Where to apply an Absolute/Relative distinction?

[keilw]

Based on input from https://reference.wolfram.com/language/tutorial/TemperatureUnits.html
or https://www.quora.com/What-are-absolute-units (the latter contradicts itself quite a bit because after saying "Measurement units can be either absolute or relative… " it continues like "However, physical measurements are often relative. ") Let's figure out, if Unit or Quantity are the best place for such Absolute/Relative indicator?

It is drafted in Quantity (mainly to find out the payload it adds on top of other elements like the measurement level) but e.g. the Mathematica/Wolfram examples are pretty clear about using specialized Unit definitions like Kelvin vs. KelvinDifference for that.

Not entirely sure, which is best, something like an attribute on Unit which could allow Units.KELVIN vs. Units.KELVIN_DIFFERENCE (I recall @desruisseaux mentioned that option once in an earlier thread) or an attribute on Quantity?

Also how should it be defined either statically or dynamically and how does it affect unit and quantity operations?

Needs #95
Needs #131

[keilw]

Is it possible to answer this?
Especially those who already participated in other parts of this "epic" like @desruisseaux, @dautelle, @filipvanlaenen or @andi-huber could you tell your preference please?
I sense from some discussions in other threads, it may change e.g. based on a particular operation. If that's a requirement, I believe, Quantity is the safer option. If placed on a Unit it should be a constant value on that particular Unit instance, even if that would mean having something like CELSIUS and CELSIUS_INTERVAL.

[keilw]

A) Define the absolute/relative indicator in Quantity

[keilw]

B) Define it in Unit

[keilw]

C) Define it somewhere else, please comment where you prefer.

[andi-huber]

As demonstrated with [1], the Mathematica way of handling this seems almost trivial to implement.
Note: That would be Option c), where the distinction (absolute vs relative) is made by using different units.

That's not my vote, because I'm not sure how well the other options could be implemented. But I have a feeling, that option c) could be the most simple one.

[1] unitsofmeasurement/indriya#164

[andi-huber]

We also need to carefully consider impact on Unit formatting and parsing. E.g. currently I'm using this JAXB-Adapter to allow for Quantities to be marshaled/unmarshaled ...

public static final class QuantityAdapter extends XmlAdapter<String, Quantity<?>>{
	
	private final UnitFormat format = SimpleUnitFormat.getInstance();

	public Quantity<?> unmarshal(String v) throws Exception {
		final String unitLiteral = Strings.substringEmbeddedBy(v, "[", "]");
		final String valueLiteral = Strings.substringEmbeddedBy(v, null, "[");
		final double value = Double.parseDouble(valueLiteral);
		final Unit<?> unit = format.parse(unitLiteral);
		return Quantities.getQuantity(value, unit);
	}

	public String marshal(Quantity<?> v) throws Exception {
		
		String formattedUnit = format
				.format(v.getUnit(), new StringBuilder()).toString();
		
		return v.getValue().doubleValue() + "[" + formattedUnit + "]";
	}

}

Having different Units that yield the same format, would break this code. So even if we choose option c) an additional flag indicating 'absolute' vs 'relative' would be required somewhere.