Format numbers so that they are easier to read

src/format.rs

@@ -3,10 +3,61 @@ use crate::units::{Metric, MetricQuantity};
 pub fn format(quantity: MetricQuantity) -> String {
 	let SiUnit(multiplier, unit) = si_unit(quantity);
 	let amount = quantity.amount / multiplier;
-
+	let amount = format_number(amount);
 	format!("{amount} {unit}")
 }
 
+fn format_number(number: f64) -> String {
+	let sign = if number < 0.0 { "-" } else { "" };
+	let number = number.abs();
+
+	// Lower the number of decimal digits as the number grows, to try
+	// to maintain four significant figures
+	let precision = if number < 1.0 {
+		4
+	} else if number < 10.0 {
+		3
+	} else if number < 100.0 {
+		2
+	} else if number < 1000.0 {
+		1
+	} else {
+		0
+	};
+
+	// Split number into integer and decimal parts for further processing
+	let formatted = format!("{number:.precision$}");
+	let mut formatted = formatted.split('.');
+	let integer = formatted.next().expect("f64 formatted with .precision$ must have a part before '.'");
+	let decimal = if precision > 0 {
+		formatted.next().expect("f64 formatted with .precision$ must have a part after '.' if precision > 0")
+	} else {
+		""
+	};
+
+	// Group the integer part into groups of three, e.g. 1000 → 10 000
+	let mut grouped = String::new();
+	let mut group_length = 0;
+	for c in integer.chars().rev() {
+		if group_length == 3 {
+			grouped.push(' ');
+			group_length = 0;
+		}
+		grouped.push(c);
+		group_length += 1;
+	}
+	let grouped: String = grouped.chars().rev().collect();
+
+	// Remove trailing zeroes
+	let decimal = decimal.trim_end_matches('0');
+
+	if decimal.len() == 0 {
+		format!("{sign}{grouped}")
+	} else {
+		format!("{sign}{grouped}.{decimal}")
+	}
+}
+
 #[derive(Debug, PartialEq)]
 struct SiUnit(f64, &'static str);
 
@@ -53,7 +104,7 @@ mod test {
 			amount: 1.5,
 			unit: Metric::Metre,
 		}));
-		assert_eq!("1000 km", &format(MetricQuantity {
+		assert_eq!("1 000 km", &format(MetricQuantity {
 			amount: 1_000_000.0,
 			unit: Metric::Metre,
 		}));
@@ -68,6 +119,34 @@ mod test {
 		}));
 	}
 
+	#[test]
+	fn numbers() {
+		assert_eq!(&format_number(0.0001), "0.0001");
+		assert_eq!(&format_number(0.001), "0.001");
+		assert_eq!(&format_number(0.01), "0.01");
+		assert_eq!(&format_number(0.1), "0.1");
+		assert_eq!(&format_number(1.0), "1");
+		assert_eq!(&format_number(10.0), "10");
+		assert_eq!(&format_number(100.0), "100");
+		assert_eq!(&format_number(1_000.0), "1 000");
+		assert_eq!(&format_number(10_000.0), "10 000");
+		assert_eq!(&format_number(100_000.0), "100 000");
+		assert_eq!(&format_number(1_000_000.0), "1 000 000");
+
+		assert_eq!(&format_number(0.00001), "0");
+		assert_eq!(&format_number(1.0001), "1");
+		assert_eq!(&format_number(1.001), "1.001");
+		assert_eq!(&format_number(10.001), "10");
+		assert_eq!(&format_number(10.01), "10.01");
+		assert_eq!(&format_number(100.01), "100");
+		assert_eq!(&format_number(100.1), "100.1");
+		assert_eq!(&format_number(1_000.1), "1 000");
+
+		assert_eq!(&format_number(-1.0), "-1");
+		assert_eq!(&format_number(-100.0), "-100");
+		assert_eq!(&format_number(-1000.0), "-1 000");
+	}
+
 	#[test]
 	fn units() {
 		assert_eq!(SiUnit(0.001, "mm"), si_unit(MetricQuantity {