Add UnitSumPack, with labels
#654
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The hardest part about adding and subtracting `Constant` instances is that we need a unit type to represent this sum, and a readable, automatically generated unit label to go along with it. The strategy is to break this into two parts: 1. `UnitSumPack<...>` will be the actual type to hold unit sum instances. End users will _never_ form this directly. We will not bother validating weird edge cases for this, because we will assume that every instance the _library_ forms is valid and well formed (at least two units, properly ordered, etc.). 2. `UnitSum<...>` is the utility that takes multiple unit instances as inputs, and produces the canonical `UnitSumPack<...>` output. That means unpacking any inputs that are already `UnitSumPack<...>`; identifying unique "unscaled units"; collecting like terms; eliminating unscaled units that get "zeroed out"; putting what's left in the right order; and, handling cases where only 0 or 1 units are left. This PR provides the first prong of this strategy. We make sure that, given well formed `UnitSumPack` instances, the result has the right magnitude, and right unit label. Helps #607.
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The hardest part about adding and subtracting
Constantinstances isthat we need a unit type to represent this sum, and a readable,
automatically generated unit label to go along with it. The strategy is
to break this into two parts:
UnitSumPack<...>will be the actual type to hold unit suminstances. End users will never form this directly. We will not
bother validating weird edge cases for this, because we will assume
that every instance the library forms is valid and well formed (at
least two units, properly ordered, etc.).
UnitSum<...>is the utility that takes multiple unit instances asinputs, and produces the canonical
UnitSumPack<...>output. Thatmeans unpacking any inputs that are already
UnitSumPack<...>;identifying unique "unscaled units"; collecting like terms;
eliminating unscaled units that get "zeroed out"; putting what's left
in the right order; and, handling cases where only 0 or 1 units are
left.
This PR provides the first prong of this strategy. We make sure that,
given well formed
UnitSumPackinstances, the result has the rightmagnitude, and right unit label.
Helps #607.