Given the design requirements the choice was how to make a class that could represent physical units. The desire for it to be a compact class drove the decision to somewhat limit what could be represented to physically realizable units.
The unit class consists of a multiplier and a representation of base units. The seven SI units + radians + currency units + count units. In addition a unit has 4 flags, per-unit for per unit or ratio units. One flag[i_flag] that is a representation of imaginary units, one flags for a variety of purposes and to differentiate otherwise similar units[e_flag]. And a flag to indicate an equation unit. Due to the requirement that the base units fit into a 4 byte type the represented powers of the units are limited. The list below shows the bit representation range and observed range of use in equations and observed usage
- meter:[-8,+7] :normal range [-4,+4], intermediate ops [-6,+6]
- kilogram:[-4,+3] :normal range [-1,+1], intermediate ops [-2,+2]
- second:[-8,+7] :normal range [-4,+4], intermediate ops [-6,+6]
- ampere:[-4,+3] :normal range [-2,+2]
- kelvin:[-4,+3] :normal range [-4,+1]
- mole:[-2,+1] :normal range [-1,+1]
- candela:[-2,+1] :normal range [-1,+1]
- currency:[-2,+1] :normal range [-1,+1]
- count:[-2,+1] :normal range [-1,+1]
- radians:[-4,+3] :normal range [-2,+2]
These ranges were chosen to represent nearly all physical quantities that could be found in various disciplines we have encountered.