newRPL: Handling of units

[Claudio L.]

(09-30-2015 04:15 PM)David Hayden Wrote:  Even addition is hard. What is 10_C + 5_F? Is it 10_C + 5_ΔF? Or 5_F + 10_ΔC? Or 10_ΔC + 5_ΔF? All of these will produce different answers.

All those things are not the same.
Because the scales have a non-null zero, 10_C is actually:
ZeroC + 10_ΔC (where ZeroC is a vector to the origin of the scale, and ΔC is a unit vector in the direction of the scale, and the _ operator is just a multiplication).
5_F = ZeroF + 5_ΔF
Therefore:
10_C+5_F = ZeroC+ZeroF+10_ΔC+5_ΔF
Adding ZeroC+ZeroF has no physical meaning (it would be just another origin of an unknown useless scale), so I like better:
10_C+5_F = 10_C- (-5_F)
ZeroC+10_ΔC - (ZeroF-5_ΔF)
(ZeroC-ZeroF) + 10_ΔC + 5_ΔF
where ZeroC-ZeroF is an actual Δ unit, so the result will be in ΔC direction.

The second case you propose is:
10_C+5_ΔF
ZeroC + 10_ΔC + 5_ΔF
ZeroC + 12.77777..._ΔC = 12.7777..._C

And this is an actual temperature, not a Δ.

Right now this latter case is not supported but will be in short. Everything else is already implemented and working like a charm.

PS: Everything else is finished, I'm just debugging/polishing the units module and I'll update the firmware image for testing in a few days.