign might be preferred for positive numbers:

# #![feature(try_from)]
use std::convert::TryFrom; // Until stabilized.

use signifix::{Signifix, Result, Error};

let format_diff = |curr, prev| -> Result<String> {
	Signifix::try_from(curr - prev).map(|diff| format!("{:+#}", diff))
		.or_else(|case| if case == Error::OutOfLowerBound(0f64)
			{ Ok("=const".into()) } else { Err(case) })
};

assert_eq!(format_diff(78_346, 57_393), Ok("+20k95".into()));
assert_eq!(format_diff(93_837, 93_837), Ok("=const".into()));
assert_eq!(format_diff(27_473, 36_839), Ok("-9k366".into()));