metrify/src/format.rs

use crate::units::{Metric, MetricQuantity};

pub fn format(quantity: MetricQuantity) -> String {
	let (amount, prefix) = si_prefix(quantity.amount);
	let unit = abbreviation(quantity.unit);

	format!("{amount} {prefix}{unit}")
}

#[derive(Clone, Copy)]
struct SiPrefix {
	prefix: &'static str,
	size: f64,
}

const SI_PREFIXES: [SiPrefix; 16] = [
	SiPrefix { prefix: "z", size: 0.000_000_000_000_000_000_001 },
	SiPrefix { prefix: "a", size: 0.000_000_000_000_000_001 },
	SiPrefix { prefix: "f", size: 0.000_000_000_000_001 },
	SiPrefix { prefix: "p", size: 0.000_000_000_001 },
	SiPrefix { prefix: "n", size: 0.000_000_001 },
	SiPrefix { prefix: "µ", size: 0.000_001 },
	SiPrefix { prefix: "m", size: 0.001 },
	SiPrefix { prefix: "c", size: 0.01 }, // Included for cm
	SiPrefix { prefix: "", size: 1.0 },
	SiPrefix { prefix: "k", size: 1_000.0 },
	SiPrefix { prefix: "M", size: 1_000_000.0 },
	SiPrefix { prefix: "G", size: 1_000_000_000.0 },
	SiPrefix { prefix: "T", size: 1_000_000_000_000.0 },
	SiPrefix { prefix: "P", size: 1_000_000_000_000_000.0 },
	SiPrefix { prefix: "E", size: 1_000_000_000_000_000_000.0 },
	SiPrefix { prefix: "Z", size: 1_000_000_000_000_000_000_000.0 },
	// Yotta and above cannot be represented exactly as a f64
];

fn si_prefix(amount: f64) -> (f64, &'static str) {
	let absolute = amount.abs();
	if absolute < SI_PREFIXES[0].size {
		let prefix = SI_PREFIXES[0];
		return (amount / prefix.size, prefix.prefix);
	}
	if absolute >= SI_PREFIXES[SI_PREFIXES.len() - 1].size {
		let prefix = SI_PREFIXES[SI_PREFIXES.len() - 1];
		return (amount / prefix.size, prefix.prefix);
	}

	// Find the correct prefix SI_PREFIX[index] such that:
	//  SI_PREFIXES[index].size ≤ absolute < SI_PREFIXES[index + 1].size
	for index in 0..SI_PREFIXES.len() {
		if SI_PREFIXES[index].size <= absolute &&
		   absolute < SI_PREFIXES[index + 1].size {
			let prefix = SI_PREFIXES[index];
			return (amount / prefix.size, prefix.prefix);
		}
	}

	unreachable!();
}

fn abbreviation(unit: Metric) -> &'static str {
	match unit {
		Metric::Metre => "m",
		Metric::Gram => "g",
	}
}

#[cfg(test)]
mod test {
	use super::*;

	#[test]
	fn quantities() {
		assert_eq!("1 m", &format(MetricQuantity {
			amount: 1.0,
			unit: Metric::Metre,
		}));
		assert_eq!("5 kg", &format(MetricQuantity {
			amount: 5_000.0,
			unit: Metric::Gram,
		}));
		assert_eq!("25.5 cm", &format(MetricQuantity {
			amount: 0.255,
			unit: Metric::Metre,
		}));
	}

	#[test]
	fn prefixes() {
		assert_eq!(si_prefix(0.000_000_000_000_000_000_0005), (0.5, "z"));
		assert_eq!(si_prefix(0.000_000_000_000_000_000_001), (1.0, "z"));
		assert_eq!(si_prefix(0.000_000_000_000_000_000_01), (10.0, "z"));
		assert_eq!(si_prefix(0.000_000_000_000_000_00_01), (100.0, "z"));
		assert_eq!(si_prefix(0.000_000_000_000_000_001), (1.0, "a"));
		assert_eq!(si_prefix(0.000_000_000_000_001), (1.0, "f"));
		assert_eq!(si_prefix(0.000_000_000_001), (1.0, "p"));
		assert_eq!(si_prefix(0.000_000_001), (1.0, "n"));
		assert_eq!(si_prefix(0.000_001), (1.0, "µ"));
		assert_eq!(si_prefix(0.001), (1.0, "m"));
		assert_eq!(si_prefix(0.01), (1.0, "c"));
		assert_eq!(si_prefix(1.0), (1.0, ""));
		assert_eq!(si_prefix(1_000.0), (1.0, "k"));
		assert_eq!(si_prefix(1_000_000.0), (1.0, "M"));
		assert_eq!(si_prefix(1_000_000_000.0), (1.0, "G"));
		assert_eq!(si_prefix(1_000_000_000_000.0), (1.0, "T"));
		assert_eq!(si_prefix(1_000_000_000_000_000.0), (1.0, "P"));
		assert_eq!(si_prefix(1_000_000_000_000_000_000.0), (1.0, "E"));
		assert_eq!(si_prefix(10_000_000_000_000_000_000.0), (10.0, "E"));
		assert_eq!(si_prefix(100_000_000_000_000_000_000.0), (100.0, "E"));
		assert_eq!(si_prefix(1_000_000_000_000_000_000_000.0), (1.0, "Z"));
		assert_eq!(si_prefix(2_000_000_000_000_000_000_000.0), (2.0, "Z"));

		assert_eq!(si_prefix(-0.000_000_000_000_000_000_0005), (-0.5, "z"));
		assert_eq!(si_prefix(-0.5), (-50.0, "c"));
		assert_eq!(si_prefix(-2_000_000_000_000_000_000_000.0), (-2.0, "Z"));
	}
}