A comparator that chains key differences with `||`, such as
`(b.score ?? 0) - (a.score ?? 0) || a.id - b.id`, ran through the
comparator sort: TimSort called the closure once per comparison. Each
term orders one number field, ascending or descending, and a term that is
0 or NaN falls through to the next, so the chain orders elements by their
keys with the first deciding. With no key NaN or -0 that order is
consistent, and a stable sort by the keys gives the comparator sort's
permutation.
The recognizer now accepts such chains of up to eight terms over number
fields of records and class instances, and a key may read an optional
number field with a number default (`x.k ?? 0`), also as a single key. The
runtime reads and checks every key of every element before it moves a
slot, then runs stable keyed passes from the last key to the first. A NaN
or -0 key, or a chain with any other term, keeps the comparator sort.
Chris Tate
f58c321c07